The total thickness of sedimentary deposits—that is, deposit laid down by the action of water on the earth's surface, and now estimated by the measurement of strata lying one over the other in various parts of the globe—tilted and exposed to view so that we can trace out their order of super-position—is about 130,000 feet. The lower half of this huge deposit contains no fossilized remains of the living things which were present in the waters which laid it down; they were soft, probably shell-less and boneless, and so no fossilized trace of them is preserved. Thus we divide the sedimentary crust into 65,000 feet of "archaic" non-fossiliferous deposit, and an overlying 65,000 feet of fossil-containing deposits.
The earliest remains of living things known are not very different from marine creatures of to-day; they are the strange shrimp-like Trilobites and the Lingula-shells found in the lower Cambrian rocks of Wales. Over them lie 65,000 feet of sedimentary deposit teaming with fossils—the petrified remains of animals and plants. The Trilobites and the Lingulas must have had a long series of ancestors leading up to them from the simplest beginnings of life—for they are highly organized creatures. But no trace of those ancestors is preserved in the 65,000 feet of sedimentary rock underlying the earliest fossils.
This great basal mass of non-fossiliferous deposit is called "the Archæan series." The 65,000 feet of deposit above it are divided by geologists into three very unequal series. The first and lowest is the Primary or Palæozoic series, occupying the enormous thickness of 52,000 feet; above these we have the Secondary or Mesozoic series of 10,000 feet, and lastly, bringing us to recent time, we have the Tertiary or Cainozoic of only 3000 feet. These three series amount in all to 65,000 feet. The Palæozoic series is more than five times as thick as the Mesozoic, and these two taken together are twenty times the thickness of the Tertiary. Each series is divided by geologists into a series of systems, distinguished by the fossils they contain, which, on the whole, indicate animals of a higher degree of evolution as we ascend the series.
The Palæozoic series include the vast thicknesses of the Cambrian, the Ordovician, the Silurian, Devonian, Carboniferous and Permian systems. The first "trilobite" is found in the lowest Cambrian rocks, and the last or most recent existed in the Permian period—after 50,000 feet of rock had been deposited. None are known of later age. The first fossil remains of a vertebrate are found in the uppermost beds of the Silurian—in "beds" (that is to say, stratified rocks) which are just half-way in position so far as the measurable thickness of the deposits are concerned, between the earliest Cambrian fossils and the sediments of the present day. To put it another way, 34,000 feet of fossiliferous rock precede the stratum (upper Silurian) in which the earliest remains of vertebrates are found. These first vertebrates to appear (others soft and destructible preceded them) are fishes—a group which, apart from this fact, are shown by their structure to present the ancestral form of all the vertebrate classes. In later Palæozoic beds we find the remains of four-legged creatures like our living newts and salamanders. The Secondary or Mesozoic series is divided into the Triassic, Jurassic and Cretaceous systems. It ends with the familiar chalk deposit of this part of the world, and is often called the age of Reptiles, because large reptiles abounded in this period. The Tertiary or Cainozoic series are divided into the Eocene, Oligocene, Miocene, Pliocene and Pleistocene systems. The huge reptiles disappear and their place is taken by an endless variety of warm-blooded, hairy animals—the Mammals—small at first, but in later beds often of great size. As we pass upwards from the Eocene we can trace the ancestry of our living Mammals such as the horse, rhinoceros, pig and elephant in successive forms. Complete skeletons are preserved in the rocks and show a gradual transition from the more primitive Eocene kinds—through Miocene and Pliocene modifications—until in the Pleistocene strata many of the species now inhabiting the earth's surface are found. A number of horizons, characterized by the special mammalian and other animal remains preserved in them, are distinguished by geologists in each of the "systems" of sands, clays and harder beds known as Eocene, Oligocene, Miocene and Pliocene. At last we arrive at the latest or most recent 250 feet of deposit, consisting of sand, clay and gravel. This is called "Pleistocene." It is only a very small fraction (1/260th) of the thickness of the whole fossil-bearing sedimentary crust of the earth—about the proportion of the thickness of a common paving-stone to the whole height of Shakespeare's cliff at Dover. This Pleistocene or post-glacial Tertiary—often now called Quaternary—has been so carefully examined that we divide it as shown on page 39 into upper, middle and lower, and each of these divisions into successive horizons (only a few feet thick) characterized by the remains of different species of animals and often by the differing implements and carvings as well as the bones of successive races of men.
When we are concerned with written history, ancient Egypt seems to be of vast and almost appalling antiquity; on the other hand, if we study the cave-men, ancient Egypt becomes relatively modern, and the first cold period and extension of glaciers, which 500,000 years ago marked the passage from Pliocene to Pleistocene, becomes our familiar example of something belonging to the remote past—beyond or below which we rarely let our thoughts wander. That is a natural result of concentration on a special study. But it has had the curious result, in many cases, of making students of ancient man unwilling to admit the discovery of evidences of the existence of man at an earlier date than that which belongs to the deposits and remains to which their life-long studies have been confined and upon which their thought is concentrated. The last 500,000 years of the earth's vicissitudes, which resulted in the 250 feet of "Pleistocene" deposit and the marvellous treasures of early humanity embedded in them, form but a trivial postscript to the great geological record which precedes it.
Fig. 11.—Horse (wall engraving), cave of Marsoulas, Haute Garonne. The drawing suggests the Southern less heavy breed as compared with Figs. 12 and 13.
