Page:A Brief History of South Dakota.djvu/16

10 washed down by the ocean, each washing making a new stratum, until finally there came a time when the ocean could not overcome the hills and the latter became high and solid earth somewhat as we now know them. In this time the earliest evidences of life appeared, in the form of snails and other low orders of creatures.

Then the ocean seems to have come back and swept down another stratum of soil from the mountain bases, and after it had again subsided came a race of monstrous reptiles, the remains of which are found quite generally over the state wherever the formation of that period is exposed. It is quite certain that at this time South Dakota was in the main a vast steaming swamp, for the climate was tropical, and out of the swamp grew tropical verdure.

For how long the reptiles reigned no one can ever know, but their period was followed by another, in which great animals, much larger than anything now in existence, roamed throughout the land. They have been given hard names by scientific men who study their remains; as titanotheres, brontotheres, and eleotheres. The titanotheres and brontotheres were evidently of the elephant or rhinoceros family, and the eleotheres were giant pigs. While remains of these animals are most common in the Bad Lands, they are found in many other localities, showing that they roamed generally throughout the state. At this time we can be very sure, from the signs which are left, that South Dakota was a great swampy, tropical plain which sloped gently down from the Black Hills on the west to the great central river flowing through the