8 GKREEN MOUNTAINS IN MASSACHUSETTS. 



ties introduced In possible faults, as well as the temptation to infer their 

 existence; and also in case of pitching- folds to get, through radiating cross 

 sections, a knowledge of the true order of bedding. 



These conditions were found well presented in the northwestern corner 

 of Massachusetts. Here the western edge of the main ridge coming down 

 from Vermont makes a sharp turn eastward around Clarksburg mountain; 

 then after resuming tor several miles a straight southerly course it curves 

 back westward to bend around the Dalton hills. Opposite this bay stands 

 Greylock mountain, which Emmons and Dana had shown to be a great 

 synclinal mass. The greater and higher part of this Greylock mass of 

 Lower Silurian rocks rises to the east of the chord of the arc that is formed 

 by this bay-like curve. Again, Hoosac mountain, east of this bay, exhibits 

 a variety of distinct rocks in folds, the axes of which show a persistent 

 northerly pitch. And in addition to this I hoped for much aid from the 

 great tunnel, which, in 1865, I had examined for the state of Massachu- 

 setts. With a length of nearly 5 miles, it pierces the mountain through its 

 whole breadth at a depth of over 1,000 feet, and the fact that the tunnel 

 was driven from both ends and from two intermediate shafts gave assurance 

 that the dumps would supply unaltered material for the petrographic study 

 of the various rocks in all their variation of habit. As there was then no 

 topographic map of the region we were obliged to locate all of our w r ork 

 by transit survey. During the first two seasons, in company with my 

 assistants, Mr. B. T. Putnam and Mr. J. E. Wolff, 1 made thorough recon- 

 naissances of the area in question, and, to obtain as much light as possible, 

 these excursions were extended southward to the Highlands east of the 

 Hudson and northward to central Vermont. 



We had found that the mass of Hoosac mountain consists of a core of 

 coarsely crystalline granitoid gneiss, overlain in some places by a conglom- 

 erate, in others by fine grained white gneisses. Above the conglomerate 

 and white gneisses we had found a great thickness of biotitic and sericitic 

 schists, containing either macroscopic or microscopic albite, in both un- 

 twinned and simple twinned crystals. At all the contacts of this Avhole 

 series there appeared distinct structural conformabilitv. 



On Clarksburg mountain we had found the same coarse granitoid gneiss, 



