4 8 EDWIN W. HUMPHREYS AND ALEXIS A. J U LI EN 



apparently, connected with some of the pegmatitic seams inter- 

 calated in the schist. The position of the dike, on the west side 

 of the overthrow in the anticline at the north end, suggests that it 

 has there acted as an obstacle against the northwestward thrust 

 of the beds and so produced the westward distortion of the upper 

 side of the fold. The pegmatite itself is an aggregate of grayish 

 quartz, white feldspar, and very little mica, of the rather uniform 

 medium texture usual in the dikes of the Bronx region, with grains 

 rarely exceeding two or three inches. 





'/V',7, 



Dike Decomposed Schist Undecomposed Schist 



Fig. i. — Relative positions of decomposed schist, undecomposed schist, and 

 pegmatite dike. 



Decay of schist. — In the eastern part of the excavation, the rock 

 was hard and sound, and needed to be blasted for removal. In the 

 western, the schist was thoroughly decomposed throughout to an 

 undetermined depth, so soft that it was easily removed with pick 

 and shovel, bluish to purple gray in color, and in texture passing 

 from a gritty aggregate almost to a clay; the latter corresponded 

 closely to the glacial clays of similar color commonly found about 

 the city. The two tracts, fresh and decayed, were separated by 

 an exceedingly sharp contact (the line A-B in the diagram), so that, 



