FRESH WATER PLIOCENE. 221 



from living specimens from the modern Clear Lake. A third abundant 

 species is BythijicIIa intcrmedUi Tryon. This shell is not known to exist in 

 Clear Lake, but has a wide distribution on the Pacific slope. The fauna 

 of Clear Lake, however, has not been systematically investigated, and Mr. 

 Stearns thinks it by no means improbable that B. intermedia still exists there. 

 A single specimen, certainly belonging to the genus risidium, and probably 

 to the species abditum Hald., is not perfect enough for specific identification. 

 There are several similarly imperfect specimens of Hclisoma '! ammon Gould 

 and an imperfect Phiim, which is either P. (fjjrina or P. heterovtropha. All 

 these are living forms. 



The age of these beds cannot of course be satisfactorily determined 

 from fresh-water shells. The most careful watch was kept for vertebrate 

 remains, but only a few fragmentary bones were discovered. These were 

 referred to Prof. O. C. Marsh, who reports finding among them the fragments 

 of a pelvis, apparently of a horse; the lower portion of a scapula, which lie 

 thinks belonged to a camel; and the head of a large femur, probably of an 

 elephant or a mastodon. These imperfect fossils, he concludes, suggest a 

 very late Pliocene age for the beds in which they occur.* The continuity 

 of life between Cache Lake and Clear Lake, with the continuity of sedimen- 

 tation mentioned above, appears to preclude the supposition that the beds are 

 older than the latter part of the Pliocene. Professor Marsh's report seems 

 to sliow conclusively that they are not recent, and that they must therefore 

 represent the close of the Pliocene. This determination is of great impor- 

 tance ; for it fixes with accuracy the age of the asperites of Clear Lake and, 

 in conjunction with other facts, determines approximately the age of the 

 asperites of Mt. Shasta. 



Distribution and age of the lavas. — Tile rcgion about Stcamboat Springs, Nev., 

 includes the Washoe district, the eruptive rocks of which, have been more 

 extensively discussed than those of any other locality on this hemisphere. 

 Li the chapter on the massive rocks it will be seen that my studies of the 

 rocks of Steamboat Springs and of the Washoe district have led me to the 

 conclusion that the younger andesites form a natural group of trachyte like 

 rocks, which I have called asperites. This same group is widely distrib- 

 uted in California. It forms a large and apparently the cliief portion of 



