260 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



contacts were we can only conjecture. But there appears to be no 

 good reason for supposing them to have been in any essential particu- 

 lars different from those obtaining for the aporhyolite at the same 

 levels. As for the porphyry contacts with the slates at deeper levels , 

 such as in the northern part of the Pine Hill area and in the Pine Tree 

 Brook Reservation, we find them characterized by the presence of a 

 peculiar and cliaracteristic basic phase which will next be described. 



Chemical characters. — A specimen of the light grey porphyry was 

 obtained from well inside a large quarried block from the eastern side 

 of Rattlesnake Hill. Thin sections showed the specimen to be almost 

 perfectly fresh, except for a little calcite. The results are the averages 

 of closely agreeing duplicates. 



