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Metamorphic igneous rocks are treated separately, though briefly, 

 but the means of discriminating between originally sedimentary and 

 originally igneous rocks are discussed with the insight due to broad 

 observation. The difficulties of discrimination are suggested in the 

 following statement : 



"It has been seen on previous pages that a large number of kinds 

 of schists and gneisses may be produced by the metamorphism of 

 sedimentary rocks ; also it has been shown that similar crystalline 

 schists may be derived from igneous rocks. It is further certain that 

 a schist or gneiss may be derived partly from sedimentary and partly 

 from igneous rocks. For instance, a metamorphosed fissile sediment- 

 ary rock, such as mica-schist or mica-gneiss, maybe injected in a com- 

 plicated way parallel to the planes of schistosity, and thus produce a 

 banded gneiss, part of which is igneous and part of which is sedi- 

 mentary. The rock may be predominantly of either one of these 

 materials. If the injected sedimentary rock be subsequently folded, 

 this will produce differential movements parallel to the banding, and 

 the igneous and aqueous bands may be merged into one another and 

 have structures so similar that it is impossible to determine what part 

 of the rock is igneous and what part aqueous. The Manhattan schists 

 of southeastern New York, and especially near Long Island Sound, 

 are a perfect illustration of a gneiss produced by the extreme meta- 

 morphism of a sedimentary schist and the subsequent parallel and 

 cross injection of granitic 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 possi- 

 ble combination of two or more of these. 



"Doubtless, in regions in which the 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." 



Stratigraphy, i. e., the sequence of formations, is deduced for pre- 

 Cambrian rocks from observations of the phenomena of structure and 

 metamorphism and other relations in detail. It thus becomes an 

 inference instead of being a primary fact of observation, as it may be 

 in rocks which are less disturbed. Van Hise, therefore, gives strati- 

 graphy a logical position at the close of his discussion of principles. 

 The evidences of stratification which are obvious in unaltered series 



