STRUCTURE V A KTATIONS 



431 



Second, fracture and faulting. Evidences of extreme rigidity and 

 brittleness are also often shown in the same beds in which crumpling 

 and corrugation are prominent. These are crossed, often abundantly, 

 by seams or veins of gray or white quartz or of pegmatitic material, 

 which run usually parallel to each other and at right angles to the 

 foliation of the schist. Yet they are confined to these thin beds of 

 hornblendic rock and are thus distinguished from the main quartz and 

 pegmatite veins which intersect all the schists of the island. Such 

 fractures plainly testify that the hornblendic beds have lain as rigid 

 masses during movements of the inclosing gneiss and have yielded only 

 by rupture along cross-planes. 



Figure 1. — Fault Breccia in Quartz Vein, two feet ivide, in Quartz-diorite Schist. 

 Locality, West One hundred and thirty-eighth street and Saint Nicholas avenue. 



An excellent example still remains open to examination on the west 

 side of Saint Nicholas avenue, about West One hundred and thirty-eighth 

 street. A nearly vertical sheet of diorite gneiss, partly epidotic, 3 to 4 

 feet in thickness, forms a wall along the sidewalk for a distance of more 

 than 20 yards in the direction of the strike, north 21 degrees east.* 

 Abundant cross-seams of quartz vary in thickness from 2 inches up to 2 

 feet. By two of these the sheet has been disjointed, and the surrounding 

 micaceous gneiss is sometimes forced deeply into or even quite through 

 the old fracture. One such fault-vein, 2 feet wide, is filled by a friction 

 breccia, made up of angular fragments of the hornblendic rock, inclosed 

 in brownish white quartz (figure 1), the walls of the fracture lined by 



*True bearing, hU in this paper having been corrected for magnetic variation. 



