914 A TREATISE ON METAMORPHISM. 



that any transformations through which sedimentary rocks go are sufficient 

 to obliterate the original differences in thick formations of greatly varying 

 lithological character. 



These principles are well illustrated by the Paleozoic metamorphosed 

 sedimentary rocks of the Green Mountains of New England. In this area 

 the Paleozoic rocks have almost completely lost all interior evidence of 

 clastic characters. It is only very rarely, where the rocks were conglom- 

 eratic, that any particles can be recognized as fragmental. However, the 

 shales and grits are transformed to mica-schists and mica-gneisses, the 

 sandstones to quartz-schists, and the limestones to marbles. The schists 

 are not evenly homogeneous and regularly schistose with a secondary 

 structure in a single direction, as is so characteristic of metamorphosed 

 igneous rocks. Each formation is composed of minor beds, these beds of 

 minor laminge. Notwithstanding the metamorphism, these retain to a 

 greater or less degree their integrity. At many places the bedded struc- 

 tures intersect the secondary schistose structures. In a larger way the rela- 

 tions of the formations to one another are those of sedimentary rather than 

 igneous rocks. No one skilled in work in the metamorphic rocks would 

 fail to recognize this Paleozoic series as of sedimentary origin. For the 

 most part these formations can be discriminated from the pre-Paleozoic 

 gneissoid granites. 



In regions of extreme metamorphism, where both igneous and aqueous 

 rocks have had long and complex histories, including the development of 

 secondary and perhaps tertiary structures, and sometimes impregnations 

 and injections, no criterion as to texture or minor plication or regular band- 

 ing is sufficient to discriminate the two. Even in regions in which the great 

 formations may indicate with a high degree of probability that a consider- 

 able portion of the material is of sedimentary origin, it may be true that 

 another considerable portion is igneous and can not be discriminated from 

 the aqueous part of the series. 



(3) A third criterion of great importance in the discrimination of meta- 

 morphosed sedimentary and igneous rocks is chemical composition. It has 

 been shown on pages 555-558 that the materials for sedimentary rocks 

 are sorted, that in general there is depletion in certain of the elements as 

 compared with the igneous rocks, and that the proportions of the elements 

 in the sedimentary rocks are therefore different from those in the igneous 



