22 B. A. Daly — Mechanics of Igneous Intrusion. 



main contacts ; the solution of the engulfed blocks is effected 

 far down in the depths of the magma — by abyssal assimila- 

 tion. The resulting syntectic magma may thus be in strong 

 chemical contrast with the adjacent wall-rock at any one level. 

 Marginal assimilation is not excluded but is considered as an 

 accessory and subordinate phase in the act of replacement. 



Contact-shattering. — It has been objected that rocks are 

 good conductors of heat and that, therefore, strong temperature 

 differences with resulting rending strains are not to be expected 

 in the shell of country-rock immediately surrounding a batho- 

 lithic magma. This objection has been recently made by an 

 expert physicist now specially engaged on petrological problems, 

 and evidently needs consideration.* The following table of 

 coefficients of absolute conductivity seems, however, to show, 

 on the contrary, that rock-matter is far from being ranked as 

 a good conductor. The table has peen made by compiling the 

 values noted in the Landolt- Bornstein's Physikalisch-chemische 

 Tabellen (1905 edition) and in Winkelmann's Handbuch der 

 Physik. The values for the rocks are of the order expected 

 in view of the familiar proofs of the extremely slow cooling 

 of lava-flows.f 



k 



Silver, about 1-0000 



Copper, " -9480 



Lead _ _ _ -0836 



Quartz -0158 



Marble '008 1 7 



Granite _._:__. '00757 -'00975 



Gneiss _ '000578--00817 



Sandstone '00304 --00814 



Basalt -00673 



Syenite _ -00442 



Glass... -00108 --00227 



Water, about .' -00130 



Paper -00031 



Flannel '00023 



Silk -00022 



Cork -00013 



Feathers -00005 74 



* Cf. A. L. Day, Science, xxv, p. 620, 1907. 



f The steepness of the possible temperature gradient in the wall-rock is 

 shown by the fact that, a few days after lava ceases flowing, one can walk 

 on its crust, although the lava just below is at red heat (700 c -950° C.) oris 

 yet hotter. For many hours or for several days the gradient at the surface 

 may equal or surpass 500° C. per foot. 



In the manufacture of calcium-carbide a mixture of limestone and coke 

 is submitted to the action of a powerful electric arc. At the end of a fur- 

 nace-run (about fourteen hours in the plant at Ottawa, Canada) the flow of 

 heat is nearly steady and the temperature gradient in the furnace is about 

 3000° C. per foot. In this ease the diffusivity of the limestone- coke mixture 

 in the interior of the thoroughly heated furnace must be well below 60 in the 

 Kelvin system of units. 



