deposited. These materials were washed down from the land to the sea, and there deposited. The several systems of Paleozoic rocks are distinguished from one another by their fossils. Thus, the fossils of the Cambrian system of rocks are sufficiently unlike those of the Ordovician system to be readily distinguished by paleontologists. Even at the beginning of the Cambrian period the range of life was great, all the great types which are living at the present time, except the vertebrates, being represented, given the vertebrates may have lived, though relics of their existence have never been found. In this period trilobites and brachiopods were perhaps the most abundant and the most characteristic life, judged by the abundance of fossils. In the rocks of the Ordovician system fish-remains have been found and also relics of air-breathing life. Mollusks, crinoids and corals lived in great profusion, in addition to types of life which predominated in the Cambrian period. Most of the oil and gas of Ohio and Indiana come from the rocks of the Ordovician age. In the Silurian period the same general types of life were prevalent, but the species are so far unlike those of the preceding period as to be readily distinguished by those who are familiar with fossils. The Devonian period is often known as the Age of Fishes, on account of the abundance of fish-remains which have been found in the rocks of this system. It is far from certain, however, that fish were more abundant than now, and the variety of fish was probably less than at the present time. The Devonian was, however, the first period when fishes were abundant, so far as now known. The oil of Pennsylvania and Canada is largely derived from beds of the Devonian age. During the Carboniferous period there were extensive marshes in the United States and in some other parts of the world, in which vegetable-matter accumulated in great quantity. These marshes were subsequently submerged, and the vegetable matter was buried by sediments and ultimately converted into coal. Most of the coal of the United States east of the Great Plains was accumulated at this period. Plant-life was abundant, but the plants were largely of types now extinct. It was during this period, as far as now known, that reptilian life began. The Permian period represents a transition stage between the Paleozoic and the Mesozoic era. In the Permian period there was extensive glaciation in Australia, South Africa and India.
The several systems of Paleozoic rocks have somewhat different distribution, and, since the area of the deposits of any period corresponds approximately with the submerged area of that period, the distribution of the several systems helps us to understand the relations of land and water during the several periods. In this way it is known that the relations of sea and land were different at different times. It would appear either that the continent repeatedly oscillated up and down, causing areas which were at one time submerged to rise to above the level of the water at another and areas which were land at one time to sink beneath the sea at another; or else that the sea-level itself fluctuated. If the sea-level rose, it would overspread the low lands; if it were lowered, it would cause areas which had been submerged to become land. How far the changes in geography were the result of land-oscillations and how far they were the result of oscillations of the sea-level has never been determined. So far as present knowledge goes, it would appear that the deep-sea bottom has at no time been land and that the areas which were alternately above and below sea-level were low when they were land, and covered by shallow water only when they were submerged.
The Mesozoic (formerly called secondary) era, as the term indicates, was the era when life intermediate between the ancient and the present existed. This era is divided into several periods, as indicated above. The Triassic formations of North America are somewhat widespread in the western third of the continent, but have but little development in the eastern part and none at all in the interior. During this period reptiles perhaps were the dominant type of life. They were not only numerous, but the individuals attained great size. The earliest remains of mammals date from this period. Marine life abounded, but departed notably from the types which had prevailed in the Paleozoic era. Vegetation was abundant, but of types now extinct or but meagerly represented. The Jurassic period followed, and the distribution of the Jurassic formations is similar to that of the Triassic. The life of this period was, however, somewhat different from that of the preceding, though the same general types abounded. The oldest remains of birds are found in the rocks of this period. The Jurassic period was followed by the Cretaceous or Chalk period. In the early part of this period chalk was not being deposited, but in the later part chalk-deposits were in process of formation in many parts of the earth. The chalk-deposits are made up, for the most part, of the shells of minute marine animals (see Chalk). The Cretaceous formations of North America are much more widespread than those of the Jurassic and Triassic periods. Their distribution indicates that a large part of the North American continent was submerged during some parts of the period. It was during the Cretaceous period that the modern types of plants and of fishes made their appearance. During the last stages of the Cretaceous period
