DEFORM A TION OF R O CKS 6 O 7 



parallel banding, as described by Hobbs in the Searls quarry. 

 In other districts parallel injections of igneous rocks occurred, 

 and here the metamorphosed schists are wedded together along 

 the fissile planes by igneous material. Such are the conditions 

 in southeastern New York, and especially on northern Manhattan 

 Island in the vicinity of New Rochelle. 



As shown in other places (pp. 600-603), there may be all 

 gradations between aqueous impregnations, through aqueo- 

 igneous deposits, to true igneous injections. Usually the origi- 

 nal rock differs in chemical and mineral composition from the 

 impregnations or injections. The process gives a distinctly 

 banded structure, which has often been mistaken for metamor- 

 phosed original sedimentary layers. Also the chemical com- 

 position may be so changed, if the amount of secondary mate- 

 rial is large, as to make it impossible to tell from its chemical 

 analysis whether the original rock is aqueous or igneous. 



If the parallel mineral impregnation, or the parallel igneous 

 injection, or the two together, had been general throughout the 

 Appalachian semicrystalline and crystalline areas, we should 

 have a vast series of crystalline schists with parallel banding, the 

 bands generally dipping to the southeast, and these layers might 

 be mistaken for beds, and thus lead to estimates of enormous 

 thickness for the series, when in reality the original sedimentary 

 becls were of no unusual thickness. This error has not been 

 made for much of the Appalachian region because in most areas 

 the metamorphism has not been so extreme but that the cleav- 

 age is detected intersecting the bedding at the sharp turns of 

 the layers on the crests of anticlines and in the troughs of syn- 

 clines. Had the metamorphism gone as far throughout the 

 region as it has in certain places, the bedding could not be dis- 

 covered, and there would be now no means of tracing out the 

 different steps of the process of modification. But in the 

 Appalachian and New England regions the steps of the process 

 of the development of cleavage and fissility, the stages of the 

 parallel impregnation and injection, and all degrees of metamor- 

 phism ma}' be observed, so that in some of the regions of most 



