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