912 A TREATISE ON METAMOEPHISM. 



aqueous. The Hudson schists and gneisses of southeastern New York, 

 especially near Long Island Sound, are a perfect illustration of rocks 

 produced -by the extreme metamorphisrn of a pelite and the subsequent 

 parallel and cross injection of granitic material. 



Another case is that in which an intrusive rock enters a fractured 

 sedimentary rock in such a manner as to give the intrusive masses roundish 

 forms. When later metamorphosed the roundish forms are dissevered, and 

 they may resemble bowlders. The frequent inference is that the rock is a 

 metamorphosed conglomerate, whereas it is a metamorphosed sediment in 

 which the pseudo-bowlders are introduced igneous material. 



From the foregoing it is clear that an inseparable schist or gneiss 

 formation may be produced from altered intrusive rocks, from altered lavas, 

 from altered tuffs, from altered sediments, and from any possible com- 

 bination of two or more of these. Doubtless in many regions in which the 

 schists and gneisses are of a very complex character a number of the 

 processes mentioned in the previous pages, and possibly others unknown, 

 must be united in order to explain all of the phenomena. 



While similar schists or gneisses may be derived from sedimentary 

 rocks or from igneous rocks, or partly from each, one who has had much 

 experience in the field is apt to have a bias toward the one or the other 

 origin in a particular instance. But if a given schist or gneiss formation 

 can at no place be traced into a form characteristic either of a sedimentary 

 rock or of an igneous rock, it is extremely hazardous to make positive 

 assertions as to its origin. 



CRITERIA FOR DISCRIMINATION. 



While the schistose igneous rocks may have foliations, bandings, and 

 lithological characters which very closely simulate those of metamorphosed 

 sedimentary rocks, it is still true of the rocks belonging to the Alg'onkian 

 and later periods that in many districts the majority of the metamorphosed 

 igneous rocks can be discriminated from the metamorphosed sedimentarv 

 rocks. 



The more important criteria for discrimination between the metamor- 

 phosed sedimentarv and igneous rocks are as follows: 



(1) The much-mashed igneous rocks are apt to have a very regular, 

 fine foliation and great uniformity of lithological character. If an igneous 



