240 EXTINCT MONSTERS 



now is, but, having since been overflowed by a vast sheet of 

 basalt, its thickness is unknown. In this basin the Miohippus 

 of Marsh first makes its appearance. 



During the later Tertiary, or Pliocene period, a vast develop- 

 ment of the horse tribe took place, so that great numbers of 

 these animals left their remains in the lake-deposits of that 

 time. The largest of these lakes had the Eocky Mountains for 

 its western border, and extended from Dakota to Texas, its 

 northern part covering the bed of the older Miocene basin. 

 Another Pliocene basin, of unknown limits, extended over the 

 older Tertiary strata of Eastern Oregon, and evidence of yet 

 others may be seen in Idaho, Nevada, and California. In all of 

 these fossil horses have been found; but the most important 

 localities are the region of the Niobrara Eiver east of the 

 mountains, and the valley of the John Day River in Oregon (see 

 sketch-map, p. 253). 



It will thus be seen how abundant is the material for tracing 

 the evolution of the horse in America. In that country the 

 Equus fraternus of Leidy is believed to be almost, if not entirely 

 identical with the Equus caballus (Linn.) of the old world. 



Only in the upper Pliocene deposits does the true horse 

 appear, and then the genealogy is complete. It roamed over 

 the whole of North and South America, and soon after seems 

 to have become extinct. There is no doubt that man and the 

 horse were contemporaneous in early days ; but it can be proved 

 beyond doubt that, at the time of the Spanish conquest, few 

 if any horses were left. It has been thought, from certain 

 references contained in old narratives, that at least in South 

 America the animal may possibly have still lingered on after 

 the coming of the Europeans. What cause can have led to its 

 extermination, it is impossible to say. The present race of wild 



