106 GEOLOGICAL SURVEY OF THE TERRITORIES. 



eral animals allied to the domestic hog, one about the size of this animal, 

 another as large as the African Hippopotamus, and a third not much 

 larger than the domestic cat. 



Five species of the Ehinoceros roamed through these marshes, ranging 

 from a small, hornless species, about the size of our black bear, to the 

 largest, which was about the size of the existing unicorn of India. ~So 

 animals of the kind now inhabit the western hemisphere. 



Among the thick-skinned animals were the remains of a mastodon 

 and a large elephant, distinct from any others heretofore discovered in 

 any part of the world. Dr. Leidy says that " it is remarkable that 

 among the remains of mammals and turtles there are none of crocodiles. 

 Where were these creatures when the shores of the ancient Dakotan 

 and jSTebraskan waters teemed with such an abundant provision of sa- 

 vory ruminating hogs?" During the tertiary period Nebraska and 

 Dakota were the homes of a race of animals more closely allied to those 

 inhabiting Asia and Africa now, and from their character we may sup- 

 pose that during that period the climate was considerably warmer than 

 it is at present. The inference is also drawn that our world, which is 

 usually called the new, is in reality the old world, older than the eastern 

 hemisphere. 



Ever since the commencement of creation, constant changes of form 

 have been going on in our earth. Oceans and mountains have disap- 

 peared and others have taken their place. Entire groups of animal and 

 vegetable life have passed away and new forms have come into exist- 

 ence, through a series of years which no finite mind can number. To 

 enable the mind to realize the physical condition of our planet during 

 all these past ages is the highest end to be attained by the study of 

 geological facts. It has been well said by an eloquent historian that 

 he who calls the past back again into being, enjoys a bliss like that of 

 creating. 



We may attempt to form some idea of the physical geography of this 

 region at the time when these animals wandered over the country, and 

 to speculate as to the manner in which their remains have been so beau- 

 tifully preserved for our examination. We may suppose that here was 

 a large fresh- water lake during the middle tertiary period ; that it began 

 nejir the southeastern side of the Black Hills, not large at first nor deep, 

 but as a marsh or mud-wallow for the gigantic pachyderms that lived 

 at the time ; that as time passed on it became deeper and expanded its 

 limits until it covered the vast area which its sediments indicate. We 

 cannot attempt to point out in detail all the changes through which we 

 may suppose, from the facts given us, this lake has passed, during the 

 thousands of years that elapsed from its beginning to its extinction, time 

 long enough for two distinct faunse to have commenced their existence 

 and passed away in succession, not a single species passing from one 

 into the other. Even that small fraction of geological time seems infi- 

 nite to a finite mind. We believe that the great range of mountains 

 that now lies to the west of this basin was not as lofty as now ; that 

 doubtless the treeless plains were covered with forests or grassy meadows 

 upon which the vast herds of gregarious ruminants cropped their food. 

 Into this great lake on every side poured many little streams from broad 

 valleys, fine ranging ground for the numerous varieties of creatures 

 that existed at that time. Large numbers of fierce carnivorous beasts 

 mingled with the multitudes of gregarious ruminants, constantly devour- 

 ing them as food. As many of the bones, either through death by vio- 

 lence or natural causes, were left in the valleys, they would be swept 

 down by the first high waters into the lake and enveloped in the sedi- 



