428 JULIEN 



Island with pegmatite and quartz, in seams and lenses along the 

 foliation, has met with one curious exception which calls for 

 explanation — the beds of actinolite, tremolite, talc, chlorite and 

 serpentinoid. The complete absence of such occlusions from 

 these, and even of siliceous increment in chemical composition, 

 stands in strong contrast with the swarm of quartz and pegma- 

 tite lenses in the adjoining beds of gneiss and quartz-diorite. 

 This freedom from pegmatitic occlusions prevails also in the 

 serpentinoid of the Hoboken-Staten Island ridge, and, I believe, 

 in the similar rocks of Westchester county. Whenever free 

 silica occurs, this is merely a late product of alteration or of de- 

 cay, secreted as chert during brucitic or ophiolitic decomposi- 

 tion of a pyroxene or amphibole, or as cellular quartz enclosing 

 limonite, during subsequent decay. 



The explanation of this fact may lie in the impervious tex- 

 tures of the original pyroxenite and of the tough and pliant, 

 derivative amphibole schists. Those developed from the gabbro, 

 however, rich in hornblende and feldspar, became brittle and 

 even granular under the kneading of orogenic movements and 

 flexures, and their shattered masses admitted free permeation 

 by pegmatitic injections. 



Granite Dikes Underlying the Serpentinoid. — An outcrop of 

 coarsely foliated pegmatite on the shore at Tompkinsville, Staten 

 Island, furnishes evidence of an extensive pegmatite intrusion 

 underneath the serpentinoid ridge. A little further north, at the 

 mouth of the Kill van Kull, the blasting of a reef in the harbor 

 enabled Dr. Arthur Hollick to make a collection which consists 

 entirely of coarse and fine-grained pegmatite, sometimes dis- 

 tinctly graphic, and occasionally a well-laminated gneissoid 

 granite. The latter structure, probably marginal, must have 

 resulted from movements while the igneous mass was still in 

 partially molten condition. According to one hypothesis, this 

 was due ''to the drag of the stiffly viscous granite magma along 

 its walls during its intrusion,"^ or, according to another, "to 

 the strain of the viscous magma upon separated crystals where 



' W. O. Crosby, Tech. Quart., Boston, XII, 1899, 95. 



