FETTKE, MANHATTAN ISVHIST OF NEW YORK 231 



feldspar. The feldspar is mostly plagioclase, an andesine, with extinction 

 angles up to 22° in sections at right angles to the albite lamellae. It is 

 optically positive. Feldspar is occasionally contained in the augite as 

 inclusions. Titanite, apatite and magnetite occur as accessory constitu- 

 ents. 



This area of foliated diorite gneiss apparently represents an intrusion 

 of rather basic igneous rock which entered prior to or else during the 

 early stages of the period of folding which involved the whole region. 

 The hornblendite, on the other hand, was intruded during the latter 

 stages or else after folding had ceased entirely. There are, therefore, 

 two periods of igneous intrusion of rocks very similar in composition 

 represented. The inclusions of diorite gneiss in the diorite itself south 

 of Croton Falls are in accord with such a hypothesis. The hornblendite 

 intrusions at Croton Falls were probably contemporaneous with that of 

 the Cortlandt series at Peekskill. The hornblendite is in turn cut by a 

 number of large dikes of granitic composition, sometimes reaching a 

 thickness of two hundred feet or more. These range from true granite 

 to coarse pegmatites, a variation of texture which often appears in the 

 same dike within a very short distance. A discussion of these granitic 

 intrusives will be taken up later. Eeference is made to them here to 

 show that the entrance of the hornblendite took place prior to the granite. 



Diorite Dikes in the Vicinity of Bedford 



Two occurrences of diorite in the form of dike-like intrusions have 

 been described by Professors Luquer and Ries from the vicinity of Bed- 

 ford in their paper on the geology of this region.^^ One of these occurs 

 along the Bedford-Long Eidge Road about two and one-half miles south- 

 east of Bedford. The rock has a dark color, medium, coarsely crystalline 

 texture and massive structure. In thin section, one observes deep green 

 hornblende, showing good prismatic cleavage, pale green augite and feld- 

 spar. Most bf the feldspar is unstriated but is optically positive and 

 therefore plagioclase. Titanite and apatite occur as accessory constitu- 

 ents. The augite apparently crystallized out before the hornblende, but 

 the two are very intimately intergrown. Both minerals are perfectly 

 fresh. Some of the feldspar has undergone slight alteration to an ag- 

 gregate of quartz, sericite and calcite. 



A similar rock occurs about two and one-quarter miles south of Bed- 

 ford. It also has a dark green color, medium coarsely crystalline texture 

 and massive structure. A thin section reveals light green hornblende 



"Am. Geol., Vol. XVIII, pp. 239-261. 1896. 



