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University of California. 



[Vol. 2. 



angular fragments of rock several inches in diameter imbedded in 

 a fine paste. This ground-mass, while of the same general char- 

 acter as that of the rocks described, is more heterogeneous, and the 

 finely comminuted rock and crystal fragments are more sharply 

 angular. It, moreover, presents no evidence of lamination due to 

 sedimentary deposition, as do the lower rhyolite tuffs. Mineralog- 

 ically, however, it is much the same, exhibiting glass and kaolin 

 with more or less chalcedony, in which are imbedded crystal frag- 

 ments of quartz, orthoclase, and plagioclase, and fragments of rocks, 

 both acid and basic, the former, of course, greatly preponderating. 

 The larger fragments of the agglomerate which have been alluded 

 to as weathering out prominently on the surface appear to fall into 

 two classes. 



The greater number are hard silicious porphyries, white, gray, 

 or green in color, with compact texture and even fracture. Exam- 

 ined in thin section they are seen to be typical microgranites; abun- 

 dant phenocrysts of quartz, orthoclase, and plagioclase, more or 

 less idiomorphic, but much rounded and embayed by corrosion of 

 the magma, and occasional crystals of brown hornblende and bio- 

 tite; a ground-mass of microgranular quartz and feldspar, sometimes 

 showing beautiful granophyric intergrowths, and inclusions in the 

 phenocrysts of sharply idiomorphic crystals of magnetite, apatite, 

 and zircon, and of gas and liquid inclusions in the form of negative 

 crystals. The minerals constituting these fragments are identical 

 with those found as more or less broken crystals in the finer por- 

 tions of the tuffs. There can scarcely be a doubt that they repre- 

 sent in massive form the magma from which the tuffs are derived. 

 The other class of fragments embraces rocks of fine-grained por- 

 phyritic texture, green to greenish gray in color, the weathered 

 surface of which is black and pitted by the removal of needle- 

 shaped phenocrysts. In thin section it has the appearance of an 

 altered quartz-andesite. Lath-shaped phenocrysts of plagioclase, 

 occasional corroded crystals of quartz, which may possibly be 

 inclusions, and lath-shaped crystals of fibrous hornblende, are im- 

 bedded in a microlitic base of feldspar, magnetite, and glass, 

 colored light green by disseminated particles of uralite and chlo- 

 rite. This rock is unlike any forms of andesite found in the 



