STJi UCTURE OF THE MO UNT WA SHING TON MA SS. 735 



extended much beyond its present limit in a southerly direction. 

 As the bed thins out it becomes more graphitic, indicating also 

 that the conditions attending its formation had here some 

 peculiar local characters. 



Metamorphic Character of the Rocks as Indicated by Microscopic 

 Studies. — The microscopic examination of thin sections of rocks 

 from Mt. Washington shows clearly that they are strongly meta- 

 morphosed elastics. Evidence has been deduced from the 

 secondary growths of feldspars, garnets, and tourmalines, as well 

 as from the relations of the different metamorphic minerals to 

 one another, to show that the orographic forces to which these 

 minerals owe their development, operated in several more or less 

 distinct periods. 1 



Summary and Conclusio?is. — What has been set forth in the 

 preceding pages agrees well with Professor Dana's views so far 

 as the northern portion of the area is concerned. In the south- 

 ern and central portions, however, where the areal and structural 

 relations are more obscure, I have arrived at very different con- 

 clusions. This has been due, not to the discovery of errors in 

 Professor Dana's observations, which have been in the main con- 

 firmed, but to the collection of a larger number of observations and 

 to the application of some structural principles which were not 

 made use of in his study. A glance at the map will show how per- 

 fectly the belt of Egremont Limestone which crosses the southern 

 portion of the mountain, is concealed where it meets the valleys. 

 This belt, the discovery of which furnished a key to the struct- 

 ure, is not at first apparent to the geologist, because at its ends 

 the boundaries of the Riga Schist coincide closely in direction 

 with and form an extension of the boundaries of the Everett 

 Schist. 



To summarize briefly the results which have been discussed 

 in the foregoing, the Mt. Washington series consists of four 

 members, which in order of age are as follows : i) Canaan 

 Dolomite, 2) Riga Schist, 3) Egremont Limestone, and, 4) 



1 Phases in the Metamorphism of the Schists of Southern Berkshire : Wm. H. 

 Hobbs. Bull. Geol. Soc. Am., vol. iv., pp. 167-178, pi. 3. 



