NORTH AMERTC1. - Roeks corresponding in position and in the general character of their organic contents with the Cambrian formations of Britain have been recognized in different parts of the United States and Canada, They appear in Newfoundland, whence, ranging by Nova Scotia and New Brunswick, they enter Canada, the northern parts of New York, Vermont, and eastern Massachusetts. They rise main along the Appalachian ridge, in Wisconsin, Minnesota, Missouri, Arkansas, Texas, and Georgia. Westwards from the great valley of the Mississippi, where they have been found in many places, they reappear from under the Secondary and younger Palaeozoic rocks of the Rocky Mountains. They have been divided by American geologists into two formations - •) Acadian, a mass (2000 feet) of grey and dark shales and some sandstones; and (2) Potsdam (or Georgian), which attains in Newfoundland a depth of 5600 feet, but thins away westward and southward till in the valley of the St Lawrence, where it was studied by Logan and his associates of the Geological Survey of Canada, it is only from 300 to 600 feet thick.
Among the organic remains of the North American Cambrian rocks fueoid casts appear in many of the sandstones, but no traces of higher vegetation. The Acadian formation has yielded primordial trilobites of the genera Paradoxides, Conocory-phe, Agnostus, and some others ; brachiopods of the genera Lingulella, Discina, Obolella, and Orthis; and several kinds of annelide-tracks. The Potsdam rocks contain a few sponges, the earliest- forms of graptolite, some brachiopods, including, besides the genera in the Acadian beds, Obolus, Cconarella, and Orthisina ; some pteropods (Ilyolites or Theca) ; two species of Orthoceras ; annelide tracks ; trilobites of the genera Conocoryphe, ,Ignostus, Dike?ocephalus, Olenell us, Ptychaspis, Ch«riocephalus, Aglaspis, and flIcenurus.
M. Barrande has called attention to the remarkable uniformity of character in the organic remains of his primordial zone over the continents of Europe and America. He published in 1871 the subjoined table, to show how close is the parallelism between the proportions in which the different classes of the animal kingdom are represented.° The important system of rocks next to be described was first investigated by the late Sir R. Murchison in Wales and the bordering counties of England. He found it to be characteristically developed over the tract once inhabited by the Silures, an ancient British tribe, and he thence chose the name of Silurian as a convenient designation. It there passes down conformably into the Tremadoc slates at the top of the Cambrian series, and is covered conformably by the base of the Old Red Sandstone.