6o2 



/. E. SPURR 



from I ^ ■"■" in diameter grading down into groundmass, which 

 is cryptocrystalline, probably a devitrified glass. 



Two selected specimens of the dike rocks have the following 

 characteristics : 



Biotite -granite -porphyry, near rhyolite (243 N.). — Like 241 N, 

 this rock has phenocrysts of all sizes, from i ^ ""^ in diameter 

 down to the groundmass. There are, however, more phenocrysts 

 in this rock than in the one just described. As in 241 N, the 

 phenocrysts have no fluxional arrangements, but a divergent one. 

 The groundmass is fine holocrystalline allotriomorphic ganular. 



Biotite- granite (242 N.). — This rock consists of grains of all 

 sizes from 6^""" in diameter down to the very minutest dimen- 

 sions. The smallest ones, which are very abundant, are about 

 .02 to .03™"^ in diameter. There is a tendency to idiomorphism 

 throughout. The smaller sizes of crystals act as mesostasis for 

 the larger ones, and these have a mesostasis of the still smaller 

 ones. The essential minerals are quartz, othoclase, and biotite, 

 with accessory hornblende, titanite, magnetite, and a little 

 striated feldspar. 



Analyses. — The chemical composition of these rocks is as 

 follows (analyst. Dr. H. N. Stokes): 



