762 A TREATISE ON METAMORPHISM. 



recognizable in a section transverse to the greatest elongation. But where 

 the metamorphism is extreme the pebbles may be so greatty flattened as to 

 allow the solutions to mingle the materials of the pebbles and matrix, and 

 thus the pebbles be wholly lost. 



But even with most extreme metamorphism by recrystallization the 

 larger structures are usually preserved, although the process may occasion- 

 ally go so far as to make the strata or even the beds indistinguishable; but 

 it rarely, if ever, goes so far as to obliterate formations. Excluding- igneous 

 action, interstratified sedimentary formations of, for instance, sand, mud, 

 and limestone, have rarely been so profoundly metamorphosed as to make 

 them indistinguishable. The composition of a formation is the most promi- 

 nent feature by means of which the original nature of a rock can be 

 recognized. Where there are metamorphic formations having the approxi- 

 mate chemical compositions of sands, muds, and limestones, we may be 

 sure that such formations, are sedimentary. Rock flowage may wholly 

 obliterate the clastic textures, and even the bedded structures; but there is 

 not sufficient kneading and intermingling of the materials to obliterate 

 major structures. That this should be so is precisely what we would 

 expect when we understand that rock flowage is accomplished not by 

 recrystallization from fusion, but by recrystallization and by granulation of 

 the individual mineral particles, the rocks remaining solids throughout the 

 transformations. 



ROCK FLOWAGE AND MASHING. 



In another place I have proposed the term "mashing" to describe 

 the process of mass deformation in the zone of rock flow." It has been 

 seen that rock flow involves the universal participation of the mineral 

 particles. The preceding study shows that this participation may be by 

 recrystallization or by granvdation, or by the two combined; but in the 

 field, where microscopical work is > not usual, it is difficult or impracticable 

 in many cases to discriminate between these two processes. Therefore it is 

 very convenient to have a word which will cover deformation by rock 

 flowage without reference to the detailed effects upon the mineral particles, 

 and for this purpose it has seemed to me that the word "mashing" expresses 



"Van Hise, C. R., Principles of North American pre-Canibrian Geology: Sixteenth Ann. Kept. 

 U. S. Geol. Survey, pt. 1, 1S96, pp. 694-696. 



