wards to the great lakes. In this vast region they have been subdivided into an older series, termed Laurentian, and a younger series, called Huronian. It thus appears that both in the Old and New World, the Archæan rocks are chiefly exposed in the northern tracts of the continents. The areas which they there overspread were probably land at a very early geological period, and it was mainly from the waste of this land that the original materials were derived, out of which the enormous masses of stratified rocks were formed.

In the southern hemisphere also ancient gneisses and other schists, referred to the Archæan system, rise from under the oldest fossiliferous formations. In Australia and in New Zealand they cover large tracts of country, and appear in the heart of the mountain ranges. It thus appears that all over the world the oldest known rocks are gneisses and similar or allied crystalline masses, having a remarkable uniformity of character.

CHAPTER XVII

THE PALEOZOIC PERIODS-CAMBRIAN

THE portion of geological history which treats of those ages in which the earliest known types of plants and animals lived is termed Palæozoic. Of the first appearance of organic life upon our planet we know nothing. Whether plants or animals came first, and in what forms they came, are questions to which as yet no satisfactory answer can be given. The oldest discovered fossils are assuredly not vestiges of the first living things that peopled the globe. There is every reason, indeed, to hope that as researches in all parts of the world are pushed into older and yet older rocks, still more ancient organisms may be discovered. But it is in the highest degree improbable that any trace of the earliest beginnings of life will ever be found. The first plants and the first animals were probably of a lowly kind, with no hard parts capable of preservation in the fossil state. Moreover, the sedimentary rocks which may have chronicled the first advent of organised existence are hardly likely to have escaped the varied revolutions to which all parts of the crust of the earth have been exposed. They have more probably been buried out of sight, or have been so crushed, broken, and metamorphosed, that their original condition, together with any fossils they may have enclosed, is no longer to be recognised. The first chapters have been, as it were, torn out from the chronicle of the earth's history.

The Palæozoic rocks, which contain the earliest record of plant and animal life, consist mainly of the hardened mud, sand, and gravel of the sea-bottom. Here and there they include beds, or thick groups of beds, of limestone composed of marine shells, crinoids, corals, and other denizens of salt water. They are thus essentially the chronicles of the sea. But they also contain occasional vestiges of shores, and even of the jungles and swamps

of the land, with a few rare glimpses into the terrestrial life of the time. Everywhere they abound in evidence of shallow water; for though chiefly marine, they appear to have been accumulated not far from land. We may believe that in the earliest periods, as at the present day, the sediment washed away from the land has been deposited on the sea-floor, for the most part at no great distance from the coast.

The land from the waste of which the Palæozoic rocks were formed lay in Europe and North America chiefly towards the north. It no doubt consisted of Archæan rocks, such as still rise out from under the oldest Palæozoic formations. As already mentioned, the north-west Highlands of Scotland, part of the table-land of Scandinavia, and most of North America to the north of the great lakes, are probably portions of that earliest land, which, after being deeply buried under later geological accumulations, have once more been laid bare to the winds and waves. We can form some conception of the bulk of the primeval northern land by noting the thickness of sedimentary rocks that were formed out of its detritus during the Paleozoic periods. The older half of the Palæozoic rocks in the British Islands, for example, is at least 16,000 feet or 3 miles thick, and covers an area of not less than 60,000 square miles. This material, derived from the waste of the Archæan rocks, would make a table-land larger than Spain, with an average height of 5000 feet, or a mountain chain 1800 miles long, with an average elevation of 16,000 feet. Of the general form and height of the northern land that supplied this vast mass of sedimentary matter nothing is known. Perhaps it was lofty; but it may have been slowly uplifted, so that its rise compensated for the ceaseless degradation of its surface.

Abundant evidence of volcanic action has been preserved among the Palæozoic rocks in the form of piles of lavas and tuffs. We find also many indications of upward and downward movements of the crust of the earth. The mere fact of the superposition of many thousands of feet of shallow-water strata, one above another, is a proof of gradual subsidence. For it is evident that the accumulation of such a thickness of sediment, and the continuance of a shallow sea over the area of deposition, could only take place during a progressive subsidence (see p. 181).

The life of the Paleozoic periods, so far as known from the fossils which have been obtained from the rocks, appears to have been far more uniform over the whole globe than at any subse

quent epoch in geological history. For instance, the same species of fossils are found in corresponding rocks in Britain, Russia, United States, China, and Australia. The climate of the globe

at that ancient date was doubtless more uniform than it afterwards became, and was probably also generally warmer. Paleozoic fossils, obtained from high northern latitudes, are precisely similar to those that abound in England, whence it may be inferred that not only was there a greater uniformity of climate, but that the great cold which now characterises the Arctic regions did not then exist.

In the earlier Palæozoic periods, the animal life of the globe appears to have been entirely invertebrate, the highest known types being chambered shells, of which our living nautilus is a representative. In the middle periods vertebrate life appeared. The earliest known vertebrate forms are fishes akin to some modern sharks and to the sturgeon, the polypterus of the Nile, and the gar-pike of American lakes. The most highly organised forms of existence upon the earth's surface in the later Palæozoic periods were amphibians. a class of animals represented at the present day by frogs, toads, newts, and salamanders. It is evident, however, that the number and kinds of animal remains preserved in Palæozoic rocks afford only an imperfect record of the animal life of these early ages. Whole tribes of creatures no doubt existed of which no trace

whatever has yet been recovered. An accidental discovery may at any moment reveal the former presence of some of these vanished forms. For example, the examination of a fossil tree-trunk imbedded among the coal-strata of Nova Scotia, led to the finding of the first and as yet almost the only traces of Palæozoic landshells, though thousands of species of marine shells, belonging to the same period, had long been known. Every year is enlarging our knowledge in these respects, but from the very nature of the circumstances in which the records of the rocks were formed, we cannot expect this knowledge ever to be more than fragmentary.

The Paleozoic rocks are divided into five systems which in the order of their age have been named; (1) Cambrian; (2) Silurian; (3) Devonian; (4) Carboniferous; (5) Permian.

CAMBRIAN.

The strata containing the earliest organic remains were formerly known as Greywacke, from the rock which is specially abundant among them. They were also termed Transition, from the sup

position that they were deposited during a transitional period, between the time when no organic life was possible on the earth's surface, and the time when plant and animal life abounded. But the late Sir Roderick Murchison, who first explored them, showed that they contain a series of formations, each characterised by its own assemblage of organic remains. He called them the Silurian system, after the name of the old British tribe-the Silures, who lived on the borders of England and Wales, where these rocks are especially well developed. This name has now been adopted all over the world as the designation of those stratified formations which contain the same or similar organic remains to those found in the typical region described by Murchison.

While the succession of the rocks and fossils was established by that geologist in South Wales, and in the border counties of Wales and England, Professor Sedgwick was at work among similar rocks in North Wales. These were at first believed to be all older than those called Silurian, and were accordingly named CAMBRIAN, after the old name for Wales, Cambria. In the end, however, it was found that throughout a large part of them the same fossils occurred, as in the Silurian series, and they were accordingly claimed as Silurian. Much controversy has since been carried on regarding the limits and names to be assigned to these rocks, and geologists are not yet agreed upon the nomenclature that should be followed. Murchison and his followers claimed the Cambrian as the lowest portion of the Silurian system, while Sedgwick and his disciples maintained that the lower half of the Silurian system should be included in his Cambrian series. There can be no doubt that the first succession of organic remains established among these ancient members of the great Palæozoic series of formations was that worked out by Murchison and named by him Silurian. But it has been found convenient to retain the name Cambrian for the oldest group of fossiliferous formations. It may be well to repeat that these words, like all those adopted by geologists to distinguish the successive rock-groups of the earth's crust, have acquired a chronological meaning. We speak, not only of Cambrian and Silurian strata and Cambrian and Silurian fossils, but of Cambrian and Silurian time. The terms are used to denote those particular periods in the history of the earth when Cambrian and Silurian strata were respectively deposited, and when Cambrian and Silurian fossils were the living denizens of sea and land.

The rocks of which the Cambrian system is composed, like those