Again: where did the clay, which is deposited in such gigantic masses, hundreds of feet thick, over the continents, come from? We have seen (p. 18, _ante_) that, according to Mr. Dawkins, "no such clay has been proved to have been formed, _either in the Arctic regions, whence the ice-sheet has retreated_, or in the districts forsaken by the glaciers." If the Arctic ice-sheet does not create such a clay now, why did it create it centuries ago on the plains of England or Illinois? The other day I traveled from Minnesota to Cape May, on the shore of the Atlantic, a distance of about fifteen hundred miles. At scarcely any point was I out of sight of the red clay and gravel of the Drift: it loomed up amid the beach-sands of New Jersey; it was laid bare by railroad-cuts in the plains of New York and Pennsylvania; it covered the highest tops of the Alleghanies at Altoona; the farmers of Ohio, Indiana, Illinois, and Wisconsin were raising crops upon it; it was everywhere. If one had laid down a handful of the Wisconsin Drift alongside of a handful of the New Jersey deposit, he could scarcely have perceived any difference between them. {p. 34} Here, then, is a geological formation, almost identical in character, fifteen hundred miles long from east to west, and reaching through the whole length of North and South America, from the Arctic Circle to Patagonia. Did ice grind this out of the granite? Where did it get the granite? The granite reaches the surface only in limited areas; as a rule, it is buried many miles in depth under the sedimentary rocks. How did the ice pick out its materials so as to grind _nothing but granite_? This deposit overlies limestone and sandstone. The ice-sheet rested upon them. Why were _they_ not ground up with the granite? Did the ice intelligently pick out a particular kind of rock, and that the hardest of them all? But here is another marvel--this clay is red. The red is due to the grinding up of mica and hornblende. Granite is composed of quartz, feldspar, and mica. In syenitic granite the materials are quartz, feldspar, and hornblende. Mica and hornblende contain considerable oxide of iron, while feldspar has none. When mica and hornblende are ground up, the result is blue or red clays, as the oxidation of the iron turns the clay red; while the clay made of feldspar is light yellow or white. Now, then, not only did the ice-sheet select for grinding the granite rocks, and refuse to touch the others, but it put the granite itself through some mysterious process by which it separated the feldspar from the mica and hornblende, and manufactured a white or yellow clay out of the one, which it deposited in great sheets by itself, as west of the Mississippi; while it ground up the mica and hornblende and made blue or red clays, which it laid down elsewhere, as the red clays are spread over that great stretch of fifteen hundred miles to which I have referred. {p. 35}