The Paleozoic era. The second volume of the geological record, called the Paleozoic (Greek, PALAIOS, ancient; Zoe, life), has come down to us far less mutilated and defaced than has the first volume, which contains the traces of the most ancient life of the globe. Fossils are far more abundant in the Paleozoic than in the earlier strata, while the sediments in which they were entombed have suffered far less from metamorphism and other causes, and have been less widely buried from view, than the strata of the pre-Cambrian groups. By means of their fossils we can correlate the formations of widely separated regions from the beginning of the Paleozoic on, and can therefore trace some outline of the history of the continents.
Paleozoic time, although shorter than the pre-Cambrian as measured by the thickness of the strata, must still be reckoned in millions of years. During this vast reach of time the changes in organisms were very great. It is according to the successive stages in the advance of life that the Paleozoic formations are arranged in five systems,—the Cambrian, the Ordovician, the Silurian, the Devonian, and the carboniferous. On the same basis the first three systems are grouped together as the older Paleozoic, because they alike are characterized by the dominance of the invertebrates; while the last two systems are united in the later Paleozoic, and are characterized, the one by the dominance of fishes, and the other by the appearance of amphibians and reptiles.
Each of these systems is world-wide in its distribution, and may be recognized on any continent by its own peculiar fauna. The names first given them in Great Britain have therefore come into general use, while their subdivisions, which often cannot be correlated in different countries and different regions, are usually given local names.
The first three systems were named from the fact that their strata are well displayed in Wales. The Cambrian carries the Roman name of Wales, and the Ordovician and Silurian the names of tribes of ancient Britons which inhabited the same country. The Devonian is named from the English county Devon, where its rocks were early studied. The Carboniferous was so called from the large amount of coal which it was found to contain in Great Britain and continental Europe.
Distribution of strata. The Cambrian rocks outcrop in narrow belts about the pre-Cambrian areas of eastern Canada and the Lake Superior region, the Adirondacks and the Green Mountains. Strips of Cambrian formations occupy troughs in the pre-Cambrian rocks of New England and the maritime provinces of Canada; a long belt borders on the west the crystalline rocks of the Blue Ridge; and on the opposite side of the continent the Cambrian reappears in the mountains of the Great Basin and the Canadian Rockies. In the Mississippi valley it is exposed in small districts where uplift has permitted the stripping off of younger rocks. Although the areas of outcrop are small, we may infer that Cambrian rocks were widely deposited over the continent of North America.