908 A TREATISE ON METAMORPHISM. 



fact, in many cases rocks of diverse origins have become so similar that as 

 yet no criteria have been devised by which they can be discriminated from 

 one another. 



It has been fully explained that many of the minerals produced by 

 metamorphic processes are the same as those produced by crystallization 

 from a magma and have identical characters. It has further been seen that 

 during metamorphism secondary textures and structures are formed in 

 rocks of all kinds. Some of the secondary textures are enlargement, cata- 

 clastic, laminated, porphyritic. Some of the secondary structures are joint- 

 ing, faulting, bedding partings, cleavage, fissility, schistosity, gneissosity. 

 At the time these textures and structures are produced the original textures 

 and structures are usually obscured, and they are not infrequently obliter- 

 ated. And even if the original textures and structures are not greatly 

 modified they are often overlooked because of the prominence of the 

 secondary textures and structures. 



Finally, where rocks of different origin but similar chemical composi- 

 tion are strong^ metamorphosed with the production of secondary textures 

 and structures, it is scarcely possible to discriminate them, for, as has been 

 fully shown, upon the original chemical composition of the rocks and upon 

 the conditions of their alterations depends the character of the resultant 

 product. Two metamorphosed rocks of different origin but similar chemical 

 composition may have practically identical minerahogical, textural, and 

 structural characters. 



DISCRIMINATION BETWEEN METAMORPHOSED SEDIMENTARY AND 

 METAMORPHOSED IGNEOUS ROCKS. 



For the purposes of stratigraphy, it is very desirable that, so far as 

 possible, the criteria be given by which metamorphosed sedimentary rocks 

 may be discriminated from metamorphosed igneous rocks. So long' as the 

 sedimentary rocks retain little modified clastic textures, either macroscopical 

 or microscopical, they can be discriminated as such. So long as the igneous 

 rocks retain the igneous textures, these may readily be recognized in thin sec- 

 tions. Also, it has been fully explained on pages 644-645, 689-690, that, 

 in so far as the alterations are those of mass-static conditions, the textures and 

 structures are not rapidly destroyed, so that it is ordinarily easy to recognize 

 with the microscope the original textures of such rocks, even if complete 



