R. A. Daly — Mechanics of Igneous Intrusion. 25 



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 /c 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, 82 

 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 result- 

 ing temperature gradient in the shell adjoining the molten 

 magma must be steep for many years after the original estab- 

 lishment of the contact."* 



Further, Less 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-conduction is variable and expansional stresses must 

 ensue. 



A rough calculation of the enormous stresses involved in all 

 these processes of differential heating ' was published in the 

 second paper of this series, where also an account is given of 

 the practical use which has been made of such stresses in 

 primitive quarrying. J Every great city conflagration leaves 

 manifold evidences of the shattering effects of the one-sided 

 heating of a rock-mass — in columns, sills, and cornices of 

 granite or sandstone. 



There seems, therefore, to be a sheer necessity for believing 

 in contact-shattering through differential heating and expan- 

 sion in the thin shell of a country-rock which encloses a large 

 body of molten magma. The evidence for the shattering is 

 often exceedingly full and clear in the field. The broad or 

 narrow belts of xenoliths so often found just inside the main 

 contacts of batholiths are very hard to explain if those batho- 

 liths are due to laccolithic injection. The blocks are charac- 

 teristically angular; they are generally not arranged with their 

 longer axes parallel, as if they had been pulled off from the 



* 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 relatively 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. 



fPhil. Trans., vol. clxxxiii A, p. 481, 1892. 



\ This Journal, xvi, p. 112, 1903; cf. Ann. Eep. State Geologist of New 

 Jersev, 1906, p. 17. 



