418 CARNEGIE INSTITUTION OF WASHINGTON. 



It is certain, as will appear presently, that great changes in the nature 

 of mineral and petrological organizations have taken place, and continue 

 to take place, in the body of the earth. Under appreciable pressure 

 these changes take on specific phases of metamorphism. Metamorph- 

 ism is here regarded as a prime factor in compression. It is perhaps 

 an even greater factor than its mate diast.^ophism, because so much 

 of compression is due to balanced stresses. Mere mechanical compres- 

 sion is probably largely replaced by metamorphic changes. In these 

 metamorphic changes, energy passes from the form of heat to the forms 

 that give organization and the reverse. The interchanges may be ac- 

 companied by increases of heat or by absorption of heat. In the main, 

 but not exclusively, exothermic action is interpreted as the passage of 

 revolutional or organizing energy into vibratory or disruptive energy; 

 endothermic action, as the reverse. The whole energj^ of organization 

 does not usually, if ever, appear in tangible form in the interchange, 

 but merely the energy-difference between the states or the combinations 

 involved. 



At the surface of the earth, exothermic changes preponderate over 

 endothermic changes, and as the exothermic changes are much the 

 more obtrusive, their preponderance has been much exaggerated; 

 endothermic action, even at the surface, is by no means unimportant. 

 A reversal of this dominance, at comparatively shallow depths, has 

 been amply shown by the researches of Van Hise and Leith and their 

 followers.^ Exothermic action is dominant in the zone of ka,ta- 

 morphism; endothermic action in the zone of anamorphism. Both 

 kinds of action, however, are present in both horizons. These actions, 

 though antithetical, are not to be looked upon as mutually exclusive. 

 Reversals of action probably occur at all horizons. ^ 



THE CONDITIONS THAT CONTROL INTERCHANGES. 



It seems to be a general principle that concentrative stress favors 

 endothermic action; while low stress, absence of stress, or dispersive 

 stress, favors exothermic action. This generalization is intended to 

 have somewhat wider scope than simple mechanical pressure and its 

 opposite, as indicated below. (It is prudent to observe, before passing 

 on, that the factors which enter into chemico-physical changes in 

 the earth are so complex that no simple law can be trusted to hold 

 universally.) It seems to be a general rule that increase of stress 

 tends toward increased divergence in the stress effects, i. e., the higher 

 the stress, the more ways of relief it forces for itself. In a small 

 sphere of gas, in open space and under mild self-compression, only 

 the simpler types of oscillation are detectable, but as mass and self- 



'Sce the chapters on katamorphism and anamorphism in Van Hise'a Treatise on Metamor- 

 phism, and Leith's and Meade's Metamorphic Geology. 



^Compare C. K. Leith, The structural failure of the lithosphere. Vice-presidential address, 

 Geol. Soc. of Amer., and Science, n.8.. Vol. LIII (Mar. 4, 1921), pp. 205-207. 



