LITHOLOGY. 2I5 



rocks which are apparently biotite gneiss. In a thin section of a gneiss 

 from Mt. Frankhn in Swanzey, deep brown hornblende, epidote, garnet, 

 and plagioclase, in addition to the other regular constituents, are found. 

 In a handsome black gneiss from Littleton, the proportion of ingredients 

 is reversed, and the biotite almost disappears, while hornblende becomes 

 very prominent. In thin sections this hornblende is very deep green in 

 color, and plagioclase and epidote are found. 



Protogene Gneiss. Associated with the chloritic granites, in the same 

 localities along the Connecticut valley, there are green gneisses which 

 differ from the granites only in structure, and need no further considera- 

 tion. These are the granitic members of the group of greenstones which 

 are characteristic of the valley. In this region, hornblendic and chloritic 

 rocks are particularly abundant; and it is interesting to note that the 

 associated granites and gneisses partake of the same general nature. 



By the gradual withdrawal of the feldspar from gneiss, this rock ap- 

 proaches in composition towards mica schist, which is next considered. 



MICA SCHIST. 



This is essentially a schistose mixture of quartz and mica ; but varia- 

 tions in the structure, in the size of the constituent crystals, in the pro- 

 portion of ingredients, and in the presence of accessory minerals, make 

 of the rocks of this class a most diversified group. The mica is often 

 muscovite, often biotite, and often both. The usual accessories are an- 

 dalusite, cyanite, fibrolite, staurolite, garnet, and tourmaline, which are 

 often so abundant as to give a character to the rock. The following are 

 the principal varieties : 



Ordinary Mica Schist. In the most common kinds of this rock the 

 mica is in quite large scales, and as the rock cleaves in the plane of its 

 lamination, the glistening mica is much the most prominent ingredient. 

 The quartz is most plainly seen on the rough fractures across the lam- 

 inae, where it appears in lenticular grains about which the mica bends. 

 Though biotite is usually present, muscovite predominates. The mica, if 

 tested, usually gives some water, as does margarodite. In thin sections 

 under the microscope, magnetite, apatite, orthoclase, hornblende, and 

 chlorite are very often found. Tourmalines abound in the schists on 



