2^8 ANNALS NEW YORK ACADEMY OF SCIENCES 



Only slight evidences of dynamic nietaniorphisni are found in the rocks 

 of the Cortlandt series.^^ The degree varies considerably among the differ-: 

 ent types, the granite showing the least, while in the diorite appreciable- 

 effects of strain are at times discernible, although they are rarely sufficient 

 to be perceptible in a hand specimen. These effects are most noticeable 

 in the vicinity of inclusions of mica schist, and it is to the borders of 

 foreign inclusions that the effects of dynamic metamorphism are usually 

 confined. Evidently, therefore, these rocks entered after or at least 

 toward the close of the period of intense folding, during which the shales 

 were converted into mica schists, because, if the intnision had taken place 

 previous to that period, one shonld discover a foliated structure similar to 

 that present in the Harrison granodiorite. 



Very marked contact metamorphism has frequently occurred in the 

 mica schist in the vicinity of the intrusions. Mention has already been 

 made of this in describing the mica schist in the vicinity of Peekskill and 

 Crugers. G. H. Williams^" has given an excellent description of a contact 

 zone from the vicinity of Crugers. The mica schist shows a constantly 

 increasing metamorphism as the intrusive rocks are approached. Garnet 

 becomes very abundant, and other contact metamorphic minerals such as 

 sillimanite, cyanite and staurolite make their appearance. 



The inclusions of schist in the igneous rocks themselves naturally also 

 show the effects of contact metamorphism. G. S. Eogers^® has come to 

 the conclusion that the emery deposits which appear at several localities 

 in the Cortlandt series are due to the more or less complete absorption of 

 such inclusions by the intrusive magma before it solidified. 



Croton Falls Hornhlendite 



A similar but smaller area of basic intrusives having very much less^ 

 variation in composition occurs in the vicinity of Croton Falls. This 

 mass is about two and a half miles long and one-half mile wide starting 

 in at a point a little south of Croton Falls and extending in the form 

 of a ridge in a northeasterly direction on the east side of the Croton 

 River. 



The rock at the northeastern end of this area is a massive dark green 

 coarsely crystalline hornhlendite. Some biotite is also visible in the 

 hand specimen along with the hornblende. In thin section, under the 

 microscope, it shows a coarse granitoid texture. In mineral composition, 



^ G. S. Rogers : Op. cit., pp. 54-55. 



^ Am. Jour. Sci.. 3rd ser., Vol. XXXVI, pp. 254-269. 1888. 



^ Op. cit., p. 81. 



