ACID DIKES IN AECHEAN. 45 



ACID DIKES IN ARCHEAN. 



Observations upon dikes of acid rocks cutting the Archean granite are 

 very few, and we may suppose this to be partly due to their occurrence in 

 isolated knobs, which prevented the determinatioiT of the relations of adjacent 

 exposures of rocks of slightly different character. Some few dikes were, 

 nevertheless, observed, and are granites varying from medium to coarse 

 grained, granolitic^ to porphyritic rocks. The porphyritic facies is the 

 most common. The dikes do not show differences from the main mass of 

 the Archean granite sufficient to warrant detailed petrographical description. 

 The following description of one mass of granite-porphyry is given, as it offers 

 good proof of its relation to the schistose border facies of the granite. In 

 this case the gneissoid rock is found as inclusions in the granite-porphyry, as 

 is illustrated in the accompanying diagrammatical sketch, fig. 4, taken from 

 a ledge in the field. In this sketch the 

 sharply outlined angular and lenticular 

 areas represent the gneiss included in the 

 granite -porphyry. The phenocrysts of 

 this granite-poi-phyry have a parallel 

 arrangement, the long direction of the 

 phenocrysts agreeing also with the trend riG.4.~Granite.porpi.yrywith inciu 



soid granite. 



of the longer axes of the inclusions. The 



banding and foliation in the inclusions strike at a right angle to the flow- 

 age structure of the granite. The lines of separation between the areas of 

 gneiss and the granite, as shown in this outcrop, are sharp, and jjoint to 

 their nature as inclusions, and such is accepted as the true explanation 

 of their angular character and sharp outlines. As this porphyritic granite 

 was intruded through the border facies of the Archean granite, these frag- 

 ments were taken up, and were so arranged as to agree with the direction 

 of movement in the intruding mass. This occurrence shows this granite- 

 porphyry to be younger than the great mass of Archean granite, whether 

 we consider the inclusions to be a border facies of the Archean granite, 

 derived from it by dynamic action, and thus of secondary origin, or to have 

 resulted from differentiation of the molten magma. 



' This term has been proposed by a committee, on nomenclature for the geologic folios of the 

 United States Geological Survey, for use iu place of " granitic." 



