FOLIATION IN THE PRE-CAM BRIAN OF NEW YORK 611 



obliged to take on the form of the elongated or flattened inter- 

 stices." 1 



It is not at all necessary to assume a very active lateral com- 

 pression of the region to account for this pressure. As suggested 

 by Cushing, 2 considerable compression of the magmas must have 

 resulted from the batholithic intrusions, which, in order to make 

 room for themselves, exerted a shouldering pressure upon the ad- 

 jacent rocks. It is believed that such a shouldering pressure within 

 the magmas was sufficient, not only to cause more or less flattening 

 and elongation of minerals during consolidation and crystallization, 

 but also to determine to a considerable extent the directions of the 

 magmatic currents and hence the resulting strike of the foliation. 

 Under the very conditions of intrusion, differential pressures must 

 have been common, thus best explaining the frequent variations 

 in degree of flattening of mineral constituents. This view does not 

 of course preclude the possibility of moderate lateral pressure 

 exerted throughout the whole region during, or even after, the 

 magma consolidation. 



Foliation of batholithic borders. — Before leaving this discussion 

 another feature of the foliation of the intrusive masses should be 

 mentioned, namely, that they often exhibit a greater degree of 

 foliation and granulation around their borders than in their interiors. 

 This phenomenon seems to be best shown in the anorthosite and 

 the gabbro, and will be discussed below. Suffice it to say here that 

 production of foliation and granulation in the congealing magmas 

 affords a more plausible explanation for the peripheral distribution 

 of such features than their production by compression of the whole 

 region. 



Summary. — During the process of intrusion, which was long 

 continued, the great syenite-granite magmatic masses were under 

 only enough lateral pressure to control the general strike of the 

 uprising magmas with consequent tendency toward parallel arrange- 

 ment of syenite-granite and invaded Grenville masses ; the foliation 

 is essentially a flow structure produced under moderate pressure 

 during the intrusion; the sharp variations of strike on large and 



1 G. F. Loughlin, U.S. Geol. Surv., Bull. 4Q2, 1912, p. 129. 



2 H. P. Cushing, New York State Mus. Bull., No. 145, 1910, p. 101. 



