THE 



JOURNAL OF GEOLOGY 



FEBRUARY-MARCH, 1899 



THE PETROGRAPHICAL PROVINCE OF ESSEX 

 COUNTY, MASS. Ill 



ROCKS OCCURRING IN DIKES 



The rocks described in the preceding part of this article are 

 cut by numerous dikes of various kinds, ranging from very acid 

 aplites to basic diabases. The last are, as is usual along the 

 Atlantic border, the most numerous ; aplites and acid granite- 

 porphyries come next, and finally there is a smaller number of rare 

 and interesting types of alkali-rich dike rocks. These have not 

 yet been fully investigated, but, as the rather large number of 

 specimens in my possession represent apparently the main types, 

 a description of them will give at least an approximate idea of 

 the intrusive rocks of Essex county. 



GRANITIC DIKES 



Aplite. — Dikes of this rock are confined almost exclusively to 

 the granite areas, at least as far as my observations permit me 

 to judge. They are usually very narrow, only a few inches in 

 thickness, and contact phenomena are not very conspicuous. 



Megascopically they are dense and fine-grained, of various 

 shades of light gray, and often showing a few small biotites. In 

 thin section they show no very remarkable peculiarities, being 

 a holocrystalline mass of small anhedra of quartz and alkali- 

 feldspar, the latter usually microperthitic. Micrographic inter- 

 Vol. VII, No. 2 , 105 



