eee. 
GEOLOGY OF THE SIERRA NEVADA. 4] 
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 37, was identical with A. Co/fusii ; 
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 Liassic.* 
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 paleontological 
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 paleontological proof of this has been obtained. If Silurian 
* Mr. Gabb remarks (Journal of Conchology, 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 the Sierra and its vicinity belong to the Lias.” 
