FETTKE, MANHATTAN SCHIST OF NEW YORK 253 



Going west from the Dover-Pawling Valley, the metamorphic effects 

 become less and less noticeable, until in the vicinity of the Hudson River 

 fossil remains can still be readily identified in the limestone, and the 

 shale has hardly been converted into a slate. The transition from a 

 gametiferous staurolitic mica schist to a phyllite takes place within a 

 distance of four and one-half miles in passing from the western margin 

 of the Dover-Pawling Valley to the eastern side of the Clove Valley. 



Such a change in so short a distance can hardly be explained on the 

 basis o:^ regional metamorphism alone. The axis of most severe orogenic 

 disturbance runs in a northeast-southwest direction through western 

 Connecticut and Massachusetts into Vermont. Here the pressure was 

 greatest, as the folding and crumpling are much more pronounced than 

 they are farther west where the beds become less disturbed. Along this 

 line of most severe disturbance a series of granitic intrusions occurred 

 at the time of the folding. These sent out radiating pegmatitic dikes 

 and sheets into the adjacent formations which must have had a marked 

 metamorphic effect upon them and have brought about the recrystalliza- 

 tion of the constituents of the shale into mica schist as already pointed 

 out in the case of the Manhattan schist. 



Professor A^an Hise^'^ has described a very similar occurrence from the 

 Black Hills of South Dakota where a great intrusive batholith of granite 

 is surrounded by sedimentary rocks which are cut by a series of radiating 

 pegmatitic dikes extending out from the central core. Remote from the 

 intrusive, the sedimentary rocks are slates, while adjacent to them they 

 are schists and gneisses. 



From the study of the transition of slates to schists north of the 

 Highlands, the following seems to be the order in which the different 

 metamorphic minerals were developed. Sericite was the first new min- 

 eral to form and was accompanied by a partial recrystallization of the 

 quartz present. The formation of chlorite may have occurred at the 

 same time. Xext biotite began to develop, the iron present in the form 

 of oxide entering into its composition. Biotite was followed by garnet. 

 Still later staurolite made its appearance. The sericite by this time had 

 reciystallized into true muscovite. Feldspar also began to develop at this 

 stage. As these changes were going on, the texture of the rock was grow- 

 ing progressively coarser. In the final stages, large quantities of feldspar 

 appeared, while the muscovite became less abundant, the former develop- 

 ing at the expense of the latter. In some of these gneissic phases, the 

 muscovite disappeared entirely. Staurolite also dropped out except for an 

 occasional grain. The garnet become (piite free from inclusions during 

 these later recrvstallizations. 



'^'U. S. Geol. Surv. Mod. XLVII, p. 724. 1904. 



