LITIIOLOGY. 217 



figure the planes of vibration of the light arc represented by the cross 

 lines. The fibrolitc pierces the scales of biotite, and takes its straight 

 course, regardless also of the quartz, and its orthorhombic character is 

 shown by the circumstance that all the fibres which are parallel to the 

 plane of either Nicol arc i^lack. On the top of Mt. Washington fibrolitc 

 is very prominent in the schists, and the masses that are formed by 

 aggregations of its fibres are quite striking in appearance. 



In the neighborhood of the Crystal cascade a fibrolitc schist occurs, 

 which is black in color, and so nearly massive that it has never been dis- 

 tinguished from the diabase that is found near it. The fibrolitc is, how- 

 ever, easily seen on close examination, and a thin section shows that the 

 rock is a fine mixture of quartz, biotite, and muscovite, with less of mag- 

 netite, pyrite, and chlorite. Through this mixture fibrolitc and garnets 

 are sprinkled. Many of the garnets are zoned by the arrangement of 

 impurities. They mostly have clear centres and impure rims. This rock 

 appears spotted on fresh fracture surfaces, and darker parts are included 

 in the lighter in such a way that it has been called a trap conglomerate. 

 It being understood that the rock is of sedimentary origin, its structure 

 is not peculiar, for such features are not uncommon in crystalline schists. 



Stanrolitic Mica Schist. Mica schists, in which staurolite crystals 

 are so abundant as to give character to the rock, occur at Enfield and 

 many other towns along the Connecticut river. The staurolites are usu- 

 ally but poorly crystallized, though exceptionally good crystals, which 

 are sometimes an inch in diameter, are found. As mica schist gradually 

 loses its micaceous character and becomes argillitic in a formation at 

 Charlestown, the staurolite becomes macled, and presents itself in the 

 peculiar form that is described on page 1 10. In the mica schists of the 

 Connecticut valley, the staurolite crystals are usually large and twinned. 

 In the argillitic schists, they are often very small and simple. 



Garnetiferous Mica Schist. Mica schists, characterized by the pres- 

 ence of many garnets, are abundant. At Springfield, well crystallized 

 garnets compose more than half the bulk of some schists. In other 

 rocks, fewer but still large crystals are found ; and in many rocks, which 

 arc not characteristically garnetiferous, this mineral occurs abundantly 

 as a microscopic constituent. Garnets are found in the same schists 

 with andalusite, staurolite, etc. 



VOL. IV. 28 



