In the Pierre shales of Colorado are limestone masses of peculiar character. Their height is greater than their width and all dimensions are of a size to be measured by feet or yards. Resisting erosion much better than the shales, they stand above the general surface. Their fallen fragments protect sloping pedestals of shale, and their positions are marked in the landscape by conical knolls or buttes. The formal resemblance of these buttes to the conical lodges, or tepees, of the Sioux and other Indians has led us to call them distinctively tepee buttes. It will be convenient also to call the masses of limestone tepee cores and their material tepee rock.
The Pierre group, as developed in the Arkansas basin outside the Rocky mountains, comprises about 3,000 feet of shales, and they hold their argillaceous character so well in approaching the . . .