IGNEOUS ROCKS OF THE SIERRA NEVADA. 405 
-idiomorphic hornblendes has crystallized in ill-defined branching 
forms. In the feldspathic material there are numerous minute 
green dots that appear to be chlorite. The analysis corresponds 
closely to that of the augitic tuff (19 S. N.) and to the compo- 
sition of some pyroxene-andesite except that soda is greatly in 
excess of potash. 
There are abundant dikes of fine-grained white granites 
(aplites) in the coarse granite of the Sierra Nevada. These in 
general have very nearly the composition of typical granites. 
They contain, as a rule, very little mica or hornblende. Ananal- 
ysis of one of these dikes is given in the table (No. 161 Sierra 
Nevada collection). The specimen was taken two miles south- 
easterly from Deadman’s Peak, in the area of the Downieville 
sheet. It will be observed that the rhyolites, Nos. 126 Amador 
and 365 Plumas, have practically the same chemical composition 
as the aplite. 
The volcanoes of Jura-Trias time in the Sierra Nevada appear 
to have been much more active than those of Palzozoic time, 
judging from the evidence offered in the preceding pages, and 
from that exhibited by the maps of the Gold Belt now being 
issued. 
Following the Jura-Trias there seems to have been a long 
period during which the volcanic action was quiescent. This 
period apparently comprised most of Cretaceous and Eocene 
time. 
The main facts concerning the age and succession of the Ter- 
tiary lavas of the Sierra Nevada have already appeared in print,’ 
and only a short statement will be here inserted, together with 
some additional information obtained lately. 
The first Tertiary eruptions, the relative age of which is clear, 
occurred apparently in Miocene time. These consisted of flows 
of rhyolite, which filled many of the old river valleys, but do not 
appear to have covered the higher ridges. The Miocene age of 
* Am. Jour Science, Vol. XLIV:, pp. 455-459; Am. Geol., Vol. XI., pp. 309-316; 
and the Fourteenth An. Rep. of the Director U. S. Geol. Survey, pp. 493-495. 
