IGNEOUS ROCKS OF THE STERKA NEVADA. 389 
formed from the porphyrites, melaphyres, etc., by dynamo- 
metamorphism. So generally have all these rocks been com- 
pressed that few outcrops can be found which do not show a 
rough schistosity. Let us add that the structure of these por- 
phyrites and melaphyres is so obscured by secondary minerals 
(uralite, epidote, chlorite, zoisite and calcite) that it is little to 
be wondered that their true nature has not been clearly under- 
stood. It is now plain that the chief part of the rocks laid down 
on the geologic map as porphyrite and amphibolite schist are 
altered forms of original surface lavas and tuffs corresponding 
to modern basalts and andesites. 
As has been stated before, the oldest known rocks in the 
Sierra Nevada, south of the fortieth parallel, are of Carboniferous 
age, and in treating of the history of the volcanic phenomena of 
the range we will commence with that period. 
In the foothills of Eldorado, Amador, and Calaveras counties 
(see Placerville and Jackson folios) there is a narrow belt of 
clay-slate in which are numerous limestone croppings, and in 
several of these Carboniferous fossils have been found.  Inter- 
bedded with the slate and limestone at several points are layers 
of conglomerate, composed of well-rounded pebbles. One of 
these layers is exposed by the road to Plymouth, in Amador 
county, northwest of Sugar Loaf, and at other points, and there 
is another similar one (possibly the same horizon) about three 
miles northwest of Golden Gate Mountain in Calaveras county. 
It is to be presumed that these conglomerates are of the age of 
the enclosing rocks, and in that case the pebbles they contain 
indicate with certainty the kind of rocks that existed before the 
Carboniferous or during an earlier portion of that period. The 
pebbles are largely of igneous rocks, although quartzite pebbles 
are present. The igneous pebbles are chiefly hornblende and 
mica porphyrites, but holocrystalline rocks, with idiomorphic 
augite, plagioclase, and leucoxene enclosed in orthoclase are also 
represented. The leucoxene appears to have formed from 
ilmenite. The plagioclase is mostly decomposed, as is also part 
of the augite. The orthoclase is largely fresh. The last-men- 
