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 Rocky 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 River 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 
