found organisms clay ooze water regions muds volcanic shells fragments
PACIFIC OCEAN DEPOSITS - the explorations of the "Challenger," "Tuscarora," and other surveying ships have in recent years given a great amount of information respecting the nature of the deposits now forming over the floor of the ocean, and the specimens collected by these expeditions have been made the subject of a careful investigation by Messrs Murray and Renard. The greet extent and depth of the Pacific Ocean make it the most suitable field for the study of the varieties of deep-sea deposits and the conditions under which they are found. The various kinds of deposits, all of which are found in the Pacific Ocean. arc classed as follows : - The terrigenous deposits are found in more or less close proximity to the land, and are chiefly made up of the triturated fragments carried down into the ocean by rivers, or worn away from the coasts by waves or currents. Those found in the deeper water surrounding the land differ from the sands, gravels, and shingles of the shore and shallow water chiefly in the smaller size of the grains and the greater abuudance of clayey matter and remains of oceanic organisms. As, however, the water becomes still deeper and the distance from land greater, the deposits assume, more and more, a deep-sea character until they pass into a true pelagic deposit.
The principal mineralogical constituents of the terrigenous deposits near continental land are isolated fragments of rocks and minerals coming from the crystalline and schisto-crystalline series, and from the elastic and sedimentary formations ; according to the character of the nearest coasts they belong to granite, diorite, diabase; porphyry, &c., crystalline schists, ancient limestones, and the sedimentary rocks of all geological ages, with the minerals which come from their disintegration, such as quartz, monoclinic and triclinic felspars, hornblende, augite, rhombic pyroxene, olivine, muscovite, biotite, titanic and magnetic iron, tourmaline, garnet, epidote, and other secondary minerals. The trituration and decomposition of these rocks and minerals give rise to materials more or less amorphous and without distinctive characters, but the origin of which is indicated by association with the rocks and minerals just mentioned.
Mixed with these arc found in many places phosphatic nodules, large quantities of glauconite, and minerals arising from chemical action probably in presence of organic matter.
Blue mad is the most extensive deposit now forming around the great continents and continental islands, and in all enclosed or partially enclosed seas. It is characterized by a slaty colour, which passes in most cases into a thin layer of a reddish colour at the upper surface. These deposits are coloured blue by organic matter in a state of decomposition, and frequently give off an odour of sulphuretted hydrogen. When dried, a blue mud is greyish in colour, and rarely or never has the plasticity and compactness of a true clay. It is finely granular, and occasionally contains fragments of rocks 2 cm. in diameter ; generally, however, the minerals which are derived from the continents, and arc found mixed up with the muddy matter in these deposits, have a mean diameter of 0.5 mm. and less. Quartz particles, often rounded, play the principal part ; next come mica, felspar, augite, hornblende, and all the mineral species which come from the disintegration of the neighbouring lands, or the lands traversed by rivers which enter the sea near the .place where the specimens have been collected. These minerals make up the principal and characteristic portion of blue muds, sometimes forming 80 per cent. of the whole deposit. Glauconite, though generally present, is never abundant. The remains of calcareous organisms are at times quite absent, but occasionally they form over 50 per cent. The latter is the case when the specimen is taken at a considerable distance from the coast and at a moderate depth. These calcareous fragments consist of bottom-living and pelagic Foraminifera, Molluscs, Polyzoa, Serpulm, Echinoderms, Aleyonarian spicules, Corals, &c. The remains of Diatoms and Radiolarians are usually present. Generally speaking, as the shores are approached the pelagic organisms disappear ; and, on the contrary, as we proceed seawards the size of the mineral grains diminishes, and the remains of shore and coast organisms give place to pelagic ones, till finally a blue mud passes into a true deep-sea deposit. In those regions of the ocean affected with floating ice, the colour of these deposits becomes grey rather than blue at great distances from land, and is further modified by the presence of a greater or les4'abmidance of glaciated blocks and fragments of quartz. These deposits are found along the coasts of North and South America, and in all the enclosed and partially enclosed seas, such as the Japan Sea, China Sea, Arafura Sea, Sulu Sea, Banda Sea, Celebes Sea, Sea of Okhotsk, &c.
At some points in tire same regions are found green muds and sands, which, as regards their origin, composition, and distribution near the shores of continental land, resemble the blue muds. They are largely composed of argillaceous matter and mineral particles of the same size and kind as the blue muds. Their chief characteristic is the presence of a considerable quantity of glaueonitic grains, either isolated or united into concretions by a brown argillaceous matter. The Foraminifera and fragments of Echinoderms and other organisms in these muds are frequently filled with glanconitic substance, and beautiful casts of these organisms remain after treatment with weak avid. At times there are few calcareous organisms in these deposits, and at other times the remains of Diatoms and Radiolarians are abundant. When these muds are dried they become earthy and of a grey-green colour. They frequently give out a sulphuretted hydrogen odour. Tire green colour appears sometimes to be due to the presence of organic matter, probably of vegetable origin, and to the reduction of peroxide of iron to protoxidc snider its influence. The green sands differ from the muds only in the comparative absence of the argillaceous and other amorphous matter, and by the more important part played by the grains of glauconitc, to which the green colour is chiefly due. Red mud is found where quantities of ochreous matter are brought down by rivers and deposited along the coast, as in the Yellow Sea, but it is most characteristic in the Atlantic off the Brazil coast of America.
In addition to the terrigenous deposits above referred to, volcanic muds and sands and coral muds and sands are found around the shores of oceanic islands either of volcanic or coral origin. The volcanic muds and sands are black or grey, and when dried arc rarely coherent. The mineral particles are generally fragmentary, and consist of lapilli of the basic and acid series of modern volcanic rocks, which are scoriaceous or compact, vitreous or crystalline, and usually present traces of alteration. The minerals are sometimes isolated, sometimes surrounded by their matrix, and consist principally of plagioclases, sanidine, amphibole, pyroxcne, biotite, olivine, and magnetic iron ; the size of the particles diminishes with distance from the shore, but the mean diameter is generally 0.5 min. Glauconite does not appear to be present in these deposits, and quartz is also very rare or absent. The fragments of shells and rocks are frequently covered with a coating of peroxide of manganese. Shells of calcareous organisms are often present in great abundance, and reinter the deposit on a lighter colour. The remains of Diatoms and Radiolarians are usually present.
Coral muds frequently contain as much as 95 per cent. of carbonate of Hine, consisting of fragments of Corals, calcareous algrc, Foraminifera, Serpulx, Molluscs, and remains of other lime-secreting organisms. There is a large amount of amorphous calcareous matter, which gives the deposit a sticky and chalky character. The particles may be of all sizes according to the distance from the reefs, the mean diameter being 1 to 2 mm., but occasionally there are large blocks of coral and large calcareous concretions ; the particles are white and red. Remains of siliceous organisms seldom make up over 2 or 3 per cent. of a typical coral mud. The residue consists usually of a small amount of argillaceous matter, with a few fragments of felspar and other volcanic minerals ; but oft' barrier and fringing reefs facing continents there may be a great variety of rocks and minerals. Beyond a depth of 1000 fathoms off coral islands the debris of the reefs begins to diminish, and the remains of pelagic organisms to increase ; the deposit becomes more argillaceous, of a reddish or rose colons., and gradually passes into a Globigerina ooze or a red clay. Coral sands con. tarn much less amorphous matter than coral muds, but in other respects they are similar, the sands being usually found nearer the reefs and in shallower water than the muds, except inside lagoons. In some regions the remains of calcareous sign predominate, and in these cases the name coralline mud or sand is employed to point out the distinction.
The extent and peculiarities of the region in which these terrigenous deposits are laid down are interesting. It extends from high-water mark down, it may be, to a depth of over 4 miles, and in a horizontal direction from 60 to perhaps 300 miles seawards, and includes all inland seas, such as the North Sea, Norwegian Sea, Mediterranean Sea, Red Sea, China Sea, Japan Sea, Caribbean Sea, and many others. It is the region of change and of variety with respect to light, temperature, motion, and biological conditions. In the surface water's the temperaturo ranges from 80° F. in the tropics to 28° F. in the polar regions. Front the surface down to the nearly ice-cold water found at the lower limits of the region in the deep sea there is in the tropics an equally great range of temperature. Plants and animals are abundant near the shore, and animals extend in relatively great abundance down to the lower limits of the region, now marked out by these terrigenous deposits. The specific gravity of the water varies much, and this variation in its turn afros the fauna and flora. In the terrigenous region tides and currents produce their maximum effect, and these influences can in some instances be traced to a depth of 300 fathoms, or nearly 2000 feet. The upper or continental margin of the region is clearly defined by the high-water mark of the coast-line, which is constantly changing through breaker actions, elevation, and subsidence. The lower or abysmal margin passes in most eases insensibly into the abysmal region, but may be regarded as ending where the mineral particles from tire neighbouring continents begin to disappear from the deposits, which then pass into an organic ooze or a red clay.
The area covered by terrigenous deposits has been called the " transitional " or " critical area," and is estimated at about two-eighths of the earth's surface, while the continents cover three-eighths, and the deep-sea deposits of the abysmal regions, which will now be considered, cover the remaining three eighths.
The true deep-sea deposits may be divided into two classes, viz., those in which the organic elements predominate, and those in which the mineral constituents play the chief part. Belonging to the former class there are Globigcrina, Pteropod, Diatom, and Radiolarian Ocses, and to the latter Red Clay.
Clobigerina ooze is the name given to all those truly pelagic deposits containing over 40 per cent, of carbonate of lime which consist principally of the dead shells of pelagic Foraminifera (GlobigeTia, OrMina, Palvinutina, Pallenia, Spliwroidina) and coceoliths and rhabdoliths. In some localities this deposit contains 95 per cent. of carbonate of lime. The colour is milky white, yellow, brown, or rose, the varieties of colour depending principally on the relative abundance in the deposit of the oxides of iron and manganese. This ooze is fine grained ; in the tropics some of the Foramin,ifera shells are macroscopic. When dried. it is pulvernlent. Analyses show that the sediment contains, in addition to carbonate of lime, phosphate and sulphate of lime., carbonate of magnesia, oxides of iron and manganese, and argillaceous matters. The residue is of a reddish-brown tinge. Lapilli, pumice, and glassy fragments, often altered into palagonite, seem always to be present, and are frequently very abundant. The mineral particles are generally angular, and rarely exceed 0.0S mm. in diameter ; monoclinic and triclinic felspars, augite, olivine, hornblende, and magnetite are the most fre quent. When quarts is present, it is in the form of minute, rounded, probably Wind-borne grains, often partially covered with oxide of iron. More rarely there are white and black particles of mica, bronzite, actinolite, chromite, glauconite, and cosmic dust. Siliceous organisms are probably never absent, sometimes forming 20 per cent. of the deposit, while at other times they are only recognizable after careful microscopic examination. In some regions the frustules of Diatoms predominate, in other the skeletons of Radiolarians.
Pteropod, ooze differs in no way from a Clobigerina ooze except in the presence of a greater number and variety of pelagic organisms, and especially in the presence of Pteropod and Heteropod shells, such as Diaeria, Atlanta, Styliola, Carina-N.4, &C. The shells of the more delicate species of pelagic Foram in ifera and young shells are also more abundant in these deposits than in a Globigerina ooze. It must be remembered that the name " Pteropod ooze " is not intended to indicate that the deposit is chiefly composed of the shells of these Molluscs, but, as their presence in a deposit is characteristic and has an important bearing on geographical and bathyme!rical distribution, it is desirable to emphasize the presence of these shells in any great abundance. it may be pointed out that there is a very considerable difference between a Globigerina ooze or a Pteropod ooze situated near continental shores and deposits bearing the same names situated towards the centres of oceanic areas, with respect both to mineral particles and to remains of organisms.
Diatom ooze is of a pale straw colour, and is composed principally of the frustules of Diatoms. When dry it is a dirty white siliceous flour, soft to the touch, taking the impression of the ' fingers, and contains gritty particles which can be recognized by the touch. It contains on an average about 25 per cent. of carbonate of lime, which exists in the deposit in the form of small Glo bigerina shells, fragments of Echinoderms and other organisms. The residue is pale white and slightly plastic ; 'minerals and fragments of rocks are in some cases abundant ; these are volcanic, or, more frequently-, fragments and minerals Conlin" from continental rock's and transported by glaciers. The fine washings consist essentially of particles of Diatoms along with argillaceous and othet amorphous matter. It is estimIted that the frnstnles of Diatoms and skeletons of siliceous organisms make up more than 50 per cent. of this deposit.
It has been already mentioned that Radiolarians are seldom, if ever, completely absent from marine deposits. In some regions they make up a considerable portion of a Clobigerina ooze, and are also found in Diatom ooze and in the terrigenous deposits of the deeper water surrounding the land. in some regions of the Pacific, however, the skeletons of these organisms make up the principal part of the deposit, to which the name Radio-lotion ooze has been given. The colour is reddish or deep brown, due to the presence of the oxides of iron and manganese. The mineral particles consist of fragments of pumice, lapilli, and volcanic minerals, rarely e::ceeding 0.07 mm. in diameter. There is not a trace of carbonate of lime in the form of shells in some samples of Radiolarian ooze, but other specimens contain 20 per cent. of carbonate of lime derived from the shells of pelagic Foraminifera. The clayey matter and mineral particles are the same as those found in the red clays, which will now be described.
Of all the deep-sea deposits red clay is the one which is distributed over the largest areas in the modern oceans. It might be said that it exists everywhere in the abysmal regions of the ocean basins, for the residue in the organic deposits which have been described under the names Clobigerina, Pteropod, Diatom, and Radiolarian oozes is nothing else than the red clay. However, this deposit only appears in its characteristic form in those areas where the terrigenous minerals and calcareous and siliceous organisms disappear to a greater or less extent from the bottom. It is in the central regions of the Pacific that the typical examples are met with. Like other marine deposits, this one passes laterally, according to position and depth, into the adjacent kind of deep-sea ooze, clay, or mud.
The argillaceons matters are of a more or less deep brown tint from the presence of the oxides of iron and manganese. In the typical examples no mineralogical species can be distinguished by the naked eye, for the grains are exceedingly fine and of nearly uniform dimensions, rarely exceeding 0.05 mm. in diameter. It is plastic and greasy to the touch ; when dried it forms lumps so coherent that considerable force must be employed to break them. It gives the brilliant streak of clay, and breaks down in water. The pyrognostic properties show that it is not a pure clay, for it fuses easily before the blowpipe into a magnetic bead.
Under the term red clay are comprised those deposits in Which the characters of clay are not well pronounced, but which are mainly composed of minute particles of pumice and other volcanic material which, ()Mug to their relatively recent deposition, have not undergone great alteration. if the analyses of red clay are calculated, it will be seen, moreover, that the silicate of alumina present as clay (2Si0.„A1,03+2H20) comprises only a relatively small portion of the sediment ; the calculation shows always an excess of free silica, which is attributed chiefly to the presence of siliceous organisms.
Microscopic examination shows that a red clay consists of argillaceous matter, minute mineral particles, and fragments of siliceous organisms. The mineral particles are for the greater part of volcanic origin, except in those cases where continental matters are transported by floating ice, or where the sand of deserts has been carried to great distances by winds. These volcanic minerals are the same constituent minerals of modern eruptive rocks enumerated in the description of volcanic muds and sands ; in the great majority of cases they are accompanied by fragments of lapilli and of pumice more or less altered. Vitreous volcanic matters belonging to the acid and basic series of rocks predominate in the regions where the red clay' has its greatest devopment; and it will be seen presently that the most characteristic decompositions which there take place are associated with pyroxenie lavas.
Associated with the red clay are almost always found concretions and microscopic particles of the oxides of iron and manganese, to which the deposit owes its colour. Again, in the typical examples of the deposit, zeolites in the form of crystals and crystalline spherules are present, along with metallic globules and silicates which are regarded as of cosmic origin. Calcareous organisms are so generally absent that they cannot be regarded as characteristic. On the other hand, the remains of Diatoms, Radiolarians, and Sponge spicules are generally present, and are sometimes very abundant. The ear-bones of various Cetaceans, as well as the remnants of other Cetacean bones and the teeth of sharks, are, in some of the typical samples far removed from the continents, exceedingly abundant, and are often deeply impregnated with, or embedded in thick coatings of, the oxides of iron and manganese. Over six hundred sharks' teeth, belonging to the genera Curclutrodon, Oxyrhina, and Lamm, and one hundred ear-bones of whales, belonging to Zipleius, Balwnoptera, Baltena, Orca, and Delphinus, along with fifty fragments of other bones, have been obtained in one haul of the dredge in the Central Pacific. The remains of these vertebrates have seldom been dredged in the organic oozes, and still more rarely in the terrigenous deposits.
The abysmal region, in which the true pelagic deposits above described arc laid down, shows a marked contrast with the "transitional" or "critical area" where the terrigenous deposits are found. The former area comprises vast undulating plains from 2 to 5 miles beneath the surface of the sea, the average being about 3 miles, here andthere interrupted by huge volcanic cones (the oceanic islands). No sunlight ever reaches these deep cold tracts. The range of temperature over them is not more than 7°, viz., from 31° to 38° F., and is apparently constant throughout the whole year in each locality. Plant life is absent, and, although animals belonging to all the great types are present, there is no great variety of form nor abundance of individuals. Change of any kind is exceedingly slow.
Leaving out of view the coral and volcanic muds and sands which are found principally around oceanic islands, the blue muds, grten muds and sands, red muds, together with all the coast and shore formations, are situated along the margins of the continents and in enclosed and partially enclosed seas. The chief characteristic of these deposits is the presence in them of continental debris. The blue muds are found in all the deeper parts of the regions just indicated, and especially near the embouchures of rivers. Red muds do not differ much from blue muds except in colour, due to the presence of ferruginous matter in greater abundance, and they are found under the same conditions as the blue muds. The green muds and sands occupy, as a rule, portions of the coast where detrital matter from rivers is not apparently- accumulating at a rapid rate, viz., on such places as the Agullms Bank, off the east coast of Australia, off the coast of Spain, and at various points along the coast of America. In the tropical and temperate zones of the great oceans, which occupy about 110° of latitude between the two polar zones, at depths where the action of the waves is not felt, and at points to which the terrigenous materials do not extend, there are now forming vast accumulations of-G/obigeriws and other pelagic. Foraminifcra, coccoliths, rhabdoliths, shells of pelagic Molluscs, and remains of other organisms. These deposits may perhaps be cal led the sediments of median depths and of warmer zones, because they diminish in great depths and tend to disappear towards the poles. This fact is evidently in relation with the surface temperature of the ocean, and shows that pelagic Forantiofera and Molluscs live in the superficial waters of time sea, whence their dead shells fall to the bottom. Clobigerina ooze is not found in enclosed seas nor in polar latitudes. In the southern hemisphere it has not been met with south of the 50th parallel. In the Atlantic it is deposited upon the bottom at a very high latitude below the warm waters of the Gulf Stream, and is not observed under the cold descending polar current which runs south in the same latitude. These facts are readily explained if it be admitted that this ooze is formed chiefly by the shells of surface organisms, which require an elevated temperature and a wide expanse of sea for their existence.
The distribution of oceanic deposits may be summarized thus. (1) The terrigenous deposits - blue muds, green muds and sands, red muds, volcanic muds and sands, coral muds and sands - are met with in those regions of the ocean nearest to land. With the exception of the volcanic muds and sands and coral muds and sands around oceanic islands, those deposits are found only lying along the borders of continents and continental islands, and in enclosed and partially enclosed seas. (2) The organic oozes and red clay are confined to the abysmal regions of the ocean basins ; a Pteropod ooze is met with in tropical and subtropical regions in depths less than 1500 fathoms, a Clubiyerineb ooze in the same regions between the depths of 500 and 2800 fathoms, a 'Radiolarian ooze in the central portions of the Pacific at depths greater than 2500 fathoms, a Diatom ooze in the Southern Ocean south of the latitude of 45° south, a red clay anywhere within the latitudes of 45° north and south at depths greater than 2200 fathoms.
As long as the conditions of the surface are the same, it might be expected that the deposits at the bottom would also remain the same. In showing that such is not the case, an agent must lie taken into account which is in direct correlation with the depth. It may be regarded as established that the majority of the calcareous organisms which make up the Globigerina and Pteropod oozes live in the surface waters, and it may also be taken for granted that there is always a specific identity between the calcareous organisms which live at the surface and the shells of these pelagic creatures found at the bottom. Clotigerina ooze is found in the tropical zone at depths which do not exceed 2400 fathoms, but when depths of 3000 fathoms are explored in this zone of the Atlantic and Pacific there is found an argillaceous deposit without, iu many instances, any trace of calcareous organisms. Descending from the "submarine plateaus" to depths which exceed 2250 fathoms, the Globigerina ooze gradually disappears, passing into a greyish marl, and finally is wholly replaced by an argillaceous material winch covers the bottom at all depths greater than 2900 fathoms.
The transition between the calcareous formations and the argillaceous ones takes place by almost insensible degrees. The thinner and more delicate shells disappear first. The thicker and larger shells lose little by little the sharpness of their content' and appear to undergo a profound alteration. They assume a brownish colour, and break up in proportion as the calcareous constituent disappears. The red clay predominates more and more as the calcareous element diminishes in the deposit. Recollecting that the most important elements of the organie deposits have descended from the sill (Tricia] waters, and that the variations in contour of the bed of the sea cannot of themselves prevent the debris of animals and plants from accnimilating upon the bottom, their absence in the red clay areas can only be explained by the hypothesis of decomposition.
Pteropod ooze, it will be remembered, is a calcareous organic deposit, in which the remains of l'teropods and oilier pelagic Al ollusca are present, though thoy do not always form a preponderating constituent, and it has been found that their presence is in correlation with the batliyinetrical distribution.
In studying the nature of the calcareous elements which are deposited in the abysmal areas, it has been noticed that, like the shells of the l'oranzinifera, those of the Thecosomatons Ptereporla, which live everywhere in the superficial waters, especially in the tropics, become fewer in number in the deposit as the depth increases. It has just been observed that the shells of FOrCI - 717421770Yr disappear gradually along a series of soundings from a point where the U/obigPrina ooze has abundance of carbonate of lime, towards deeper regions ; but it is also noticed that, when the sounding-rod brings up a graduated series of sediments from it declivity descending into deep water, among- the calcareous shells those of the Pteropods and Ileteropods disappear first in p•oportion as the depth increases. At depths less than 1400 fathoms in the tropics a Pteropod ooze is found with abundant remains of Ileterolatds and Pteropods ; deeper soundings then give a Olobicrina ooze without these 'Molluscan remains ; and in still greater depths, as has been said above, there is a red clay in which caleareons organisms are nearly, if not quite, absent.
In this manlier, then, it is shown that the remains of calcareous organisms are completely eliminated in the greatest depths of the ocean. For if such be nut the case, why are all these shells found at the bottom in the shallower depths, aria not at all in the greater depths, although they are equally abundant on the surface at both places? There is reason to think that this solution of calcareous shells is due to the prestnee of carbonic acid throughout all depths of ocean water. It is well known that this substance, dissohred in water, is an energetic sol vent of calcareous matter. The investigations of Buchanan and Dittmar have shown that carbonic acid exists in a free state in sea water, and Dittmar's analyses also show that deep-sea water contains more lime than surface water. This is a confirmation of the theory which regards carbonic acid as the agent concerned in the total or partial solution of the surface shells before or immediately after they reach the bottom of the ocean, and is likewise in relation with the fact that in high latitudes, where fewer calcareous organisms are found at the surface, their remains are removed at lesser depths than where these organisms arc in greater abundance. It has been shown that sea water itself has some effect in the solution of carbonate of lime, and further it is probable that the immense pressure to which water is subjected in great depths may have an influence on its chemical activity. Objections have been raised to the explanation here advanced, on account of the alkalinity of sea water, but it may be remarked that alkalinity presents no difficulty which need be here considered (Dittmar, Pleys. Uleem. Chu11. Ear., part i., 1884).
This interpretation also explains how the remains of Diatoms and Radiolarians (suffice organisms like the Foramiaifera) are found in greater abundance in the red clay than in a Glob*rhea ooze. The action which suffices to dissolve the calcareous matter has no effect upon the silica, and so the siliceous shells accumulate. Nor is this view of the case opposed to the distribution of the Pteropod ooze. At first it would be expected that the Foraminifera shells, being smaller, would disappear from a deposit before the Pteropod shells; but if it be remembered that the latter are very thin and delicate, and, for the quantity of carbonate of lime present, offer a larger surface to the action of the solvent than the thicker, though smaller, qiobigeriva shells, this apparent anomaly will be explained.
The origin of these vast deposits of clay is a problem of the highest interest. It was at first supposed that these sediments were composed of microscopic particles arising from the disintegration of the rocks by rivers and by the waves on the coasts. It was believed that the matters held in suspension were carried far and wide by currents, and gradually fell to the bottom of the sea. But the uniformity of composition presented by these deposits was a great objection to this view. It can be shown that mineral particles, even of the smallest dimensions, continually set adrift upon disturbed waters must, owing to a property of sea water, eventually lie precipitated at no great distance from land. It has also been supposed that these argillaceous deposits owe their origin to the inorganic residue of the calcareous shells which are dissolved away in deep water, but this view has no foundation in fact. Everything seems to show that the formation of the clay is due to the decomposition of fragmentary volcanic products, whose presence can be detected over the whole floor of the ocean.
These volcanic materials are derived from floating pumice, and from volcanic ashes ejected to great distances by terrestrial volcanoes, and carried far by the winds. It is also known that beds of lava and of tufa are laid down upon the bottom of the sea. This assemblage of pyrogenic rocks, rich in silicates of alumina, decomposes under the chemical action of the water, and gives rise, in the same way as do terrestrial volcanic rocks, to argillaceous matters, according to reactions which can always be observed on the surface of the globe, and which are too well known to need special mention here.
The universal distribution of pumice over the floor of the ocean is very remarkable, and would at first appear unaccountable; but when the fact that pieces of pumice have been known to float in sea water for a period of over three years before becoming sufficiently waterlogged to sink is taken into consideration, it will be readily understood how fragments of this material may be transported by winds and currents to an enormous distance from their point of origin before being deposited upon the bottom. Fragments of pumice are dredged in the greatest profusion in the red clay of the Central Pacific, and much less abundantly in the organic oozes and terrigenous deposits. This is owing to the rate of deposition being much slower in the former than in the latter, where the rapid accumulation of calcareous and siliceous organisms and continental debris masks their presence.
The detailed microscopic examination of hundreds of soundings has shown that the presence of pumice, of lapilli, of silicates, and of other volcanic minerals in various stages of decomposition can always be demonstrated in the argillaceous matter.
In the places where the red clay attains its most typical development, the transformation of the volcanic fragments into argillaceous matter may be followed step by step. It may be said to be the direct product of the decomposition of the basic rocks, represented by volcanic glasses, such as hyalomelan and tachylite. This decomposition, in spite of the temperature approximating to zero (32° F.), gives rise, as an ultimate product, to clearly crystallized minerals, which may be considered the most remarkable products of the chemical action of the sea upon the volcanic matters undergoing decomposition. These microscopic crystals are zeolites lying free in the deposit, and are net with in greatest abundance in the typical red-clay areas of the Central Pacific. They are simple, twinned, or spheroidal groups, which scarcely exceed half a millimetre in diameter. The crystallographic and chemical study of them shows that they must be referred to christianite. It is known how easily the zeolites crystallize in the pores of eruptive rocks in process of (fig. 5), have been formed at the expense of the decomposing volcanic matters spread out upon the bed of that ocean.
In connexion with this formation of zeolites, reference may be made to a chemical process which gives rise to the formation of nodules of manganiferous iron. These nodules are almost universally distributed in oceanic sediments, but are met with in the greatest abundance in the reel clay. This association tends to show a common origin. It is exactly in those regions where there is an accumulation of pyroxenic lavas in decomposition, containing silicates with a base of manganese and iron, such for example as augite, hornblende, olivine, magnetite, and basic glasses, that manganese nodules occur in greatest numbers. In the regions where the sedimentary action, mechanical and organic, is, as it were, suspended, and where everything shows an extreme slowness of deposition, - in these calm waters favourable to chemical reactions, ferro-manganiferous substances form concretions around organic and inorganic centres.
These eoncentrations of ferric and manganic oxides, mixed with argillaceous materials whose form and dimensions are extremely variable, belong generally to the earthy variety or wad, but pass sometimes, though rarely, into varieties of hydrated oxide of manganese with distinct indications of radially fibrous crystallization.1 The interpretation necessary, in order to explain this formation of manganese nodules, is the same as that admitted in explanation of the formation of coatings of this material on the surface of terrestrial rocks. These salts of manganese and iron, dissolved in water by carbonic acid, then precipitated in the form of earl onate of protoxide of iron and manganese, become oxidized, and give rise in the calm and deep oceanic regions to more or less pure ferro.manganiferous concretions. At the same time it must be admitted that rivers may bring to the ocean a contribution of the, same substances.
Among the bodies which, in certain regions where reel clay predominates, serve as centres for these manganiferous nodules are the remains of vertebrates. These remains are the hardest parts of the skeleton - tympanic, bones of whales, beaks of Ziphius, teeth of sharks ; and, just as the calcareous shells are eliminated in great depths, so all the re mains of the larger vertebrates are absent, except the most resistant portions. These bones often serve as a centre for the manganese iron concretions, being frequently surronnded by layers several centimetres in thickness (fig. 8). In the same dredgings in the red-clay areas some sharks' teeth and Cetacean ear-bones, some of which belong to extinct species, are surrounded with thick layers of the manganese, and others with merely a slight coating.
.c The cosmic spherules incidentally referred to under the description of red clay may be here described in greater detail. If a magnet be plunged into an oceanic deposit, especially a red clay from the central parts of the Pacific, particles are extracted, some of which are magnetite from volcanic rocks, to which vitreous matters are often attached ; others again are quite isolated, and differ in most of their properties from the former. The latter are generally round, measuring hardly 0.2 MM., usually smaller; their surface is quite covered with a brilliant black coating having all the properties of magnetic oxide of iron ; often there may be noticed clearly marked upon them cup-like depressions (figs. 7 and 8). If these spherules be broken down in an agate mortar, the brilliant black coating easily falls away and reveals white or grey metallic malleable nuclei, which may be beaten out by the pestle into thin lamelbe. This metallic centre, when treated with an acid solution of sulphate of copper, immediately assumes a coppery coat, thus showing that it is native iron. lint there are some malleable metallic nuclei extracted from the spherules which do not give this reaction ; they do not take the copper coating.
Chemical reactions show that they contain cobalt and nickel ; very probably they constitute an alloy of iron and these two metals, such as is often found in meteorites, and whose presence in large quantities hinders the production of the coppery coating on the iron. G. Rose has shown that this coating of black oxide of iron is found on the periphery of meteorites of native iron, and its presence is readily understood when their cosmic origin is admitted. Indeed, these meteoric particles of native iron in their transit through the air must undergo combustion, and, like small portions of iron from a smith's anvil, be transformed either entirely or at the surface only into magnetic oxide, and in the latter case the nucleus is protected from further oxidation by the coating which thus covers i t.
Offe may suppose that meteorites in their passage through the atmosphere break into numerous fragments, that incandescent particles of iron are thrown off all round them, and that these eventually fall to the surface of the globe as almost impalpable dust, in the form of magnetic oxide of iron more or less completely fused. The luminous train of falling stars is probably due to the combustion of these innumerable particles resembling the sparks which fly from a ribbon of iron burnt in oxygen, or the particles of the same metal thrown off when striking a flint. It is easy to show that these particles in burning take a spherical form, and are surrounded by a layer of black magnetic oxide.
Among the magnetic grains found under the same conditions as those just described are other spherules, which are referred to the chondres, so that, if the interpretation of a cosmic origin for the magnetic sphernles with a metallic centre were not established in a manner absolutely beyond question, it almost becomes so when their association with the silicate spherules, which will now be described, is taken into account. It will be seen by the microscopic details that these spherules have quite the constitution and structure of ch,ondres To frequent in meteorites of the most ordinary type, and on the other hand they have never been found, as far as is known, in rocks of a terrestrial origin ; in short, the presence of these spherules in the deep-sea deposits, and their association with the metallic spherules, are matters of prime importance.
Among the fragments attracted by the magnet in deep-sea deposits are distinguished granules slightly larger than the spherules with the shining black coating above described. These are yellowish-brown, with a bronze-like lustre, and under the microscope it is noticed that the surface, instead of being quite smooth, is grooved by thin lamellae. Their dimensions never attain a millimetre, generally they are about 0.5 min. in diameter ; they are never perfect spheres, as in the case of the black spherules with a metallic centre ; and sometimes a depression more or less marked is to be observed in the periphery. Illicit examined by the microscope it is observed that the lamella: which ,compose them are applied the one against the other, and have a radial eccentric disposition. It is the loafy radial structure (rudialbliittrig), like that of the chondres of bronzite, which predominates in these spherules. The serial structure of the chondres with olivine is observed 11)11011 less rarely, and indeed there is some doubt about the indications of this last type of structure. Fig. 9 shows the characters and texture of one of these spherules magnified 25 diameters. On account of their small dimensions, as well as of their friability due to their lamellar structure, it is difficult to polish one of these spherules, and it has been necessary to study them with reflected light, or to limit the observations to the study of the broken fragments, These spherules break up following the lamellar, which latter are seen to be extremely fine and perfectly transparent. In rotating between crossed nicols they have the extinctions of the rhombic system, and in making use of the condenser it is seen that they have one optic axis. It is observed also that when several of these lamellae are attached they extinguish exactly at the same time, so that everything tends to show that they form a single indi victual.
In studying these transparent and very thin fragments with the aid of a high magnifying power, it is observed that they are dotted with b•own-black inclusions, disposed with a certail) symm etry, and showing somewhat regular contours ; these inclusions are referred to magnetic iron, and their presence explains why these spherules of bronzite are extracted by the magnet. It should he observed, however, that they are not so strongly- magnetic as those with a metallic nucleus.
They are designated bronzite rather than enstatite, because of the somewhat deep tint which they present ; they are insoluble in hydrochloric acid. Owing to the small quantity of substance, only a qualitative analysis could be made, which showed the presence in them of silica, magnesia, and iron.
The study of deep-sea deposits suggests some interesting conelusions. It has been said that the debris carried away from the land accumulates at the bottom of the sea before reaching the abysmal ( regions of the ocean. It is only in exceptional cases that the finest 1 te•rigenous materials are transported several hundred miles from the shores. 1 n place of layers formed of pebbles and elastic elements with grains of considerable dimensions, which play so large a part in the composition of emerged lands, the great areas of the ocean basins are covered by the microscopic remains of pelagic organisms, or by the deposits coming from the alteration of volcanic products. The distinctive elements that appear in the river and coast sediments are, properly speaking, wanting in the great depths far distant from the coasts. To such a degree is this the case that in a great number of soundings, from the centre of the Pacific for example, no mineral particles on which the mechanical action of water had left its imprint have been distingnished, and quartz is so rare that it may be said to be absent. It is sufficient to indicate these facts in order to make apparent the profound differences which separate the deposits of the abysmal areas of the ocean basins from the series of rocks in the geological formations. As regards the vast deposits of red clay, with its manganese concretions, its zeolites, cosmic dust, and remains of vertebrates, and the organic oozes which are spread out over the bed of the Central Pacific, Atlantic, and Indian Oceans, have they their analogues in the geological series of rocks? if it be proved that in the sedimentary strata the true pelagic sediments are not represented, it follows that deep and extended oceans like those of the present day cannot formerly have occupied the areas of the present continents, and as a corollary the great lines of the oceanic basins and continents must have been marked out from the earliest geological ages.
Without asserting that the terrestrial areas and the areas covered by the waters of the great ocean basins have had their main lines marked out since the commencement of geological history, it is a fact proved by the evidence of the pelagic sediments that these areas have a great antiquity. The accumulation of sharks' teeth, of the ear-bones of Cetaceans, of manganese concretions, of zeolites, of volcanic material in an advanced state of decomposition, and of cosmic dust, at points far removed from the continents, tends to prove this. There is no reason for supposing that the parts of the ocean where these vertebrate remains are found are more frequented by sharks or Cetaceans than other regions where they are never, or only rarely, dredged from the deposits at the bottom. When it is remembered also that these ear-bones, teeth of sharks, and volcanic fragments are sometimes incrusted with two centimetres of manganese oxide, while others have a mere coating, and that some of the bones and teeth belong to extinct species, it may be concluded with great certainty that the clays of these oceanic basins have accumulated with extreme slowness. It is indeed almost beyond question that the red-clay regions of the Central Pacific contain accumulations belonging to geological ages different from our own. The great antiquity of these formations is likewise confirmed in a striking manner by the presence of cosmic fragments, the nature of which has been descriocd. In order to account for the accumulation of all these substances in such relatively great abundance in the areas where they were dredged, it is necessary to suppose the oceanic basins to have remained the same for a vast period of time.
The sharks' teeth, ear-bones, manganese nodules, altered volcanic fragments, zeolites, and cosmic dust are met with in greatest abundance in the red clays of the Central Pacific, at that point on the earth's surface farthest removed from continental land. They are le;s abundant in the Radiolarian ooze, are rare in the Globigerina, Diatom, and Pteropod oozes, and have been dredged only in a few instances in the terrigenous deposits close to the shore. These substances are present in all the deposits, but owing to the abundance of other matters in the more rapidly forming deposits their presence is masked, and the chance of dredging them is reduced. The greater or less abundance of these materials, which are so characteristic of a true red clay, may be regarded as a measure of the relative rate of accumulation of the marine sediments in which they lie. The terrigenons deposits accumulate most rapidly ; then follow in order Pteropod ooze, Globigerina ooze, Diatom ooze, Radiolarian ooze, and, slowest of all, red clay.
From the data now advanced it appears possible to deduce other conclusions important from a geological point of view. In the deposits due essentially to the action of the ocean, the great variety of sediments which may accumulate in regions where the external conditions are almost identical is very striking. Again, marine faunas and floras, at least those of the surface, differ greatly, both with respect to species and the relative abundance of individuals, in different regions of the ocean ; and, as their remains determine the character of the deposit in many instances, it is legitimate to conclude that the occurrence of organisms of a different nature in several beds is not an argument against the synchronism of the layers which contain them. In this connexion may be noted the fact that in certain regions of the deep sea no appreciable formation is now taking place. Hence the absence, in the sedimentary series, of a layer representing a definite horizon must not always be interpreted as proof either of the emergence of the bottom of the sea during the corresponding period, or of an ulterior erosion.
The small extent occupied by littoral formations, especially those of an arcnaceous nature, and the relatively slow rate at which such deposits are formed along a stable coast, are matters of importance. In the present state of things there does not appear to be anything to account for the enormous thickness of the elastic sediments making up certain geological formations, unless the exceptional cases of erosion which are brought into play when a coast is undergoing constant elevation or subsidence are considered. Great movements of the land are doubtless necessary for the formation of thick beds of transported matter like sandstones and conglomerates. Arenaceous formations of great thickness require seas of no great extent and coasts subject to frequent oscillations, which permit the shores to advance and retire. Along these, through all periods of the earth's history, the great marine sedimentary phenomena have taken place.
The continental geological formations, when compared with marine deposits of modern seas and oceans, present no analogues to the red clays, Radiolarian, Globigerina, Pteropod, and Diatom oozes. On the other hand, the terrigenous deposits of lakes, shallow seas, enclosed seas, and the shores of the continents reveal the equivalents of the chalks, greensands, sandstones, conglomerates, shales, marls, and other sedimentary formations. Such formations as certain Tertiary deposits of Italy and the Radiolarian earth from Barbados, where pelagic conditions are indicated, must be regarded as having been laid down rather along the border of a continent than in a true oceanic area. The white chalk is evidently not a deep-sea deposit, for the Foram/diem and fragments of other organisms of which it is largely composed are similar to those found in comparatively shallow water not far from land. The argillaceous and calcareous rocks recently discovered by Dr Guppy in the upraised coral islands in the Solomon group are identical with the deposits now forming around oceanic islands. Regions situated similarly to enclosed and shallow seas and the borders of the present continents appear to have been, throughout all geological ages, the theatre of the greatest and most remarkable changes ; in short, all, or nearly ail, the sedimentary rocks of the continents would seem to have been built up in areas like those now occupied by the terrigenous deposits.
During each era of the earth's history the borders of some lands have sunk beneath the sea and been covered by marine sediments, while in other parts the terrigenous deposits have been elevated into dry land, and have carried with them a record of the organisms which flourished in the sea of the time. In this transitional area there has been throughout a continuity of geological and biological phenomena.
From these considerations it will be evident that the character of a deposit is determined much more by distance from the shore of a continent than by actual depth ; and the same would appear to be the case with respect to the fauna spread over the floor of the present oceans. Dredgings near the shores of continents, in depths of 1000, 2000, or 3000 fathoms, are more productive both in species and individuals than dredgings at similar depths several hundred miles seawards. Again, among the few species dredged in the abysmal areas farthest removed from land, the majority show archaic characters, or belong to groups which have a wide distribution in time as well as over the floor of the present oceans. Such are the Hexaetinellida, Bradziopoda, Stalked Crinoids and other Echinoderms, &a.
As already mentioned, the "transitional area" is that which now shows the greatest variety in respect to biological and physical conditions, and iu past time it has been subject to the most frequent and the greatest amount of change. The animals now hying in this area may be regarded as the greatly modified descendants of those which have lived in similar regions in past geological ages, and some of whose ancestors have been preserved in the sedimentary rocks as fossils. On the other hand, many of the animals dredged in the abysmal regions are most probably also the descendants of animals which lived in the shallower waters of former geological periods, but migrated into deep water to escape the severe struggle for exisience which must always have obtained in shallower waters influenced by light, heat, motion, and other favourable conditions. Having found existence possible in the less favourable and deeper water, they may be regarded as having slowly spread themselves over the floor of the ocean, but without undergoing great modifications, owing to the extreme uniformity of the conditions and the absence of competition. Or it may be supposed that, in the depressions which have taken place near coasts, some species have been gradually carried down to deep water, have accommodated themselves to the new conditions, and have gradually- migrated to the regions far from land. A few species may thus have migrated to the deep sea during each geological period. In this way the origin and distribution of the deep-sea fauna in the present oceans may in some measure be explained. In like manner, the pelagic fauna and flora of the ocean b is most probably derived originally from the shore and shallow water. During each period of the earth's history a few animals and plants have been carried to sea, and have ultimately adopted a pelagic mode of life.
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