MOUNT GREYL6CK. 181 



PETROGRAPHY. 



The petrographic character of the beds of these formations will now be 

 described with the aid of Mr. J. E. Wolff's notes on the microscopic sections, 

 which have been briefly summarized. 



THE VERMONT FORMATION. 



As the beds of tins formation are only represented 1)}- one or two out- 

 crops in the Greylock area they will only be described in connection with 

 Stone hill in Appendix A. 



THE STOCKBRIDGE LIMESTONE. 



The lower limestone is a coarsely < >r finely crystalline limestone or mar- 

 ble, usually white, but often banded or mottled, and in places entirely dark 

 grey, and there argillaceous. South of and near the South Adams quar- 

 ries it is very quartzose, and at the south end of Stone hill there are grad- 

 ual passages from limestone to quartzite, the rock consisting of an "aggre- 

 gate of calcite grains with rarely a small grain of feldspar and of quartz." 

 About Williamstown and along the Green river north of Sweet's corners, the 

 limestone is very fine grained, and has a hardness intermediate between 

 that of quartzite and limestone, and contains occasional quartz grains. 

 This fine-grained quartzose limestone may be more characteristic of the 

 base of the horizon, but pure quartzite occurs near the top. 



The coarse crystalline limestone is often so micaceous as to resemble a 

 gneiss. 1 A specimen from a point a little southeast of the North Adams 

 reservoir was found to consist of "coarse grains of calcite interbanded with 

 lnuscovite and biotite, and containing occasional porphyritic crystals of 

 feldspar. The feldspars contain inclusions of muscovite, rutile, pyrite, etc. 

 There are occasional grains of quartz. Some fragments of feldspar are 

 microcline, and the calcite cuts across these grains," indicating the possibility 

 of replacement by calcite. The limestone about Sugarloaf mountain is also 

 quite micaceous. Prof. Dewey speaks of the flexibility of this micaceous 

 limestone from New Ashford. 2 Lenses and seams of quartz are not infre- 

 quent. Prof. Emmons noticed the occurrence of albite in the limestone of 



1 See E. Hitchcock. Final Report Geology of Massachusetts, p. 569. 



2 "Notice of the flexible or elastic marble of Berkshire county." Am. Jour. Sci., 1st ser., vol. 9, 

 1825, p. 241. 



