ANUESITIC (JLASS. 153 



not universal constituent of these rocks. The occurrence of biotlte and 

 pyroxene, unaccompanied by a trace of hornblende, is somewhat excep- 

 tional in eruptive rocks; at least I am unacquainted with any area so large 

 as that at Clear Lake wdiich is thus characterized. In other respects these 

 andesites are not peculiar, showing porphyritic plagioclases with augiteand 

 hypersthene in a groundmass of feldspar grains and magnetite, sometimes 

 holocrystalline and sometimes accompanied by glass. In some cases where 

 biotite makes its appearance the pyroxene is found to be exclusively hyper- 

 sthene, suggesting that the mica in a sense replaces or represents augite, but 

 augite and biotite sometimes appear in the same slide. One specimen shows 

 augite, hypersthene, biotite, and a few sharply defined, vividly polarizing 

 olivines. The groundmass in this case contains some base, but no apprecia- 

 ble amount of pyroxene, and there is nothing basaltic in the appearance of 

 either the specimen or the slide. The porph3-ritic feldspars of these rocks, 

 as determined by optical methods, are referable to labradorite and probably 

 io part to andesine, while the microlitic feldspars show the extinctions of 

 oligoclase. 



At t.wo localities on the southerl}- slope of Mt. Konocti dacite is found. 

 One of these is about half way up the slope, the other near the foot. It 

 is remarkable that cinnabar is associated with each and that from the 

 higher occurrence, known as the Uncle Sam mine, considerable quantities 

 of ore liave been taken. The rock at each of these spots is much decom- 

 posed, but what remains of the feldspar is all triclinic and the ferromag- 

 nesian silicates were certainly principally pyroxene. No distinct evidence 

 of hornblende or mica is perceptible, but it is not impossible that some of 

 each existed. 



Andesitic glass. — (jlosely associated with the more or less glassy asperites 

 of Clear Lake are large areas of volcanic glass. This glass is often 

 opaque, excepting in very thin splinters, and possesses a high luster, as if 

 charged with metallic oxides. Sparsely distributed in it, and forming cer- 

 tainly less than 1 per cent, of the mass, are sometimes crystalline grains. 

 Such a specimen is No. 13, Clear Lake, collected half a mile south of Kel- 

 sevville. Under the microscope it is found to be a mass of light-brown 

 glass, full of excessively minute, black trichites, usually arranged in stellar 



