tertiary cretaceous species series american formations upper feet
NORTH AMERICA. - The recent surveys of the western territories of the United States have greatly increased our knowledge of the Cretaceous system on the American continent, where it is now known to cover a vast expanse of surface, and to reach a thickness of sometimes 10,000 feet_ Sparingly developed in the eastern States, from New Jersey into South Carolina, it spreads out over a wide area in the south, stretching round the end of the long Palaeozoic ridge from Georgia through Alabama and Tennessee to the Ohio ; and reappearing from under the Tertiary formations on the west side of the Mississippi over a large space in Texas and the south-west. Its maximum development is reached in the western States and Territories of the Rocky Mountain region - Wyoming, Utah, and Colorado. Cretaceous rocks have likewise been detected in Arctic America near the mouth of the Mackenzie River.
Much controversy has been carried on among American geologists regarding the upper limit of the Cretaceous system, some maintaining, from the character of the plants and of the shells, that the great plant-bearing series termed the Lignitic is of Tertiary age, others insisting, from the occurrence of true Cretaceous shells in and above the Lignitic series, that it must belong to the Cretaceous system. In the Upper Missouri region Hayden and Meek established the following subdivisions : - American Cretaceous fossils include the earliest dicotyledonous plants yet found on this continent, upwards of 100 species having been found, of which one-half were allied to living American forms. Among them are species of oak, willow, poplar, beech, maple, hickory, fig, tulip-tree, sassafras, sequoia, American palm (Salted), and eyeads. The more characteristic mollusca are species of Terebratula, Ostrea, Gryphtect, laweeranzus, Hippurites, Radiolites, Ammonites, Scaphites, IIamites, Baculites, Belemnites, Ancyloecras, and Turrilites. Of the fishes of the Cretaceons seas 97 species are known, comprising large predaceous representatives of modern or osseous types like the salmon and saury, though cestracionts and ganoids still flourished. But the most remarkable feature in the American Cretaceous fauna, as at present known, is the great number, variety, and size of the reptiles. According to the enumeration of Cope, who includes, however, in his list the Lignitic group here placed among the Tertiary formations, there are known at present 18 species of deinosaurs, 4 pterosaurs, 14 crocodilians, 13 sauropterygians or sea-saurians, 48 testudinates (turtles, &c.), and 50 pythonomorphs or sea-serpents. One of the most extraordinary of these reptilian forms was the Elasmosaurus - a huge snakelike form 40 feet long, with slim arrow-shaped head on a swan-like neck rising 20 feet out of the water. This formidable sea-monster "probably often swain many feet below the surface, raising the head to the distant air for a breath, then withdrawing it and exploring the depths 40 feet below without altering the position of its body. It must have wandered far from laud, and that many kinds of V fishes formed its food is shown by the teeth and scales found in the position of its stomach" (Cope). But the real rulers of the American Cretaceous waters were the pythonomorphie saurians or sea-serpents. Some of them attained a length of 75 feet or more. They possessed a remarkable elongation of form, particularly in the tail ; their heads were large, fiat, and conic, with eyes directed partly upwards. They swam by means of two pairs of paddles, like the flippers of the whale, and the eel-like strokes of their flattened tail. Like snakes they had four rows of formidable teeth on the roof of the mouth, which served as weapons for seizing their prey. But the most remarkable feature in these creatures was the unique arrangement for permitting them to swallow their prey entire, in the manner of snakes. Each half of the lower jaw was articulated at a point nearly midway between the ear and the chin, so as greatly to widen the space between the jaws, and the throat must, consequently, have been loose and baggy like a pelican's. Nine species of birds have been obtained from the American Cretaceous rocks. Three of these belonged to the order of Natatorcs or swimmers, which includes our modern gulls, ducks, and geese ; four were Gralke or waders ; while two belonged to a long extinct order, and united certain ichthyic and reptilian characters with those of birds. (See Cope, Report of U.S. Geol. Surv. of Territories, vol. ii., 1875 ; Marsh, American Journ. Science, 3d ser., i. to iv.; Leidy, Smithsonian Contributions, 1865, No. 192 ; Lesquereux, Cretaceous Flora, Report of U.S. Geol. Sure. of Territories, vol. vi.) The close of the Secondary periods was marked in the west of Europe by great geographical changes, during which the floor of the Cretaceous sea was raised partly into land and partly into shallow marine and estuarine waters. These events must have occupied a vast period of time, so that, when sedimentation once more began in the region, the organic remains of the Secondary ages had (save in a few low forms of life) entirely disappeared and given place to others of a distinctly more modern type. In England, the interval between the Cretaceous and the next geological period represented there by sedimentary formations is marked by the abrupt line which separates the top of the Chalk from all later accumulations, and by the evidence that the Chalk seems to have been in some places extensively denuded before even the oldest of what are called the Tertiary beds were deposited upon its surface. There is evidently here a considerable gap in the geological record. We have no data for ascertaining what was the general march of events in the south of England between the eras chronicled respectively by the Upper Chalk and the overlying Thanet beds.
Here and there on the Continent a few scraps of evidence are obtainable which help to fill up this gap. Thus, on the banks of the Meuse at Maestricht, a series of shelly and polyzoan limestones with a conglomeratic base (Systeme Maestricletien of Dumont, who places it above his Senonian system in the Upper Cretaceous series) contains a mingling of true Cretaceous organisms with others which are characteristic of the older Tertiary formations. It contains, for example, the characteristic Upper Chalk crinoid, Bourgeticrinus ellipticas, in great numbers ; also Ostrea vesicularis, Bactdites Faujasii, Belemnitella mucronata, and the great reptile 3-fosasaurus ; but with these occur such Tertiary genera as Voluta, Fasci,o/aria, and others. At Faxoe, on the Danish island of Seeland, the uppermost member of the Senonian series contains in like manner a blending of well-known Upper Chalk organisms with the Tertiary genera Cypr(ea, Oliva, and Mitra. In the neighbourhood of Paris also, and in scattered patches over the north of France, a formation known as the pisolitic limestone occurs, which was formerly classed with the Tertiary formations, seeing that its fossils had more affinities with later than with older rocks. But the discovery in it of numerous distinctively Upper Cretaceous forms has led to its being placed at the top of the Senonian series, from which, however, it is marked off by a decided unconformability, for it rests on a denuded surface of., the White Chalk. These fragmentary-formations are interesting, in so far as they help to show that, though in western Europe there is a tolerably abrupt separation between Cretaceous and Tertiary deposits, there was nevertheless no real break between the two periods. The one merged insensibly into the other ; but the chronicles of the intervening ages have been in great measure destroyed.
In entering upon the Tertiary series of formations, we find ourselves upon the threshold of the modern type of life. The ages of lycopods, ferns, cycads, and yew-like conifers have passed away, and that of the dicotyledonous angiosperms - the hard-wood trees and evergreens of to-day - now succeeds them, but not by any sudden extinction and recreation, for, as we have seen (ante, p. 359), some of these trees had already begun to make their appearance even in Cretaceous times. The ammonites, baculites, and other cephalopods, which had played so large a part in the molluscan life of the Secondary periods, now cease. The great reptiles, too, which in such wonderful variety of type were the dominant animals of the earth's surface, alike ou land and sea, ever since the commencement of the Lias, now wane before the increase of the mammalia, which advance in ever-augmenting diversity of type until man appears at their head.
The name Tertiary, given in the early clays of geology before much was known regarding fossils and their history, has retained its hold on the literature of the science. It is sometimes replaced by the term Cainozoic (recent life), which expresses the great fact that it is in the series of strata comprised under this designation that most recent species and genera have their earliest representatives. Taking as the basis of classification the percent-age of living species of neollusca found in tire different groups of the Tertiary series, Lyell proposed a scheme of arrangement which has been generally adopted. The older Tertiary formations, in which the number of still living species of shells is very small, where, in fact, we seem to see as it were the first beginnings of the modern life, he named Eocene (dawn of the recen-t), including under that title those parts of the Tertiary series of the London and Paris basins wherein the proportion of existing species of shells was only per cent. The middle Tertiary beds in the valleys of the Loire, Garonne, and Dordogne, containing 17 per cent. of living species, were termed Miocene (less recent).. The younger Tertiary formations of Italy were included under the designation Pliocene (more recent), because they contained a majority or from 35 to 95 per cent. of living species. Thisnewest series, however, was further subdivided into Older Pliocene (35 to 50 per cent, of living species) and Newer Pliocene (90 to 95 per cent.). This classification, with various modifications and amplifications, has been adopted for the Tertiary group not of Europe only but of the whole globe.
As the North American development of the Tertiary series differs in so many respects from that of Europe, it will be most conveniently considered by itself after the European classification has been described.