LITHOLOGY. 217 
figure the planes of vibration of the light are represented by the cross 
lines. The fibrolite 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 are black. On the top of Mt. Washington fibrolite 
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 fibrolite 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 fibrolite 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 fibrolite 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. 
Staurolitic 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 110. 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 
are 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 
