The Palaeozoic is the oldest of the three main groups into which the normal fossiliferous strata are divided; it forms the first legible volume of the earth's history, and in interpreting it speculation and hypothesis play a much less prominent part than in the pre-Cambrian volume. The Palaeozoic rocks are conglomerates, sandstones, shales, and limestones, with quite extensive areas of metamorphic rocks, and associated igneous masses, both volcanic and plutonic. The thickness of these rocks is very great, estimated in Europe at a maximum of 100,000 feet. This does not imply that such a thickness is found in any one place, but that if the maximum thicknesses of each of the subordinate divisions be added together, they will amount to that sum. In this country more than 25,000 feet of Palaeozoic strata are exposed in the much folded and profoundly denuded Appalachian Mountains, but in the Mississippi valley they attain only a fraction of that thickness. These rocks are, in the vast majority of cases, of marine origin, but some fresh-water beds are known, and very extensive swamp and river deposits have preserved a record of much of the land life of the era, especially of its later portions.

That there must have been land-surfaces is abundantly shown by the immense thickness and extent of the strata, all of which were derived from the waste of the land. Both in Europe and in North America, the land areas were prevailingly toward the north, and are doubtless indicated, in part, by the great regions of the pre-Cambrian meta-morphic rocks. The general character of the Palaeozoic beds shows that they were, in large measure, laid down in shallow water in the neighbourhood of land. Their great thickness indicates, 2N 545 further, the enormous denudation which the land areas underwent. The calculation has not been made for this country, but for Great Britain Geikie states that the lower half of the Palaeozoic group represents the waste of a plateau larger than Spain and 5000 feet high, cut down to base-level.

Very widespread disturbances of the earth's crust before the beginning of the Palaeozoic era and at its close have produced well-nigh universal unconformities with both the underlying pre-Cambrian and the overlyingMesozoic rocks; at only a few points are transitional series found.

Early in Palaeozoic time were established the main geographical outlines which dominated the growth of the North American continent, - a growth which was, for the most part, steady and tranquil. These conditions may be briefly stated as the formation of a great interior continental sea, divided from the Atlantic and the Pacific by more or less extensive and variable land areas. There are thus three principal regions of continental development: those of the Atlantic and Pacific borders and the interior. In addition, the eastern border is subdivided by pre-Cambrian ridges into subordinate areas of deposition. At the present time the surface rocks over the eastern half of the continent are prevailingly Palaeozoic, extending chiefly southward and southeastward from the great pre-Cambrian mass of the north.

Palaeozoic time was of vast length, perhaps exceeding that of the combined Mesozoic and Cenozoic eras.

The subdivisions of the Palaeozoic are very clearly marked, locally often by unconformities, but on a wide scale by the changes in the character of the fossils. There is some difference in the practice concerning these divisions, not as to their limits or order of succession, but merely as to their rank, whether certain ones should be called systems (periods) or series (epochs). This is a difference more about names than facts. The successive steps of organic and geographical development are best displayed by dividing the group into six systems, or periods, which are as follows, beginning with the oldest: 1. Cambrian; 2. Ordovician; 3. Silurian; 4. Devonian; 5. Carboniferous; 6. Permian. By many geologists the Ordovician and Silurian are comprised in one system, and the Carboniferous and Permian in another; but the present tendency is in favour of maintaining all six as equal in rank. It must not be supposed that these systems represent equal spaces of time as measured by the thickness of rocks, or equal geographical extent; on the contrary, they are very unequal in both these respects.

The classification means that the six systems, or periods, stand for approximately equivalent changes in the character of the animals and plants.