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. 
