Page:Popular Science Monthly Volume 9.djvu/693

Rh The mammoth, or woolly elephant, the woolly rhinoceros, the cavelion, the cave-hear, the reindeer, and the musk-ox, inhabited Britain till the ice drove them south. When the climate became tolerable again, the mammoth and rhinoceros, the bison and the lion, reoccupied our lowlands; and the hippopotamus from Africa and Spain wandered over the plains where now the English Channel flows, and pastured side by side with animals which have long since retreated to Norway and Canada.

When the ages necessary for all these changes is allowed for, we have not, even yet, got beyond the latest period into which the history of the globe has been divided. Under the tertiary deposits lies the chalk, a thousand feet in thickness, which is composed of the shells of minute animals, which must have been deposited age after age at the bottom of a deep and still ocean, far out of reach of winds, tides, or currents. Recent dredgings in ocean-depths have proved beyond a doubt that the greater part of the Atlantic Sea floor is now being covered by a similar deposit. It must have taken ages to form, and, if the geologists are right in their estimate of the slow rate of upheaval, many more ages to become elevated above the ocean-bed where it lay. Not only once, but many times, the chalk was alternately above and beneath the waves. It is separated by comparatively thin and partial deposits of sand and clays, which show that it has been at many different points in succession a sea-shore cliff. The chalk is not flat, as it must have been at the sea-bottom; it is eaten out into holes by the erosion of the sea-waves, and upon it lie flints, beds of shore-shingle, beds of oysters lying as they grew, water-shells standing as they lived, and the remains of trees. Yet, again, there lie upon the chalk sands, such as those of Aldershot and Farnham, containing in their lower strata remains of tropical life, which disappeared as the climate became gradually colder and colder, and the age of ice once more set in. Everywhere about the Ascot Moors the sands have been ploughed by the shore-ice in winter, as they lay awash in the shallow sea, and over them is spread in many places a thin sheet of ice-borne gravel. All this happened between the date of the bowlder clay and that of the New Red Sandstone on which it rests.

We need not follow the geologist through the lower systems which overlie the metamorphic rock. The Oolite contains remains of plants and animals now extinct, the most remarkable being huge reptiles; the Triassic has fossils like the Oolite; and the Permian has remains like those in the coal on which it rests. Then follow the coal-measures, the fossil remnants of tropical vegetation; the Old Red Sandstone, with fossils principally of fishes and shells; the Silurian, in which are found the earliest forms of life; and, lastly, the hard and crystalline rocks, devoid of fossils, which are supposed to be the earliest constituent mass of our planet.

Sir Charles Lyell and his followers allege that the rate at which