20 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



Counting from below upward, the Algonkian rocks may be divided 

 into four groups: 



1. Hinsdale gneiss. — This is a group of gray biotite-gneisses, generally 

 quite coarse and with the jet-black biotite in distinct, elongate patches, 

 granitoid and yet well foliated. The broad, fresh cleavage surfaces of the 

 feldspar are often strongly curved from pressure. These gneisses weather 

 with exceptional rapidity and seem to be calcareous. 



3. Hinsdale limestone. — The coarsely crystalline chondrodite-limestones 

 form a concentric band around the older gneisses, marked by a series of 

 abandoned limekilns, for tlie rock was economically important before the 

 opening of the "Western Railroad." 



3. Lee gneiss. — This is a heavy black hornblende- or hornblende-biotite- 

 gneiss. 



4. Washington e/neiss. — A broad band of rusty graphitic blue-quartz 

 gneiss forms the outer circle of this Algonkian nucleus. It is in the main 

 a biotite-gneiss, but with little mica, and rusty from the decomposition of 

 hornblende, pyrite, pyrrhotite, and a ferruginous dolomite. In the whole 

 circuit graphite is a never-failing accessory, especially in the upper por- 

 tion. The graphite mine at Washington, except for the size of some of the 

 constituents, suggests the Ticonderoga g-raphite mines. Very coarse calcite, 

 graphite in broad, thin, hexagonal plates, coarse white salilite, large green 

 pyroxene and hornblende masses, groups of finely terminated pistachio- 

 green pyroxenes, brown sphene, and garnets, followed paragenetically by 

 coarse calcite with phlogopite, and this by quartz, are some of the points of 

 resemblance. 



Another equally per.sistent and characteristic constituent of these 

 gneisses is a blue quartz in flat lamina' 1 to 3°""- in thickness, which has 

 often so deep a tint of rich purplish l)lue as to furnish beautiful cabinet 

 specimens, and is so aljundant as to form more than three-fourths of the 

 mass of the rock. 



Everywhere in the outer circuit of the Algonkian rocks a band having 

 the above peculiarities lies below the lowest beds of the Cambrian con- 

 glomerate-gneiss, viz, blue quartz formed in place, disseminated graphite, 

 beds of the heavy black hornblende-gneiss, and a general abundance of 

 hornblende and a very general rustiness, all associated with intervening 

 bands of a common biotite-ffneiss. 



