224 Major-Gen. McMahon—The " Culm " at Bude. 



Where there is shearing or voluminal compression, heat is 

 generated, and molecular activity is increased. But the " energy 

 set free " may be small in amount, and may be lost by conduction ; 

 and thus, to quote Mr. Harker's words, it is " easy to imagine 

 conditions under which any amount of contortion may be produced 

 without any metamorphisni of the rocks so affected." 



But there are some geologists who go beyond Mr. Harker, and 

 contend that pressure produces metamorphisni even in cases where 

 the rock-masses do not yield to the pressure exerted on them ; and 

 where, consequently, there is neither shearing, voluminal compression, 

 or " movements .in the rock-masses." 



Mr. Harker is under a misapprehension in supposing that I call 

 "in question the researches of M. Spring and others on the physical 

 and chemical changes produced under the action of high pressures." 

 I spoke of M. Spring's experiments as " instructive and valuable." 

 What I call in question is the way Spring's experiments are some- 

 times applied by others. The net results of M. Spring's experiments 

 are well summarized by Prof. Judd in one of his papers as follows : 

 " The researches of Spring, van 't Hoff, Keicher, and others, have 

 shown the effects of pressure in bringing the molecules of solid bodies 

 sufficiently close to one another for chemical affinity to operate 

 between them." But it is obvious that chemical affinity can only 

 operate when the substances brought into contact with each other 

 are substances which have a chemical affinity for each other. I fail 

 to see that M. Spring's experiments throw any light on what I con- 

 sider the most important class of metamorphic changes that take 

 place in a rock ; namely, metachemic changes. When minerals 

 during the progress of metamorphism have simply undergone para- 

 morphic changes, there is little difficulty in accepting the view that 

 these changes may have been brought about by pressure ; but, when 

 we have metachemic changes, that is to say, when a mineral of one 

 chemical composition is converted into a mineral of a different 

 composition, it is clear that pressure alone cannot account for 

 the change set up. We need in order to explain these changes 

 to call in the aid of some agent, such as water, to carry to the 

 mineral in progress of alteration the chemical elements required 

 which it does not itself contain; and to remove the chemical 

 elements which it has to get rid of in whole or in part. The chemical 

 elements required for the new mineral niay exist in other minerals 

 in some other part of the rock, but they need to be extracted from 

 those minerals by one set of chemical reactions, and conveyed to the 

 spot where they are made over to the mineral in progress of 

 metamorphic change by another set of chemical reactions. For 

 these operations we require water or some other agent. This circu- 

 lating fluid then — water or other — is the active agent in promoting 

 these changes, and all that compression, or shearing, can do is 

 to provide the heat, or other form of energy, necessary to start or 

 facilitate the requisite chemical reactions. 



The agency of water, or some other circulating medium, is, as I 

 have said above, necessary to account for metachemic changes ; heat, 



