METAMORPHOSED SEDIMENTARY AND IGNEOUS ROCKS. 913 



mass gradually varies in composition in its different parts, as is so often the 

 case with differentiated magmas, the mashed varieties grade into one 

 another. The complete absence of minor plications, indicating the absence 

 of a parallel structure before the schistosity was produced, is most striking. 

 It is therefore concluded that the finely, regularly laminated, homogeneous 

 schists are usually of igneous origin. It should perhaps be remarked that 

 this is a reversal of the interpretation frequently made at the present time, 

 and which was almost universally made a few years ago. 



The converse proposition, that the banded and heterogeneous schists 

 and gneisses are of sedimentary origin, can not be made; for the igneous 

 rocks, intrusive and extrusive, in a given area may have had an original 

 banded character, or may have had great complexity in their parts, as a 

 result of which their mashed equivalents show a banding. This is particu- 

 larly likely to be true of lavas and tuffs, and may be true even for intrusive 

 masses. Further, it has been seen that the mashed, originally homo- 

 geneous, igneous rocks may have become cleaved or fissile, and that along 

 the secondary structure impregnations or injections may occur and the rock 

 thus become banded. If a new mashing now takes place which develops a 

 tertiary structure cutting the secondary one, the minor plications of the 

 secondary structure may simulate similar plications of sedimentary strata. 



(2) The metamorphosed sedimentary rocks are not usually so changed 

 as to obliterate all evidence of their original condition. This is true even 

 of the metamorphosed pelites. Minute variations in the coarseness of the 

 laminae are usually sufficient to produce minor plications which somewhere 

 will be found to intersect the secondary structure. Moreover, in a larger 

 way, thick pelite formations are apt to contain some beds of grit, sandstone, 

 or limestone. These give schists of different kinds, and each of these is 

 different from the metamorphosed mudstone. 



Variations in original lithological character are still more prominent 

 when great formations are considered. It has been seen that a sandstone 

 formation may be transformed into a quartz-schist or a micaceous quartz- 

 schist. Limestone may be transformed into marble, bearing greater or 

 less quantities of silicates, depending largely upon its original purity. 

 The arkoses and shales may be transformed into schists and gneisses. 

 Widely disseminated graphite and the presence of ferruginous beds have 

 been regarded as indicating a sedimentary origin. There is no evidence 

 mon xlvii — 04 58 



