:;<is The American Geologist. May. 1 m 
Quartz- Poephi kitk. 
In Amador, Calaveras and Tuolumne counties there are 
considerable areas of pre-Tertiary rocks usually showing 
abundant porphyritic-quartzes. These rocks are usually light 
greenish in color, weathering nearly white. Considerable 
portions of these areas are schistose, but even then the por- 
phyritic quartzes are usually plainly discernible. 
In Calaveras and Amador counties dike-like areas of quartz- 
porphyrite or quartz-porphyrite-schist were noted in the dia- 
base and amphibolite-sehist areas. Some of these are shown 
on the Jackson atlas sheet. The quartz-porphyrites contain 
'2.H f / r to 3.4% of lime, and usually more soda and potassa. 
Qdartz-Porphyry. 
In Plumas county there are two belts of white-schistose 
quartzose rocks that do not differ in general appearance from 
the quartz-porphyrite-schist just described. One of these 
areas forms a belt a mile or less in width and about twelve 
miles long, in the Grizzly mountains, lying just east of a belt 
of argillite that appears to be a continuation of the Grizzly 
formation of Mr. Diller, which is Silurian in age: the other 
belt forms the summit of Eureka peak and continues thence 
south into Sierra county. It is the rock about Wade's lake, 
where it contains numerous dikes of diabase as previously 
noted, and of the Sierra buttes. The quartz-porphyry con- 
tains usually less than 1 '/, of lime, and as much or more po- 
tassa than soda. 
The distinction between the quartz-porphyries of Plumas 
county and the quartz-porphyrites of the Jackson atlas sheet 
rests on a slender basis of chemical composition and may be 
discarded later. 
The quartz-porphyry of the Grizzly mountains may. how- 
ever, prove to be Paleozoic in age. This is suggested by its 
association as noted above, with presumably Silurian argillite. 
The quartz-porphyry is in part, at least, as late or later than 
some of the diabase known to be .Jura-Trias in age. The 
long belt extending from Eureka peak to the Sierra buttes 
is probably Jura-Trias in age. as has been noted under the 
M ilton series - . 
