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. [Illustration: 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.] [Illustration: FIG. 12.–Horse (wall engraving) outlined in black, cave of Niaux (Ariège).] [Illustration: FIG. 13.–Horses: _A_, wall engraving (cave of Hornos de la Péna). _B_, wall engraving from cavern of Combarelles. _C_, engraved on reindeer antler (Mas d'Azil). Note the halter in _A_ and in _C_; also note the heavy head and face of _B_ like that of Prejalvski's horse.]