THIRD HYPOTHESIS OF DERIVATION 



45: 



ticularly in regard to serpentine. A similar view, already quoted, has 

 been taken by F. J. H. Merrill, who has given much study to these rocks. 

 L. M. Luquer and H. Ries, in 1896, were inclined to consider hornblendic 

 schists at Bedford, in the northern part of Westchester county, as possi- 

 bly metamorphosed diabase dikes.* The first careful discussion of this 

 hypothesis in reference to Manhattan island was presented by J. F. 

 Kemp in 1897, before the New York Academy of Sciences, and on De- 

 cember 28, 3898, before the Geological Society of America. To him I 

 am indebted for the use of analyses and many suggestions. More re- 

 cently D. H. Newland, in a paper on the serpentines of this region, f 

 has also referred to the igneous character of the anhydrous ferro-magne- 

 sian silicates, regarded as the original rocks. 



In reference to such a genetic relationship the following characteristics 

 demand attention : 



CHEMICAL COMPOSITION OF THE SCHIST 



Its remarkable uniformity in physical features — texture, mineralog- 

 ical constitution and structure — testifies to a probable uniformity in com- 

 position, a well known character of eruptive material in dikes ejected 

 from a common magma ; yet more definite appears the approach of 

 Jouet's analysis (IV) to those of two igneous rocks of exceptionally basic 

 constitution, stated below : 



XXI. Gabbro. Northwest Minnesota. Stokes. X 



XXII. Diabase greenstone. Lower Quinnesec falls, Wisconsin. R. B. 

 Riggs. ^ 



This consisted of brown hornblende, chlorite, epidote, and quartz, with 

 menaccanite, leucoxene, traces of feldspar, and pyrite. 





XXI. 



XXII. 



Si02 



45.66 



16.44 



.66 



13.90 



7.23 



11.57 



.41 



2.13 



.07 



43.80 



Al.O, 



16.08 



T^eA 



FeO 



9.47 

 10.50 



CaO - 



7.81 



MgO 



K2O 



6.54 

 .34 



Na^O 



1.96 



Ignition loss 



3.99 



CO, 



.08 





98.07 



100.57 



* Am. Geologist, vol. xviii, 1896, pp. 241, 247. 



+ School of Mines Qixart., N. Y., vol. xxii, 1901, p. 409. 



X W. S. Bayley : Jour. Geol., vol. 1, p. 712. 



g U. S. Geol. Survey, Bull. no. 62, 1890, p. 91. 



