Geological N-otes on the Sierra Nevada, — Turner. 297 
ently of the Archaean contacts the fact that an irruptive 
magma may invade the country rock in thin parallel sheets 
alternating with thin sheets of the rock invaded. 
GEOLOGICAL NOTES ON THE SIERRA NEVADA. 
By H. W. Turner, Washington. D. ('. 
[Concluded from page 249.] 
PART II. 
THE IGNEOUS ROCKS. 
The classification adopted below is practically that of 
Rosenbusch. It is now generally conceded, and I think by 
Rosenbusch himself, that the classification of igneous rock- 
on the basis of time is not an altogether scientific one. The 
rocks called porphyrites are in fact the product of ancient 
volcanoes and correspond in the main to modern andesites. 
Some of the rocks classed as diabase do not appear to differ 
in structure or in original composition from modern doleritea 
and augite-andesites. The science of petrography is now in 
a transition stage and the classification here adopted will 
doubtless not obtain a few years later. In the Sierra Nevada, 
however, the time classification is very useful. All. or nearly 
all. of the rocks classed as intrusive are Jura-Trias in age or 
older. During the Cretaceous period the igneous forces in 
this district seem to have been quiescent, and do not appear 
to have been very active again till in Neocene ( Miocene and 
Pliocene) time. 
The pre-Cretaceous volcanic rocks are all of them altered. 
usually highly so. They have been subjected to pressure, 
rendered in places thoroughly schistose, and folded in with 
pre-Cretaceous sediments. Nearly every hard specimen of 
these ancient volcanic rocks will he found on microscopic ex 
animation to contain epidote, uralite, or chlorite or other 
secondary products. They also differ from the Tertiary vol- 
canic rocks in containing numerous gold quartz veins and 
other metalliferous deposits. 
The Tertiary volcanic rocks on the other hand lie with a 
marked unconformity in nearly horizontal beds or sheets on 
