474 A. A. JULIEN AxMPHIBOLE SCHISTS OF MANHATTAN ISLAND 



avenue, and at West One hundred and seventy-fifth street, at end of 

 Highbridge — admit of another possible explanation. It seems likely 

 that on vertical pipes or vents of the dikes, often thin, and on upright 

 parts of their apophyses the first effect of downward pressure has been 

 wavy distortion and crumpling. Then followed their sidewise bending, 

 parting, and rolling out on the foliation plane into zigzag corrugations, 

 represented by part, at least, of the sheets showing this characteristic 

 structure (number 4 in plate 61) in almost every outcrop. The mode 

 in which such changes may have progressed in deformation of a 

 dike and its apophyses is illustrated in two purely theoretical diagram- 

 matic sketches (figure 8), with reference to the actual outcrop at West 

 One hundred and nineteenth street (plate 61). The literature of dikes 

 is extensive, but we have as yet scanty knowledge as to their earlier 

 phases of distortion and metamorphism during conversion into schist, 

 and to this these ancient dikes of Manhattan may offer a contribution. 

 It will be further suggested in the summar\^ of conclusions that these 

 intrusive forms may have been of secondarj^ character, of an entirely 

 different origin and period, produced during progress of metamorphism 

 of old igneous inclosures. 



It may be added that the extent of such distortion of dikes included 

 within these strata must also indicate a corresponding mashing and ob- 

 literation of the original bedding structure. Thus the adjoining mica- 

 ceous and pegmatitic gneisses have assumed an imitative foliation with 

 the same strike, though much contorted — for example, in Morningside 

 park, along the Speedway, and elsewhere. In these it is now hopeless to 

 search either for the original alteration of quartzose and of micaceous 

 layers, suggestive of sedimentary origin, or for microscopic remnants of 

 detrital structure. This is confirmed by the microscopic examination 

 of the micaceous gneiss already presented (see page 427). 



EVIDENCES OF CONTACT ALTERATION 



In general. — The survival of results of contact action would be a 

 desirable confirmation of the igneous origin of the hornblende schist, 

 though their absence would not conflict with that hypothesis, in view of 

 the common observation elsewhere of such absence of alteration b}^ dikes 

 crossing rocks of quartzose constitution, such as sandstones and gneisses, 

 and even more sensitive materials — for example, shales and coal. There 

 are three characteristic features to be looked for along the plane of junc- 

 tion between a dike and the country rock. 



Sharp line of demarcation. — The blending of a sedimentar\' bed is rarely 

 absent at one point or another along its margin with the contiguous 



