MALLARD HILLS. 591 



scope, the rhyolitic character of these rocks is seen in the fact that the 

 groundmass is sphserulitic. The larger sphserulites consist usually of radiat- 

 ing fibrous material grouped around a dark centre. 



In the valley to the north of these hills, whose waters run into the Snake 

 River, is a Tertiary deposit, in which were found curious menilite-like 

 fragments of white porcelain-like material, having a somewhat vitreous 

 lustre, which, by the naked eye, can be seen to be full of little needle- 

 shaped fragments of black glass. In these, the microscope detects a tuface- 

 ous structure and the remains of diatoms. Along the northern slopes of 

 the hills, just beyond the limits of the map, are numerous warm springs. 

 The ridge which runs northwest from the Mallard Hills, forming the north- 

 eastern boundary of Bone Valley, is composed of the same dark brownish- 

 red rock found in the main group. 



Bone Valley. — In Bone Valley, which forms an isolated interior basin, 

 are deposits of white Tertiaries, which have been largely eroded away. On 

 the surface of this valley were found a number of vertebrae and fragments 

 of the jaws of fossil horses, evidently originating in these Tertiary beds, and 

 which determine their age as that of the Pliocene. 



These vertebrate remains were not found in place, but lying loose upon 

 the surface of the valley; they were thoroughly petrified, but in general too 

 imperfect for specific determination, although not water-worn, or showing 

 any evidence of having been brought from a great distance; moreover, 

 remains of light-colored limestone adhering to some of the specimens proved 

 that they originated in the limestone beds of the Tertiary deposits of this 

 enclosed valley. They were submitted to -Prof Joseph Leidy, who pro- 

 nounced them identical with those obtained from the Pliocene Tertiaries of 

 the Niobrara River. Those which were recognized and figured by' him 

 were: 



Protohippus perditus,^ an extinct equine animal, represented, among the 

 specimens collected, by a fragment of an upper jaw with two molars, frag- 

 ments of the lower jaw with one molar each, fragments containing canine 

 and incisors, some vertebrae, the end of a humerus, a pastern, and a coronary 

 bone : 



^ Leidy, Exiinct Mammalian Fauna, 1869, 275. 



