176 MINERALOGY AND LITHOLOGY. 



calcite, which is colored by a little of some chloritic substance. The 

 mass of the rock is composed of very minute particles ; and the quartz 

 can be distinguished from the orthoclase by shutting off the light from 

 beneath, when the feldspar, being partially decomposed, appears white 

 and opaque, and the quartz, being still fresh, appears dark and clear. 

 The feldspar forms the larger part of the rock. It is rather striking to 

 notice that, just beside one of these felsite dykes, separated from it by 

 merely a partition wall of the crystalline schists, there is a dyke of a 

 black rock which a thin section shows to be a very fine-grained and con- 

 siderably altered diabase. 



A specimen from a felsite dyke at Bemis brook is brownish yellow in 

 color, and so very compact that it resembles jasper, but under the micro- 

 scope, though it is seen to be very fine, its felsitic character is observed. 

 Here and there, in the specimens of this felsite, a crystal of quartz or 

 orthoclase is seen, and their sparing presence introduces one stage of the 

 easy transition from felsite to porphyry. A large dyke in Bartlett is 

 composed of a felsite which, when microscopically examined, is found to 

 be porphyritic in its character. 



Some rocks in New Hampshire have been called felsites, which differ 

 essentially from them. In Albany there are some light red rocks, very 

 fine in texture, and spotted with minute little black dots. When sections 

 are cut from these rocks, they are found to consist of orthoclase crystals 

 quite well formed, and of some size, and the black specks are found to 

 be of hornblende. The rocks are only fine-grained sienites. 



Some very compact fine-grained rocks which resemble felsites are 

 interstratified with the schists of the Connecticut valley. They are dis- 

 tinguished from quartzite by the circumstance that they fuse, and they 

 have therefore been called felsites. Under the microscope these rocks 

 show the constituents of argillitic mica schists, to which they are related, 

 and from which they differ in the less amount of the micaceous ingre- 

 dient, which accounts for the more massive condition. 



The eruptive felsites often appear schistose; but this is a peculiarity 

 which is often noticed, and is merely a secondary structure which has 

 been induced in them. 



