188 



GREEN MOUNTAINS IN MASSACHUSETTS. 



Fig. 73 represents a slightly enlarged section of a specimen of the felds- 

 pathic schists, which may be regarded as petrographically and structurally 

 typical of this formation. 



From all the foregoing the transitional lithologic character of the for- 

 mations is manifest.' In the Stockbridge limestone there are passages from 

 limestone to quartzite and to schist. In the Berkshire schist the rock 

 is often calcareous. In the Bellowspipe limestone there are transitions 

 from limestone to calcareous schist, and from these to noncalcareous 

 schists and to quartzite and gneiss. Mr. Wolff's microscopic examinations 

 indicate that this feature is due in part to various replacements and other 



TatiHIiA.V 



Mid Saddle Ball, 



le quartz lamina-. 



Fig. 73. — Thin section of albitic Bericite-echist from locality ">4L', between Greylock s 

 enlarged 1J diameters. A typical specimen of the Greylock schist, showing the minute plicai 



the slip cleavage with the alhite interspersed. (From a photograph.) 



chemical changes at the time of or subsequent to metamorphism, as well as 

 in part to variations in the character of the original sediments. 



THICKNESS. 



The numerous folds, and the fact that they are sometimes compressed 

 and overturned, not to mention the difficulties arising from cleavage, render 

 exact measurements of thickness very difficult, if not impossible, in the 

 Greylock area, but approximations can be obtained. The figures appended 

 to the following table are given only as estimates based upon the sections. 

 The difference in the estimates arises in part from the varying amount of 

 thickening in plication (Stauung). As thickening in consequence of plica- 



1 Prof. J U. Dana refers to this in several of his papers on the Taconic rocks. 



