COMPOSITION OF GRANITE-PORPHYEY. 229 



die of a dike about 30 feet in width. It is probably slightly more acidic 

 than the mass of the rock. The groundmass of the rock is made up of an 

 aggregation of quartz and feldspar, the former crystallized in regular dihexa- 

 hedrons. Biotite is present, but no hornblende, and in the hand specimens 

 there is only a slight development of ferro-magnesian silicates. Both rocks 

 analyzed carried a trace of chlorine. It will be seen that the rock from the 

 central mass carries a higher percentage of all bases, except potash, than 

 the narrow dike. The latter probably represents fairly well the contact 

 rocks of the larger dikes. 



It is possible that in a careful study of these rocks with reference to 

 the development of crystallization it might be shown that the ferro-magne- 

 sian minerals exhibited a tendency to segregate in the central or less rapidly 

 crystallizing portion of the dike, due to differentiation in the chemical 

 composition of the molten lava. It may be well to mention here that in 

 connection with these miles of porphyry dikes there are no evidences of 

 any recent volcanic action. The granite-porphyries and the rhyolites seem 

 to be wholly independent of each other as regards their mode of occurrence 

 and their loci of eruption. 



