I08 NEW YORK STATE MUSEUM 



Some have thought that the Manhattan Inwood-Lowerre series are 

 Precambrian sediments corresponding to this later point in the 

 column, but, if they are not, then there is nothing known after the 

 Grenville. 



Post-Grenville volcanism. To this period belongs a complicated 

 igneous history, and the results are so confused and overlapped that 

 it is impossible to determine all the steps. It is clear that there has 

 been repeated igneous intrusion. The largest and most fundamental 

 step, however, was the development of a great granite bathylith 

 which seems to have undermined a very large area of which this 

 district is simply a small part. Its many successive stages of 

 activity, accomplished most of the complexity of the Highlands 

 region. Not all the igneous representatives are granite, however, 

 and there is a wide range in the age of individual units ; but the 

 most reasonable conception, and the most important one for a work- 

 ing understanding of the history of the district as a whole, is that 

 which connects the greater part of the igneous history with the 

 normal development of a single great bathylith which was itself 

 undergoing extensive changes for an exceedingly long time. 



Probably the various individual units, distinguished now by their 

 petrographic quality and which differed originally in capacity to 

 metamorphose country rock, are essentially differentiates of the same 

 plutonic mass. Perhaps one sees in such a regional occurrence an 

 illustration of both recurrent activity and continued differentiation. 

 In no other way can one feel justified in attempting to correlate 

 the igneous formations of this district with those of such widely 

 separated regions as northern New Jersey and the southern Adiron- 

 dack's. It does not seem reasonable, otherwise, that in such a number 

 of cases the petrographic habits would be similar enough in these 

 separate districts to indicate identity and that they should match as 

 perfectly as they do. 



Igneous invasion in widely separated localities could not be 

 expected to present so many points of similarity if there were not 

 one control for all the districts. This control might very well be 

 the successive differentiation stages of a single great bathylith from 

 which the different igneous members came. If this conception is 

 taken as a .starting point, then the suggested correlations look 

 reasonable enough, for a particular type of activity might develoj) 

 in all the regions affected a similar igneous expression. After an 

 epoch of comparative quiet, presumably with continued differentia- 

 tion, a rejuvenation of activity might normally be expected to 

 exhibit a similar expression again in whatever districts this new 



