



' 



the ]><>r\ ' repUt. / 



.-Trt*linf:, giUx-. '. rnij>-. 



junctio 

 ;\i\<\ l>> 

 in tl -'allio o, 



' 



ner in 

 probably with 



The in* 

 in flue nee on I 

 . ounding T.. 



- 

 ijrri-- 



. 

 nev. 



I 

 . 

 I 

 rnev. -aet- 



. 



. as w are 



formed fro. Occasion. ; e as to 



es into a brownish glass. 



TV- roved most useful in studying question* 



.ic metamorphism. or that dne to "earth -stresses/ 

 >n hy movement has .sometimes been .so great as to 

 obi:' - or even wholly, the original structure of a rock. 2 



The intense pressures must produce some elevation of tempera- 

 ture and increase the solvent action of water, so that the original 

 constituents of the rock are destroyed, partiallyj if not wholly, and 

 at a later stage new minerals are produced. It has been shown that 

 many gneisses and schists (though not all) have been formed by 

 crashing or shearing from igneous rock. e.g. gneiss from granite, 

 hornblende schist from dolerite. In the former case, the crushing 

 of the felspar, the formation of white mica and free quartz from 

 its dust. 3 the effects produced on the other minerals, can all be 

 studied under the microscope ; and in the latter the conversion 

 of augite into hornblende. This, however, may be brought about by 

 more than one cause, and each probably produces effects which can be 

 distinguished. These questions, however, on which many experienced 

 petrologists have been engaged for at least fifteen years, are much 

 too difficult and technical to be discussed in a book of this character : 

 enough to say that heat, pressure, and water, singly and conjoir. 

 produce important changes in rocks, many of which can now be 

 identified. 



1 Bonney, Quart, fourn. Geol. Scr. xliv. (1888 . p. 11. 



2 Tresca, ' Flow of Solids,' Proc.. Inst. Mech. Eiitj. 1878, p. 301. 



3 A minute hydrons mica, often called aericite, ^eems to form i-eadilj in an 

 argillaceous rock nnder preaaure. The ailky-Iooking -ilate<< itn which the name 

 phyllite is restricted by aome authors! are largely composed of it. 



