736 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



ments, the pre-emption temperature of the country-rock and the temperature o£ 

 the magma itself, are partly indeterminate. If the former be regulated by the 

 normal law of the vertical distribution of the isogeotherms, that temperature 

 will be about 200° C. at a depth of four miles below the earth's surface — 

 a rather liberally estimated average depth for the upper limit of a granitic 

 magma chamber. If we assume that the temperature of an intruding 

 magma is approximately that at which the rock resulting from its crystallization 

 becomes thinly molten under plutonic pressures (an assumption apparently 

 justifiable irom the known properties of lavas and notwithstanding the presence 

 of mineralizing agents), there should occur by conduction at the molar contact, 

 a rise of temperature in the invaded formation, of something like 1000° C. 

 That would mean a cubic expansion in the solid rock of between 2-5 per cent 

 and 3-0 per cent, corresponding to a linear expansion of about 0-9 per cent. 

 The force required to prevent that degree of expansion is equal to the amount 

 of pressure required to compress the rock by the same amount. The coefficient 

 of compressibility for ordinary crystalline and well-cemented sedimentary rock 

 is not far from that of glass, viz.: about 0- 0000025 per atmosphere of pressure. 

 The pressure of more than 10,000 atmospheres, or about 75 tons to the square 

 inch, would be required to prevent the expansion of rock raised 1000° C. in 

 temperature.* However great the expansion transverse to the plane of the molar 

 contact may be, a large proportion of the force of expansion must pass into the 

 form of compressive strain, developing lines of force in the plane of the contact. 

 The integrity of the rock must be destroyed, for its crushing strength would 

 hardly average as much as 20 tons to the square inch. The action would be 

 complicated and intensified by the variable values of heat-conduction in the 

 invaded formation which is always more or less heterogeneous. 



It has been objected that rocks are good conductors of heat and that, there- 

 fore, strong temperature differences with resulting rending strains are not to 

 be expected in the shell of country-rock immediately surrounding a batholithic 

 magma. The following table of coefficients of thermal conductivity seems, 

 however, to show, on the contrary, that rock-matter is far from being ranked 

 as a good conductor. The table has been made by compiling the values noted 

 in the Landolt-B'orn stein'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 



* Through a mistake in placing a decimal point the pressure was greatly over- 

 stated in the second paper on the ' Mechanics of Igneous Intrusion/ 



t The steepness of the possible temperature gradient in the wall rock i6 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°-950°C.) or is 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 sub- 

 mitted to the action of a powerful electric arc. At the end of a furnace 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. 



