COMPOSITION OF GRANITE-PORPHYEY. 229 



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

 than tlu' mass of the rock. The yroiindmass of the rock is made uj) of an 

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

 hedrous. Biotite is jn-esent, but no hornblende, and in tlio; 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 liigher percentage of all bases, except pota.sh, 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 refei'ence to 

 the development of crystallizatiou it might be shown that the feiTo-iTiagne- 

 sian minerals exhibited a tendency to segregate in the centi'al or less rapidly 

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

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

 connection with these miles of poi-j)hyry dikes there are no evidences of 

 any recent volcanic action. The granite-porphyries and therhyolites seem 

 to be wholly independent of each other as regards their mode of ( >ccurrence 

 and their loci of eruption. 



