64 GKEEN MOUNTAINS IN MASSACHUSETTS. 



THE SToCKBRIDOE LIMESTONE. 



The next rock is the limestone found in Hoosic valley at the base of 

 Hoosac mountain and covering the valley west to the base of the Greylock 

 mountain mass. It occurs in contact with the Vermont quartzite and with 

 both the Berkshire and Boosac schists at several places in the valley. 



The rock is generally a coarsely crystalline white marble banded with 

 layers of yellow muscovite or dark graphitic substances, anil containing 

 layers of bluish quartz. Layers of quartzite an- frequent in the limestone 

 and the change from one to the other is gradual. Microscopically the lime- 

 stone consists of »rains of calcite, a few of quart/., flakes of mica, etc. 



It lias been mentioned that one variety of the tine grained white gneiss 

 often contains considerable calcite, thus forming in some sense a transition 

 between the Stockbridge limestone and the Vermont gneiss A much more 

 perfect transition is found between the limestone and Hoosac schist. The 

 best case of this kind is found in the "Cove," in Cheshire, where the ground 

 is Idled with large angular blocks of this rock, which occurs in place in one 

 ledge. These rocks resemble a micaceous white limestone filled with little 

 dark grains or imperfect crystals of feldspar. In the slide the rock is com- 

 posed of a. mass of calcite grains, with here and there single grains of 

 quartz, or an aggregate of several grains, plates of muscovite and often of 

 chlorite and biotite, and large porphyritic feldspar grains in single crystals 

 or simple twins, very rarely showing polysynthetic twinning. These feld- 

 spars contain inclusions of mica, quartz, iron ore, rutile, and calcite, and 

 are in every way identical with the albites of the albite-schists, although 

 the exact species of plagioclase has not been determined. The calcite seems 

 to play the part which the quartz does in the schists: it sends tongues into 

 the feldspars, or cuts them in two, and gives one the impression by its in- 

 clusions in the feldspar and its occurrence with the quartz and mica that it 

 is of contemporaneous origin with the feldspar, mica, and quartz. Rutile 

 needles, and masses of ore (ihnenite.') occur in curved bands in these feld- 

 spars. Small irregular masses of microcline occur sometimes among the 

 quartz grains of the rock. 



On the Grreylock side of the valley about 300 yards west of Maple 

 Grove station there occur outcrops of a similar teldspathic limestone. Part 



