USE OF TERM "OPHITIC^ 667 



ophitic has come into wide usage, but it is now used with either one of two 

 different meanings. Some writers follow the original definition and the prac- 

 tice of its author in applying the term to all rocks having plagioclase in lath- 

 shaped crystals of early formation. Others, apparently misunderstanding a 

 much later description of the texture, 34 confine the term to those rocks whose 

 lath-shaped feldspars are poikilitically inclosed by large anhedra of pyroxene. 

 It seems evident from both the original definition and from the usage of the 

 author that, while such a poikilitic arrangement is included, it is not essential 

 to the texture ; and it appears that pyroxene itself is equally non-essential ac- 

 cording to the usage of the author, since he says the last mineral to crystallize 

 is generally pyroxene, and, moreover, applies the term ophitic to a rock from 

 Greenland 35 in which the pyroxene is replaced by native iron. 



It appears, therefore, that the term ophitic should be used in the broad 

 sense already defined, for which usage it has clekr priority over all other 

 terms. Furthermore, if a rock with ophitic texture has glass in the ground- 

 mass the texture may properly be called intersertal, which thus designates a 

 variety of the ophitic texture. It appears also that the terms divergent-strah- 

 lig-kornig of Lossen, radiolitic of Loewinson-Lessing, and diabasic and doleritic 

 of Rosenbusch are needless synonyms of ophitic. Finally, it may be suggested 

 that if luster-mottled be considered too cumbrous a term to describe a texture 

 which is at once ophitic and poikilitic, it might well be called poikilophitic. 



Then was read 



CHEMICAL COMPOSITION AS A CRITERION IN IDENTIFYING METAMORPHOSED 



SEDIMENTS 



BY EDSON S. BASTIN * 



[Abstract.] 



This paper called attention to the small number of definite statements, even 

 of a qualitative character, in geological literature, as to the nature and value 

 of chemical criteria in distinguishing schists of sedimentary from those of 

 igneous origin. Quantitative statements are wholly wanting. 



By compiling a large number of analyses of pelitic sediments the writer 

 showed the nature of the chemical changes involved in their metamorphism. 

 He then proceeded to contrast the composition of the pelitic schists and 

 gneisses with that of their allies among igneous rocks. The calculation of the 

 "norm" of a schist and its classification, according to the quantitative system 

 of Cross, Iddings, Pirsson, and Washington, was pointed out as a convenient 

 method for making such comparisons. 



. These statistical studies brought out not only the character of the chemical 

 criteria which may be used, but gave a quantitative measure of their value. 

 The paper concluded with the application of these criteria to certain selected 

 schist and gneiss analyses. 



34 A. Michel Levy : Structures et Classification des Roches firuptives, Paris, 1889, p. 26. 



35 C. R., vol. xcii, 1881, p. 891, and Ann. Ch. Phys., vol. xvi, 1879, p. 505. 

 x Introduced by G. 0. Smith. 



