596 /. E. SPURR 



164 N^, on account of the multiplication of phenocrysts, which 

 show the same great variety in point of size as the rock just 

 described. The same angular fragments of more basic lava also 

 occur. The phenocrysts are quartz, orthoclase, and a striated 

 feldspar; the latter tested twice by the Fouque method proves 

 to be albite. The orthoclase was optically determined as 

 such. 



Rhoylite ( 162 N^.). — In this rock the groundmass is fine holo- 

 crystalline, coarse enough to enable one to distinguish the 

 mosaic of quartz and feldspar. This groundmass contains the 

 same fragments of more basic lava that have already been 

 described, and also encloses broken phenocrysts of mostly 

 unstriated feldspar. 



Granite -p or phy ry , fi?ie-grai?ied (169 N.). — In this rock the 

 groundmass is fine granular, sometimes granophyric, and con- 

 rists chiefly of quartz and orthoclase. The phenocrysts are 

 abundant and consist of quartz and feldspar, with chlorite and 

 epidote which are derived from the decomposition of ferromag- 

 nesian minerals. The feldspar phenocrysts are partly orthoclase 

 and partly a striated feldspar, which, determined by the Foque 

 method, proves to be oligoclase-albite. 



Biotite- granite, medium- grained (171 N.). — In this rock the 

 grains are of two distincts sizes, one many times larger than the 

 other. The larger grains have a tendency to idiomorphism, the 

 smaller grains to allotriomorphism. The smaller grains are- 

 included between the interlocking larger ones and may be con- 

 sidered as forming an overgrown groundmass, partly crowded 

 out by the multiplication and joining of phenocrysts, which are 

 represented by the larger grains. The minerals of the rock are 

 quartz, feldspar, biotite, and magnetite. The feldspar is almost 

 entirely orthoclase, with some microcline and albite. 



Gra?iiie, coarse (175 N.). — The structure of this rock is like 

 that of the preceding, only coarser. It contains many perfect 

 idiomorphic crystals of feldspar, often touching and almost inter- 

 locking, and smaller crystals of bleaching biotite and ragged pale 

 green hornblende, the last perhaps secondary. These minerals 



