238 ANNALS NEW YORK ACADEMY OF SCIENCES 



intricately folded as is the schist itself. On the other hand, they do not 

 show much evidence of having come in prior to the folding since ; had that 

 been the case, evidences of considerable crushing and recrystallization of 

 the coarse pegmatite would be expected. The crushing and recrystalliza- 

 tion, however, fail, as the texture of the sheets and lenses is practically 

 the same as that of the dikes which were intruded later and which are not 

 involved in the folds. It seems reasonable, therefore, to believe that the 

 first intrusions of granite and pegmatite accompanied the period of fold- 

 ing itself. 



In the case of the Manhattan schist, the shales which were converted 

 into mica schist during this period of folding yielded most readily along 

 planes parallel to the bedding and naturally the early intrusions followed 

 these lines of weakness, giving rise to the intrusive sheets and injected 

 lenses which were drawn out and pinched off during the folding. In the 

 case of the Inwood limestone, conditions were somewhat different. This 

 was a more massive formation, and the bedding planes were not particular 

 lines of weakness. Ver\^ few intrusives entered parallel to them. The 

 magma rose through fissures and gave rise to true dikes where it solidi- 

 fied. These dikes are usually of fairly large size when they do occur, but 

 they are not as abundant as in the mica schist (PI. IX, Fig. 1). 



The intrusive activity continued during a long interval of time, extend- 

 ing even beyond the period of folding. The later intrusions took the 

 form of dikes which often cut one another, showing that some came in 

 earlier than others, thus emphasizing the fact that igneous manifestations 

 continued for a long time after the folding had ceased. The relations are 

 not surprising because the pegmiatites represent the final differentiation 

 products of the great masses of granite. 



In the case of igneous intrusions so richly supplied with water and other 

 mineralizers as the pegmatites must have been, rather marked contact 

 metamorphic effects would naturally be expected, especially in the case of 

 the limestones. In their field occurrence, however, this does not appear 

 to be the case. The dikes of pegmatite ten feet or more thick, apparently 

 have produced no contact metamorphic effects on the limestone whatever. 

 The explanation for this may be the one which Dr. E. Weinschenk^® has 

 given, namely, that when the pressure at the time of the intrusion is suffi- 

 ciently great the CO., of the calcite and dolomite does not have an oppor- 

 tunity to escape, and hence the silica cannot combine with the lime and 

 magnesia to form silicates. Occasionally the schist in immediate contact 

 with the pegmatite becomes very rich in garnet. These contact phases of 



<« GrundziiRP der Oesteinsknndo, I Teil, p. 105. 1002. 



