178 W. II. HOBBS — METAMORPHISM OF BERKSHIRE SCHISTS. 



was not a continuous process, but occurred in stages, of which there must 

 have been several. It was in one of the later of these stages that the 

 staurolite was developed. 



The importance of the enlargement of mineral fragments in clastic 

 rocks as a factor in their alteration by metamorphism, has been empha- 

 sized by Irving and Van Hise in their papers on the rocks of the Lake 

 Superior region. This study presents a somewhat different phase of the 

 subject and adds an instance of their occurrence in rocks which have 

 been more profoundly metamorphosed. The investigation here outlined 

 is not completed. The interesting problems of the chemical nature of 

 the reactions involved in the development of the feldspars and their 

 secondary enlargements, and of the other porphyritic constituents, will 

 require for its solution a separation and chemical examination of the dif- 

 ferent constituents. 



In conclusion, I would acknowledge my indebtedness to Dr G. H. 

 Williams and Dr J. E. Wolff for valuable suggestions and criticism. 



Explanation of Plate. 



Sections of garnetiferous porphyritie Schist from the southeast Slope of Mount Washington, 



showing secondary Growths of Feldspar. 



Locality (number 3104) on road between Joyceville and Plantain Pond. 



Figure 1. — A = Secondary growth of feldspar, of which the core has a rounded 



outline. 

 B = Feldspar growth with two distinct enlargements indicated by 



different extinction angles. The core has a micropegmatite structure. 



Crossed nicols. X 77. 

 Figure 2. — A = Simply twinned feldspar core with an untwinned enlargement. 



Crossed nicols. X 48. 



