IGNEOUS ROCKS OF THE SIERRA NEVADA. 391 
other elongated areas called diabase and porphyrite (db), on 
the Placerville map in the vicinity of these hornblende-porphy- 
rite areas, are distinct tuffs and volcanic agglomerates, and there 
can be no reasonable doubt that the igneous fragments they con- 
tain were derived from volcanoes existing at the time the beds 
were forming. As hornblendic and augitic porphyrite fragments 
are very common in the agglomerates, we may safely assume that 
eruptions of porphyrite (old andesite) took place in the Carbon- 
iferous time in the foothill region. Exactly similar evidence is 
presented in the same belt of Carboniferous rocks in the area of 
the Jackson sheet. There are numerous streaks of fragmental 
porphyrites that are interbedded with the slates and evidently 
contemporaneous with them. In mapping these fragmental areas, 
the writer in places found much difficulty in deciding whether to 
lay down certain masses as pyroclastic rocks or as ordinary sedi- 
ments, for there is a gradation from one to the other in places. 
The rule followed was to regard those layers as igneous that were 
chiefly composed of igneous fragments. 
The most positive evidence of volcanic eruptions during the 
Carboniferous period has been brought forward by Diller,’ who 
described a bed of tuff near Genesee Valley at the north end of the 
range in which Upper Carboniferous fossils were found. Mr. T. W. 
Stanton kindly collected a specimen of this tuff for the writer, on 
one side of which are shell impressions. A thin section shows 
numerous plagioclase phenocrysts andsome non-twinned feldspars, 
probably orthoclase, in an altered groundmass containing very 
abundant minute fibers of a brightly polarizing secondary mineral, 
perhaps uralite. A partial analysis of this rock (No. 80S. N.) 
made by Augustus Wedderburn under the direction of Professor 
Chas. E. Munroe, of the Columbian University, shows 5.01 per 
cent. of potassa, and 1.94 per cent. of soda. It is probably a 
trachyte. 
The fossiliferous Upper Carboniferous beds on Little Grizzly 
Creek2 are interbedded with porphyritic volcanic material more 
t Bulletin Geological Society America, Vol. III., p. 374. 
2American Geologist, Vol. XIII., p. 230, and Fourteenth Annual Report, U.S. 
Geological Survey, p. 448. 
