Page:Popular Science Monthly Volume 9.djvu/692

664 the upheavals and subsidence of strata which characterize the earth's crust were produced suddenly, and by violent agencies, the school to which he belonged were little likely to attempt to fix a date for the creation of the world. To their minds the facts of geology gave no evidence as to time. It is, therefore, to Sir Charles Lyell and his followers that we must turn for an estimate of duration drawn from the "testimony of the rocks."

It is impossible to deny that periods of very vast duration must have elapsed while the changes took place of which we see the traces. If, for instance, we search below the sand on English shores, we find, perhaps, a bed of earth with shells and bones; under that, a bed of peat; under that, one of blue silt; under that, a buried forest, with the trees upright and rooted; under that, another layer of blue silt, full of roots and vegetable fibre; perhaps, under that, again, another old land-surface, with trees again growing in it; and, under all, the main bottom clay of the district. In any place where bowlder clay crops out at the surface—in Cheshire or Lancashire, along Leith shore near Edinburgh, or along the coast of Scarborough—it will be found stuffed full of bits of different kinds of stone, the great majority of which have nothing to do with the rock on which the clay happens to lie, but have come from places many miles away. On examining the pebbles, they will prove to be rounded, scratched, and grooved, in such fashion as to show that at some period they have been subjected to a grinding force of immense violence. Among the pebbles in the clay, and on plains far away from mountains, are found great rocks of many tons in weight. They were carried on the backs of icebergs, which, at some time, covered the now temperate regions of the earth, and were dropped by the melting ice either in the shape of pebbles, as moraines of ancient glaciers, or as bowlders stranded when the icebergs melted in the lowlands.

Such evidence points to vast periods of more than arctic winter, which must have endured for many thousand years. But in close juxtaposition with these glacial shells and pebbles lie remains which tell of tropical climates that alternated with the dreary ages of ice. Fossil plants and the remains of animals prove that all Northern Europe was once warmer than it is now; that England bore the flora and fauna of the torrid zones. Underneath London there lies four or five hundred feet of clay. It is not ice clay; it belongs to a later geological formation, and was, in fact, the delta of a great tropical river. The shells in this clay are tropical—nautili, cones, fruits, and seeds of nipa palms, now found only at Indian river-mouths; anona-seeds, gourd-seeds, acacia fruits; the bones, too, of crocodiles and turtles; of large mammals allied to the Indian tapir, and the water-hog of the Cape. All this shows that there was once, where London stands, a tropical climate, and a tropic river running into the sea. We find in it the remains of animals which existed before the Ice age.