1 86 EDSON S. BASTIN 



the actual contacts are not exposed. It is associated with a border 

 zone of diorite and gabbro surrounding, and practically contempo- 

 raneous with, a large granite mass of probable Devonian age. This 

 association, however, may be accidental, and the Cortlandite may 

 be a considerably later intrusion. 



IV. PORPHYRITIC GRANITE FROM SOMERSET COUNTY 



The specimen here described was collected by Dr. George Otis 

 Smith in the course of a reconnaissance trip in the summer of 1905, 

 and is worthy of brief notice because of its unusually coarse por- 

 phyritic character. Bowlders of this rock are common in Somerset 

 County, and in going up the Carrabassett valley they become more 

 and more plentiful, the rock being found in place in Highland Plan- 

 tation. Here it forms the divide between Dead River and Car- 

 rabassett River, both tributaries of the Kennebec. Where crossed 

 by the county road it shows tabular feldspars 3 to 5 inches in length, 

 and is exposed for several miles. The porphyritic granite does not 

 extend into the adjacent townships to the east and west, and that 

 seen both to the north and south of this locahty is more of the normal 

 type. The coarsely porphyritic rock seems to be characteristic 

 of an area a few miles square, which does not, however, represent 

 a peripheral zone, but is more probably a separate intrusive mass 

 in a region quite thoroughly intruded with granite. 



The specimen examined by the writer came from well within 

 this area. It shows a number of phenocrysts of feldspar, tabular 

 parallel to the h pinnacoid, and lying in approximately parallel 

 positions. The largest of these is 7 inches long, 3 inches wide, 

 and I to f of an inch thick. In most of the crystals a very pro- 

 nounced zonal structure is shown on these side pinnacoid faces, 

 and is seen to be due partly to the greater abundance of muscovite 

 along certain layers, and partly to slight differences in tint which 

 probably correspond to slight variations in composition. A micro- 

 scopic examination of material from the center and from various 

 points on the periphery of the crystal, using Wright's' adaptation 

 of Schroeder's method, showed that there was no considerable or 

 progressive change in composition from the center outward. The 



I F. E. Wright, American Journal of Science, Vol. XVII (1904), pp. 385-87. 



