GEOLOGV OF THE WEST POINT QUADRANGLE, NEW YORK 35 



It was not sufficiently fluid and vigorous. This type of behavior 

 seems to characterize the later units rather than the earlier ones, 

 or it may be that the portion of the older unit now exposed, was 

 not so likely to show this behavior. Such a hab,it might belong to 

 the upper portion of a mass more prominently than to the deeper 

 portions. 



The Cortlandt series as a whole is still less capable in this respect. 

 Blocks of gneiss and schist involved in Mohegan granite are sharp- 

 margined and not materially different from the rock at a distance 

 from the granite, and the contact of the principal Cortlandt mass 

 with the inclosing schist shows so little effect on either rock that 

 typical specimens of the formations may be taken within a few 

 inches of the margin. 



It appears, therefore, that magmatic dift'erences in the igneous 

 units have been important factors in producing some of the rock 

 variety of the Highlands, and it is a distinct step toward a compre- 

 hension of the meaning of the whole series and an appreciation of 

 the genesis of some of these types to distinguish these different 

 habits or capacities of different magmas. 



Magmatic differentiation. Every large igneous unit in this quad- 

 rangle shows such continuous gradations within itself and the variety 

 of facies is so great that it is difficult to escape the conclusion that 

 magmatic differentiation is responsible for some of them. In the 

 older and larger masses in particular, these gradations include wide 

 divergence in mineral proportion or composition, difference in texture 

 from medium fine to extremely coarse and difference in structure 

 from massive to streaked or banded, or from uniform massive to 

 bunchy pegmatitic and primary veined structure. 



Not every formation exhibits all these habits to equal degree, but 

 all exhibit the tendency. The Peekskill or Mohegan granite undoubt- 

 edly shows the least tendency in this direction, but even in it there 

 are so many grades of color that the product of the quarries on a 

 single property is assorted for marketing purposes into several 

 grades. The variability shown by the Cortlandt series is explained 

 more easily as a differentiation effect than as anything else. In this 

 series norites, gabbros, peridotites, diorites and quartz-diorites grade 

 into one another most intimately while dikes of syenitic or granophyric 

 composition and even the Peekskill-Mohegan granite itself are judged 

 to be differentiates of the same magmatic mass. This formation is 

 less confused than are some of the more ancient granites, and its 

 differentiates are readilv distinguished as such. In the older mem- 



