R. A. Daly — Mechanics of Igneous Intrusion. 113 



the known properties of lavas and notwithstanding the presence 

 of mineralizin^^ agents), there should occur by conduction at 

 the molar contact, a rise of temperature in the invaded forma- 

 tion, of something like 1000° C. That would mean a cubic 

 expansion in the solid rock of between 2*6 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. 

 Assuming that the thermal expansion should so occur that the 

 volumetric change would be exactly in the reverse sense of the 

 volumetric change observed during compression tests for solids, 

 and assuming that the just mentioned coefficient of compressi- 

 bility should apply at very high pressures, it follows that the 

 inconceivable pressure of more than one million atmospheres, 

 or about 8000 tons to the square inch, would be required to 

 prevent the expansion of rock raised 1000° C. in temperature. 

 How^ever 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, devel- 

 oping lines of force in the plane of the contact. If only one 

 per cent of the total force of expansion were applied in that 

 plane, the integrity of the rock must be destroyed, for the 

 crushing strength of rock is in no case as much as fifteen tons 

 to the square inch. 



It is extremely difficult, if not impossible, to imagine the 

 enormous and complicated stresses set up in this way. 

 Although the heat of the intrusion would be conducted out- 

 wards in all directions from the igneous body, the supposition 

 is reasonable that the temperature reigning only a few hundreds 

 of meters from the intrusive would be very much lower than 

 that at the contact. Differential expansion and consequent 

 intense shearing stresses far above the breaking strain of rock- 

 material might thus be produced. That action would be com- 

 plicated and intensified by the variable values of heat-conduc- 

 tion in the invaded formation which is always more or less 

 heterogeneous. The following table of relative values calcu- 

 lated from Everett's "Units and Physical Constants" (1879, 

 pp. 101 and 103) shows the importance of differential conduc- 

 tion in different rocks. For ease of comparison all the other 

 values are referred to the conductibility (taken as unity) of 

 calcareous sandstone. 



*This Journal, April, 1903, p. 274. 



