346 E. L. Troxell — Early Pliocene One-Toed Horse. 



fossils. He identified and described the Miocene formation, 

 but in the region east of his district he erroneously assumed 

 that the rocks were of Cretaceous age ; the fact is that a later 

 formation exists there and the fauna indicates the Early Plio- 

 cene. 



The rock is entirely of sandstone and most of it is very fine- 

 grained. The variety of the grains, their rounded form and 

 the absence of larger components, suggest the probable eolian 

 origin of the formation, although the sand was shifted and 

 finally deposited by running water. The lower strata are 

 much harder because the grains are much more firmly cemented 

 together ; the rock of the main quarry, on the other hand^ is 

 not so compact, but is soft enough to be cut with a knife. 

 Under the microscope the cementing substance, which in this 

 case is calcium carbonate, has a filmy appearance and incom- 

 pletely fills the open spaces between the grains of sand. 

 Where the sandstone is very indurated it stands out in bold 

 cliffs, and bowlders of considerable erosional resistance cover 

 the slopes. The alternate harder and softer areas give rise to 

 differential weathering and in some places result in deep natural 

 caves. That this formation was built up by a stream is quite 

 certain, for the sand, the irregularity of the bedding planes, 

 the discontinuous layers, the water-worn bones and the posture 

 of the complete skeleton, all show this. 



In one of the canyons there appeared to be an irregular line 

 marking the boundary of an unconformity and at the bottom 

 of the P. lullianus quarry itself, there was a floor of hard 

 sandstone only a few inches beneath the complete skeleton. 

 There was also other vertebrate material, consisting mostly of 

 teeth and small bones, resting on this floor or only an inch or 

 two above it. The peculiar association of this fragmentary 

 material, most of which was water worn, with the complete 

 skeleton, suggests the secondary deposition of these broken 

 parts along with the original deposit of the whole skeleton. 

 The same torrent which washed out and broke up the other 

 bones, once buried, may have engulfed the colt which is now 

 the type of the new species. There is a strong probability 

 that this less consolidated formation is a channel deposit, rest- 

 ing upon and within the Upper Miocene or Earliest Pliocene, 

 but itself of later age, for we frequently find patches of a later 

 formation occupying the old valley of some prehistoric river. 

 Since the Pliocene is commonly considered a period of uplift 

 and also of semi-arid conditions, there would be few streams 

 and no great amount of stream action ; we, therefore, consider 

 this one of the rare deposits representing the period. 



Associated fauna. — Rhinoceroses are represented by frag- 

 ments in the quarry and by more complete bones in the neigh- 



