70 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



was so kind as to guide ine to it. Just south of where tlie tunnel frosses 

 the west hue of Florida the Becket gneiss wraps around the Arehean 

 nucleus of Hoosac Mountain and the feldspathic mica-schist wraps around 

 the Becket gneiss, and Dr. Wolff called my attention to the fact that the 

 mica-schist is at base a rather dark garnetiferous mica-schist. 



Thickness. — I have not been able to form an opinion concerning the 

 thickness of the Hoosac schists from this region. They ai-e cut across in 

 the Hoosac Tunnel and are 7,000 feet thick measured horizontally, and 

 this with a mean dip of 35° would give 4,000 feet for their thickness, but it 

 is not certam that the section is free from repetitions or faults. 



THE MIDDLEFIELD AREA. 



The band of Hoosac schist continues from the point where it leaves 

 Mom-oe, in Franklin County, a little east of south across Berksliire County 

 to the point where it enters Middlefield, in Hampshire Countj^; and in all this 

 distance (24 miles) I have found the bed to maintain its character unchanged. 

 There is a garnetiferous mica-schist stratum at the base, and above this a 

 heavy bed of a feldspathic mica-schist fairly well deserving this name, as it 

 is not so albitic as that in Monroe. It changes everywhere rather suddenly 

 into the stratum above, the change consisting only in the disappearance of 

 the porphyritic albites and part of the mica, the rocks being otherwise alike. 



In both the mica is very extensively hydrated and greasy to the feel, 

 and the rocks have been called talcose schists, talcoid schists, and, lastly, 

 hydi'omica- or sericite-schists. It is generally baiTcn, hv\i in some bands 

 is garnetiferous. The garnets are uniformly quite large — 12 to lo™"' may 

 be an average — and are widely scattered through large beds of the rock, 

 not often massed together in a single layer, as in the calciferous mica- 

 schist. They are almost always trapezohedra, while in the last-mentioned 

 schist they are rhombic dodecahedra. They are often surrounded by a layer 

 of chlorite, which has sometimes wholly replaced the garnet, and scattered 

 bunches and scales of the same green mineral appear everywhere on the 

 cleavage surfaces of the schists, distinguishing this and the Rowe, Sa\'oy, 

 and Hawley schists from all others in the series. The micaceous minerals 

 are generally present in but small quantity, and much of the rock could 

 be described as a micaceous quartz-schist. 



