Page:Popular Science Monthly Volume 69.djvu/425

Rh formations belonging to the four great rock systems. At the top is the bluish limestone which weathers into fantastic buttresses and pinnacles, many of them so dizzily perched as to give a constant challenge to the winds or to man to topple them from their precarious position and hurl them a thousand feet below. My companion could hardly find time to sleep, so great was his delight in prying off great blocks of stone and listening to their terrible crunch and roar as they fell hundreds of feet, striking a ledge here and there and finally crushed to powder, or, still in gigantic mass, they rested a thousand or more feet below. This top formation, which Powell called the 'Fortification limestone' is the 'Upper Aubrey' limestone. It is about five hundred feet thick.

Next below it, can be seen the great white four hundred feet wide band of Upper Aubrey sandstone which stretches like a ribbon in sight for fifty miles and more to the east and west. Its walls are even more precipitous than those of the Aubrey limestone.

Below it are a thousand feet of shelving red sandstone which form the 'Lower Aubrey.' These three formations—the Upper Aubrey limestone and sandstone, and the Lower Aubrey sandstone—constitute the Upper Carboniferous system which is so familiar to the inhabitants of the Mississippi Valley states as the source of coal, but which in this region is not coal bearing.

Following the Upper Carboniferous a shelf of shales leads out to the precipitous wall of the great 'Red Wall' limestone, which, with