REPORT OF THE CHIEF ASTRONOMER 739 



SESSIONAL PAPER No. 25a 



follows that thermal diffusivity in rock decreases with rising temperature even 

 faster than the conductivity decreases. For rock heated to 1000° or 1200° C. k 

 may not be more than 100 in the Kelvin system of units. 



It seems safe to assume, first, that the diffusivity of the gradually heated 

 wall-rock may vary from 275 or less to 100 or 150; secondly, that the average 

 diffusivity of an 80-foot shell heated during the first year by adjacent molten 

 magma, will be no greater than 200. If k be regarded as averaging 200 for all 

 periods greater than one .year, the four columns showing values of u in the 

 table will serve if t is, respectively, 2, 8, 32, and 200 years. 



As a result of somewhat rigorous calculation, then, it appears certain that 

 the heating of wall rock by plutonic magma must progress with great slowness 

 and that the resulting temperature gradient in the shell adjoining the molten 

 magma must be steep for many years after the original establishment of the 

 contact.* 



Further Lees has proved that rocks have highly variable coefficients of 

 conductivity, some species possessing coefficients twice as high as those of other 

 species. f It is also well known that bedded or schistose rocks conduct heat 

 along and across their structure-planes at quite different rates. Where, there- 

 fore, the wall rocks about a batholithic mass are heterogeneous, the heat conduc- 

 tion is variable and expansional stresses must ensue. 



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 

 sadden rending and fracturing rather than by any form of rock flow. Still a 

 third portion of the energy might become potentialized as in Rupert'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 country-rock had 

 precipitated the destruction, repeating on a large scale the destruction of a 

 Rupert's drop. 



Experiments and certain observations made in rock-quarries throw 

 light on one of the more important and simpler methods by which disruption of 

 the country-rock may take place. A short statement of the facts derived from 

 each kind of study will be found in the writer's second paper on the ' Mechanics 

 of Igneous Intrusion' (Amer. Jour, of Science Vol. 16, 1903, p. 114.) 



Every city conflagration leaves manifold evidences of the shattering effects 

 of the one-sided heating up of a rock mass — in columns, sills, and cornices of 

 granite and sandstone. Telling illustrations have recently been published by 



* By using the same Fourier equation it is not difficult to show that the loss of 

 thermal energy which a magma suffers by conduction into the country-rock is rela- 

 tively small, even after the lapse of two or three hundred thousand years. The long 

 duration of the magmatic period in a slightly superheated plutonic mass of large size 

 becomes easily understood. 



t H, Lees^ Phil. Trans., Vol. 183 A, 1892, p. 481. 



X A. A. Julien, Jour. Franklin Inst., Vol. 147, 1899, p. 382. 



25a — vol. iii — 48 



