GEOLOGY OF THE SIERRA NEVADA. 37 
at that time on the Estate, afterwards found specimens of the Auce/a and 
Pholadomya described in the Geology of California, Vol. I., by Mr. Meek, 
who had little hesitation in referring them to the Jurassic. This seemed 
highly satisfactory evidence, so far at least as the age of a portion of the 
auriferous slate series was concerned, and it was immediately after this that 
the article in the American Journal of Science was published, to which 
reference has already been made, and which contained the first distinct 
recognition of the Secondary age of a part of the heart of the auriferous 
belt. 
After the publication of Geology, Vol. I, the work of tracing out the 
detailed geology of the gold-bearing rocks in the counties next adjacent to 
Mariposa in the north was continued by the writer with the assistance of 
Mr. A. Rémond, through the region lying between the Merced and the 
Stanislaus rivers. These explorations had for a result the discovery of numer- 
ous fossils, at various points along the line of outcrop of the same fossiliferous 
beds of dark-colored slate, in which they had first been discovered on the 
Mariposa Estate. These fossils were all unmistakably of Secondary age, 
and consisted of Ammonites and Belemnites, and especially of the Aucella 
Erringtont, which latter was quite abundant in some places.” 
Still farther north, near Spanish Flat, about six miles north of Placerville, 
in the fine-grained slates, several specimens of unmistakable Belemnites have 
been obtained by various members of the Geological Survey. At several 
other localities along the auriferous slate belt, specimens of Ammonites have 
been found. One was obtained at Wilkinson’s Ranch near White Rock sta- 
tion, on the Pacific and Sacramento Valley railroad, about seven miles east of 
Folsom. The most interesting locality of Secondary fossils discovered in the 
auriferous slate series, north of Mariposa County, is at Colfax, on the line of 
the Central Pacific railroad. The exact place was in a cut on the road, at 
Station 2,777, Section 53, one mile west of the town of Colfax. Two specimens 
of Ammonites were obtained here, when the road was constructing, and were 
examined and described by Mr. Gabb, under the name of Ammonites Colfaxi ; 
the description, with figure, will be found in the American Journal of Con- 
chology, Vol. V. p. 7. Another specimen of an Ammonite was brought to 
the office of the Survey as having been obtained at Robinson’s Ferry on the 
Stanislaus River. 
* The localities in which these fossils were found extend along a line of about twelve miles in length, 
between the Mariposa Estate and Coulterville. They are all in the immediate vicinity of the Great 
Quartz Vein. 
