ON THE MIGRATION OF MATERIAL DURING THE 

 METAMORPHISM OF ROCK-MASSES. 



The researches of numerous geologists during the last two 

 decades have placed at our disposal a large amount of informa- 

 tion respecting the metamorphism of rocks, and from the facts 

 thus collected we are now in a position to draw conclusions which 

 we may expect to have a wide application. The important 

 changes that affect the character of rock-masses divide roughly 

 into two classes. 



First, there are those dependent on meteoric agencies. These 

 changes, though not necessarily superficial in the ordinary sense, 

 are due in the first place to the action of circulating waters in 

 communication with the atmosphere, and as a rule they involve 

 the addition or subtraction of various ingredients or the transfer- 

 ence of material from one place to another. The ordinary 

 "weathering" effects illustrate the removal of alkalies and silica, 

 the addition of water, oxygen, carbonic acid, etc. We must also 

 include the processes which have given rise to many crystalline 

 limestones and quartzites, serpentine-rocks, dolomites, iron- 

 stones, and jaspers, and even (as appears from Van Hise's 

 researches in the Penokee region) some mica-schists and fine- 

 grained gneisses. The characteristic of almost all these trans- 

 formations is that they are metasomatic as well as metamorphic. 



Secondly, we have those transformations more usually under- 

 stood by the term metamorphism : viz., dynamic metamorphism, 

 due to high pressure operating upon rock-masses, and thermal 

 metamorphism, due to high temperature, whether produced by 

 an intrusion or by the mechanical generation of heat. In these 

 various cases of metamorphism proper, metasomatism is rather 

 the exception than the rule. I shall deal here with thermal meta- 

 morphism only, and shall draw my data chiefly from the rocks 

 surrounding the large igneous intrusions of the English Lake 

 District, investigated by Mr. Marr and myself, but the conclusions 



are confirmed in other areas. 



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