924 A TREATISE ON METAMORPHISM. 



In proportion as the igneous rocks are metamorphosed it is difficult to 

 apply the above criteria. Where they are no longer distinguishable from 

 the sedimentary rocks the criteria given for determining structural relations 

 are not available. 



RELATIONS OF ROCK FLOVAGE TO MOUNTAIN MAKING. 



The principles of metamorphism worked out in the previous chapters 

 have an important bearing upon theories of mountain making. Various 

 authors have advocated expansion theories of mountain making. Probably 

 the most notable of these is Reade. His explanation of the rise of moun- 

 tains is that the volume of the thick deposits of sediments increases in 

 consequence of the rise of the isogeotherms. a It is perfectly clear from 

 the previous chapters that the alterations of the zone of anamorphism 

 result in very important contraction. Not only is this true of the deep- 

 seated zone, but in the zone of katamorphism the volume of one great 

 class of sediments, the pelites, is certainly decreased both by consolidation 

 and by dehydration. It may perhaps be conceded that the reactions of 

 metamorphism, if all of geological time be taken into account, have 

 somewhat expanded the volume of a very superficial belt of the earth. 

 But in the deep-seated zone, where pressure is dominant, the reactions are 

 taking place in a reverse sense, and as these have prevailed throughout 

 geological time and to unknown depths, it can hardly be doubted that 

 the contraction due to these deep-seated reactions is of vastly greater 

 importance than the sum total of expansion due to superficial causes. 



Perhaps it will be well to recall the facts bearing on the amount of 

 contraction which takes place in consequence of the reactions of the zone 

 of anamorphism. 



It has been seen on page 364 that the average specific gravity of the 

 minerals formed in the zone of anamorphism is about 18 per cent greater 

 than that of those which develop in the zone of katamorphism ; but for 

 the rocks of the zone of anamorphism not all the minerals have passed to 

 the products characteristic of that zone. Therefore, one can not premise 

 that in the metamorphism of sedimentary rocks there is a diminution of 

 volume as great as this. However, it is probable that the average of the 

 chemical reactions of dehydration and silication and mechanical consolida- 



« Reade, T. Mellard, Origin of mountain ranges: London, 1886, pp. 1-359. 



