IGNEOUS ROCKS OF THE SIERRA NEVADA. 413 
Brégger in a late paper on the basic igneous rocks of Gran, 
in Norway, writes as follows :* 
We have consequently in the basic rocks of Gran.a remarkable example 
of the fact that one and the same magma fart/y without essential differentia- 
tion has been pressed up to a higher level, and there has crystallized out as 
large boss-masses (in the form of olivine-gabbro-diabase), Zartly has been 
differentiated at a deeper-seated level into a basic magma (which by its out- 
burst has formed sheets and dikes with porphyritic structure, camptonites), 
and into a more acid residuary magma (which in the final eruptions has given 
rise to sheets and dikes of bostonite). This differentiation (into camptonites 
and bostonites) has partly also taken place in the dike and sheet-fissures 
themselves afer passing up into a higher level. 
This certainly confirms Iddings’ law in a remarkable degree. 
The chemical analyses of the igneous rocks of the Sierra 
Nevada given in the two tables, pages 403 and 407, are arranged 
as nearly as the writer is able in their order of succession. 
Considering now the Sierra Nevada as a whole as a petrographic 
province, there are certain relations which seem to suggest that 
Iddings’ law may be applied here. 
The oldest of the pre-Tertiary rocks which have a wide dis- 
tribution are the augitic tuffs and breccias. These are chiefly 
of an intermediate character. The serpentines (originally peri- 
dotites and pyroxenites) are at a number of points clearly 
intrusive in, and therefore later than, these AMSG woNMs, — Ite 
seems to be also true that the quartz-mica-diorite (granodiorite ) 
is later than the serpentine. This relation has already been 
noted’ to the southeast of Placerville, and the quartz-mica-dior- 
ite of Indian Valley (Downieville sheet) sends out a dike-like 
protrusion into the serpentine where exposed in the bed rock of 
the Indian Hill gravel mine. In the bed of Mill Creek, one and 
one-half miles northeast of Big Bar Hill (Bidwell Bar atlas 
sheet), a dike of biotite-granite cuts the tremolite and chlorite 
schists which are altered forms of pyroxenites, and are in this 
section associated with serpentine as part of the same rock 
mass. About two and one-half miles south of Big Bar Hill 
* Quart. Jour. Geol. Soc., Vol. L., p. 29. 
2See Am. Geol., Vol. XI., p. 310. 
