114 E. A. Daly — Mechanics of Igneous Intrusion. 



Rock. Conductibility. 



Calcareous sandstone _ 1-00 



Clay slate 1-24 



Slate, across cleavage 1 -60 



Trap 1-97 



Micaceous flagstone, across cleavage 2*09 



Limestones (mean) _ 2*44 



Granites (mean) 2*5 1 



Sandstones and hard grit (dry) _ 2*58 



Sandstones and hard grit (wet) 2'80 



Slate, along cleavage _ _ _ 2*84 



Micaceous flagstone, along cleavage 3 '00 



Sandstone of Craigleith quarry _ 5*06 



The table illustrates the well known fact that, in a rock-rnass 

 possessing the plane-parallel structure, the rate of conduction 

 is widely different according as the heat passes along or across 

 the planes of schistosity or cleavage. Such differential conduc- 

 tion and consequent differential expansion may be held respon- 

 sible for the opening of fissures for the entrance of apophysal 

 igneous matter in spite of the general tendency of the expansion 

 to close preexisting cavities in the country-rock. The apoph- 

 yses themselves by virtue of their own high temperature 

 must hasten the destructive action. 



Part of the stress-energy set free might be added to that of 

 injection and expended in the minute crumpling of relatively 

 plastic bedded country-rock. Another portion is conceivably 

 expended in irregular and perhaps very complete shattering of 

 the rock, which by that action is relieved from the strains by 

 sudden rending and fracturing rather than by any form of 

 rock-flowage. Still a third portion of the energy might become 

 potentialized as in Kupert's drops, Bologna glasses or certain 

 slickensided rock surfaces,* and only finally expressed as a 

 shatter-force after sudden faulting or other shock in the coun- 

 try-rock had precipitated the destruction, repeating on a large 

 scale, the destruction of a Rupert's drop. 



Exfoliation at plutonic molar contacts. — The complexity of 

 these mighty interacting forces is such that the shattering pro- 

 duced by them cannot be referred to simple strain-categories. 

 Experiments and certain observations made in rock-quarries 

 seem to throw light on one of the more important and simpler 

 methods by which disruption of the country-rock may take 

 j)lace. A short statement of the facts derived from each kind 

 of study will serve to place the question of the efficiency of 

 this process in a clearer light. 



Keade has given so concise an account of his experiments on 

 the differential expansion of stone that his descriptions of cer- 



* A. A. Julien, Jour. Franklin Inst., cxlvii, 1899, p. 382. 



