GEOLOGY OF THE SIERRA NEVADA. 



41 



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pages. Mr. Gabb, however, inclined to the opinion that the Colfax Ammo- 

 nite was closely allied to A. Solaris ; he also considered that the specimen 

 from Robinson's Ferry, mentioned on page 87, was identical with A. Co/faxii; 

 and, chiefly on the strength of this rather unsatisfactory evidence, he thought 

 it not unlikely that the rocks in which these fossils were found were Lmssic. # 

 It would seem probable, from the fact of the undoubted occurrence of the Tri- 

 assic in Plumas County, that this formation is also represented in the aurif- 

 erous belt farther south ; but it does not appear that any palueontologieal 

 evidence has yet been obtained which would make it possible to assert this 

 as an established fact. 



The case, then, stands thus in reference to the geological age of the sedi- 

 mentary strata of the bed-rock of the western slope of the Sierra : The ex- 

 istence of the Alpine Trias in Plumas County is a well-established fact, and 

 this member of the series, so widely developed in Nevada and along the 

 Pacific coast, may make up a considerable part of the auriferous belt farther 

 south, but there is no proof of this. The Jurassic also has been shown to 

 occur in Plumas, and in Mariposa County, as well as at various points inter- 

 mediate between these two localities. The specimens obtained in Nevada 

 County, at Colfax, and from Robinson's Ferry, near the line between Cala- 

 veras and Tuolumne counties, are referred by Mr. Gabb, with some doubt, to 

 the Liassic. The occurrence of Jurassic fossils at several localities along a 

 belt of rock in close proximity with the Great Quartz Vein proves beyond a 

 doubt the Mesozoic age of a portion, at least, of the most productive part of 

 the auriferous belt. No rocks older than the Trias have been found anywhere 

 on the western slope of the Sierra, except at certain localities of limestone, 

 in Shasta and Butte counties, known to be of Carboniferous age. Through 

 the whole range of the gold-mining region occur detached portions of what 

 appears once to have been a continuous belt of this rock, and it would seem 

 likely that this is the continuation of the Shasta and Butte County lime- 

 stone ; but no palEContological proof of this has been obtained. If Silurian 



* Mr. Gabb remarks (Journal of Concliology, Vol. V. p. 5) : "A large proportion of the stratified rocks 

 of the western slope of the Sierra Nevada appear referable to the Jurassic formation, while at least one 

 small tract in Nevada [in the mining district of Volcano, about thirty miles southeast of Walker's Lake] 

 yields fossils of this age in a reasonably good state of preservation. From the paucity of species, and none 

 being referable to known forms, we were unable, at the time of publication, to do more than designate the 

 great group of the Mesozoic era to which they belonged. The discovery of two Ammonites, closely allied 

 to known European species, together with other characteristic forms, lead us now to believe that all of the 

 at present known Jurassic rocks of tin 1 Sierra and its vicinity belong to the Lias." 



