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University of California . 



[Vol. 3. 



some cases being reduced to ovoid patches showing zonary band- 

 ing. A few small well shaped laths occur, but yield no proper 

 sections upon which extinction angles could be measured. 

 Olivine is not a prominent constituent. It occurs chiefly as 

 crystal fragments, often containing embayments filled with micro- 

 lites. Augite occurs in even smaller amount than the olivine, 

 differing, however, from the olivine, in that it shows sharp 

 idiomorphic boundaries . 



The third specimen from Volcano Creek is a rather light gray 

 lava of somewhat vesicular habit. Occasional crystals of plagio- 

 clase and olivine are visible to the unaided eye. The microscope 

 reveals a holocrystalline structure with a porphyrinic development 

 of plagioelase, augite, and olivine. The phenoerysts are con- 

 tained in a pilotaxitic groundmass consisting of feldspar micro- 

 lites, numerous granules of augite and patches of twinned 

 plagioelase. Small stringers of magnetite are quite numerous. 

 The structure tends to grade into the intersertal. The plagioelase 

 exists in three generations. That of the first crystallized in 

 exceedingly large phenoerysts. They are not numerous, and 

 have undergone severe corrosion and embayment. Those of the 

 second period occur in well developed laths. The maximum 

 extinction angle is -40°, indicating a labradorite of medium 

 basicity. Augite is present in large amount in sharply idio- 

 morphic forms. The pyroxene is dark brown and often shows 

 a surrounding rim of lighter colored matrix. A subordinate 

 amount of pale green olivine also occurs, but is poorly developed. 



