OCCLUSION OF IGNEOUS ROCK 393 



terized by zigzag folding down to minute corrugation, have 

 been elsewhere discussed. 



As to its enrichment by quartz, while this has been doubtless 

 derived through general diffusion from adjoining pegmatite 

 masses, its partial indebtedness to internal reaction must be ad- 

 mitted. A common opinion would attribute it entirely to re- 

 constitution of minerals, during passage from a pyroxenic to a 

 hornblendic rock.^ Most definite evidence on this point has 

 been given by comparison of two analyses, of a dolerite and of 

 its derivative hornblende schist, in the dike at Scourie, Scot- 

 land " — almost alike except for the gain of two per cent, of 

 silica in the schist. 



It will be inferred from previous statements that some forms 

 of our diorite schist have become so thoroughly disguised by 

 metamorphism that only careful study of earlier phases of the 

 transition will enable recognition of the relationship. A rare 

 form in the Woodward Collection ^ is a highly quartzose actino- 

 lite-diorite (No. 153, from aqueduct-shaft at West 179th Street 

 and loth Avenue), which bears to the eye an extraordinary resem- 

 blance to an enstatite-diorite, and also reminds one of the con- 

 stitution and structure shown in the thin section of amphibole- 

 gabbro from HoUmiihle.* 



The frequent distribution of scapolite and wollastonite, in 

 small quantity, among the gneisses of the Island, may yet 

 prove to be of igneous relationship, as in Norway, Canada and 

 the southwestern Adirondacks. 



Diorite Bed on Spuyten Diiyvil Creek. — Many peculiarities 

 of structure characteristic of an occluded igneous intrusion are 

 illustrated in one bed, as yet undescribed, the largest remaining 

 on Manhattan Island, along the shore of Spuyten Duyvil creek. 

 It has been referred to by Cozzens, in 1843, as the site of a 



1 C. Callaway, Quart. Jour. Geol. Soc, XLIII, 1887, 528. 



2 J. J. H. Teall, idem, XLI, 1885, 135-137. 



3 I have been indebted to Mr. Anthony Woodward of the American Museum of 

 Natural History for opportunity to examine his collection of specimens of rocks 

 from Manhattan Island, now at Rutgers College, New Jersey ; and to this I shall 

 bave other occasion to refer. 



* J. Lehmann, " Enst. d. altkryst. Schiefergesteine, Atlas," Tafel XXII, Fig. I. 



