DISTRIBUTION OP AMPHIBOLE ROCKS 429 



sections of these schists which have been prepared have I been able to 

 detect any remnant of a pyroxene. At least there appears to be insuffi- 

 cient evidence as to the identification of any of our rocks as pyroxenic. 



DISTRIBUTION OF AMPHIBOLE ROCKS 



These schists have been observed only on the northern part of the 

 island, the greater part of whose surface, shown on the map (plate 60), 

 is occupied by bared gneisses in beds tilted up almost everywhere at very 

 high angles, with a general strike of about north 28° east. On this map 

 the outcrops marked with an asterisk (*) are those which are now covered 

 up through the growth of the city — most of them on the east side of the 

 island — but it is shown that a great number still remain open to obser- 

 vation and may be easily visited by the geologist interested in this sub- 

 ject. Some are likely to remain always exposed, particularly those in 

 Central, Riverside, and Morningside parks and the Speedway. It will 

 also be shown that even a far wider distribution of altered schist is 

 suggested by the numerous exposures of biotitic schist and gneiss. To 

 assure oneself of this relationship, a study of the two rocks in the out- 

 crops from West One hundred and seventeenth to One hundred and 

 twenty-second streets would be profitable. 



A linear arrangement of many exposures along the strike is apparent, 

 but these were in most cases found to be interrupted, without connection 

 in the interspaces. 



DioRiTE Schist and Hornblende Schist 



OCCURRENCE AND RELATIVE ABUNDANCE 



These schists far exceed all other amphibole rocks in abundant and 

 wide distribution on the island and call for special description. They 

 were early recognized by Cozzens,* who defined some of their principal 

 outcrops. They occur in the gneisses mostly as rather thin seams or 

 lenses, from a few inches to 2 or 3 feet in thickness. On the sides of 

 street cuttings their thinned out margins may often be traced out in 

 establishment of a universally lenticular form. In places they may reach 

 20 feet or more in thickness, but then generally become quartzose and 

 gneissoid — dioritic or hornblende gneiss — and more or less biotitic, so 

 passing into black biotite gneiss, in which hornblende may be abundant, 

 sparingly distributed, or entirely absent. 



STRUCTURE VARIATIONS 



In structure these diorite and hornblende schists, as well as their 

 related gneisses, described further on, display two seemingly contra- 



*A geological history of Manhattan or New York Island, 1843, pp. 12, 15. 

 LX— Bull. Giol. Soc, Am., Vol. 14, 1902 



