3^ Mew YORK STATE kUSEUM 



This is the condition of the simplest series on the northern margin 

 of the quadrangle, but a long wedge of the same series occurring 

 in the Peekskill valley region has been still more modified, so that 

 the shale-slate member is there a phyllite, with bedding structure 

 completely destroyed, the limestone member is crystalline limestone, 

 and the quartzite is completely recrystallized. This, however, is 

 the normal dynamio-metamorphic effect. There is, as far as the 

 writers know, no reason to attach great importance to contact 

 metamorphism of this simpler series. 



In the case of the mica-schists of the Manhattan-Inwood series, 

 however, reorganization of the most elaborate sort has been accom- 

 plished. A coarse-grained schistose rock has been developed which 

 extends in repeated belts from the margin of this quadrangle to 

 New York City and is injected and impregnated on a most elaborate 

 scale. It may be taken to represent an intermediate type of 

 metamorphic rock involving both dynamic and igneous influences. In 

 many localities, the recrystallization effect is dominant and the other 

 either insignificant or lacking, whereas in other places igneous 

 influence and introduction have changed the character of the rock. 

 Everywhere all trace of the original bedding is destroyed. Every- 

 where all the original material has been recrystallized and great 

 variety in quality and structural habit has resulted. 



It has often been held that the Manhattan schist owes its com- 

 plexity, especially its extreme recrystallization, more to igneous 

 influence than to its dynamic history, and that the difference between 

 the Manhattan schist and the Hudson slates is dependent upon this 

 difference of field relationship. To what degree this may be true 

 is discussed more fully under the topic Correlation. At this point 

 it is sufficient to mention this additional cause of metamorphic com- 

 plexity represented by these rocks. 



The most elaborately metamorphosed formation is the Grenville 

 series of ancient sediments, which are limestone, schists and gneisses 

 in their present condition. They are the extreme in the direction of 

 metamorphism indicated above, where contact, injection and impreg- 

 nation effects are dominant over the simple dynamic and recrystal- 

 lization processes. It is perfectly apparent, however, that these 

 rocks have been modified extensively b}- dynamic influences. Most 

 of the dynamic history is beHeved to be earlier than the igneous 

 intrusion, but there has been so much subsequent modification that 

 it is not possible to discriminate between the metamorphic habit 

 developed before any of the igneous rocks were present and that 



