Pirsson and Washington — Geology of Red Hill, N. H. 447 



In most cases this has been ascribed to differentiation within 

 the mass itself and various agencies have been appealed to as 

 causes deemed sufficient to explain such differentiation. Others 

 have viewed these changes in a mass of igneous rock as caused 

 by the melting up and absorption of the enveloping rocks with 

 which it has come in contact. 



Unfortunately in the present instance the place where the 

 evidence usually lies, the contact between the two, is practically 

 everywhere covered up and the evidence, if there is any, con- 

 cealed. The fact that the country rock is a granite-gneiss and 

 much more acid than the average of the syenite, seems to point 

 clearly in favor of the latter view, and so do the very acid 

 dikes of aplitic alaskose found in the granite-gneiss on the east 

 side and departing radially outward from it. But if we seek 

 this origin for these dikes, how then shall we explain the com- 

 plementary very basic ones of camptonose which accompany 

 them side by side ? For these are much more basic than any 

 rock of the mass or its surroundings, and their origin must be 

 ascribed to processes of magmatic differentiation. But in this 

 case they must have acid complementary forms which we can 

 readily see in the aplitic alaskose. Again, the presence of the 

 acid paisanal liparose dikes in the very center of the nephelite- 

 syenite cannot be explained by digestion of the bordering rocks 

 and again points to magmatic differentiation. 



But if differentiation has occurred, as these cases seem to 

 prove, it may well have caused the changes observed in the 

 main mass itself without the necessity of seeking some other 

 agency, though it is possible to conceive as suggested by Daly,* 

 for the origin of certain granites, that the effect is the com- 

 bined one of assimilation and differentiation. Since the 

 evidence the contact might afford is hidden, this must however 

 remain a purely speculative hypothesis. 



Sheffield Scientific School of Yale University, 

 New Haven, Conn., Feb. 1907. 



*This Journal, xx, p. 185, 1905. 



