REPORT OF THE CHIEF ASTRONOMER 727 



SESSIONAL PAPER No. 25a 



with illustrations of this fundamental fact. Barrois' maps of Britanny are 

 eloquent for all the eases. Numerous examples may be found in the Boundary 

 map sheets. (See also pages 292, 299, 302, 426-30, 465, and 495-99.) 



It is a truism that batholiths generally have wider aureoles of contact 

 metamorphism than laccoliths, or than other injected bodies. In apparently 

 all cases, as we have noted, the intensity of the metamorphism is greatest at the 

 roof, less at the wall, of both stocks and batholiths. The explanation is almost 

 surely found in the tendency of the volatile constituents of the magma to collect 

 at the roof. Since stocks are generally cupola-like masses at batholithic roofs 

 we can readily understand the fact that the aureoles of heavy metamorphism 

 about stocks may be wider than the wall-contact aureoles of even very large 

 batholiths. But it remains true that the degree of contact metamorphism 

 exhibited in batholithic aureoles is often much less than that often shown at 

 dikes and sheets, if account be taken of the volumes of magma involved. 



This fact becomes understood by assuming that these injected bodies were, 

 at the time of intrusion, much hotter than the average batholithic magma was 

 when its visible molar contact was established. The mere fact that dike and 

 sheet magma could penetrate miles along relatively narrow fissures in the earth's 

 crust speaks for some amount of superheat. The presence of quartz instead 

 of tridymite (inversion point about 800° C.) in the vast majority of batholiths 

 proves a very low temperature for their magmas as these crystallized at the 

 visible contacts. The low temperature at that stage is likewise proved by the 

 evidence of enormous viscosity in the magma during the crystallizing period. 

 Such viscosity must be assumed because of the suspension of foreign blocks 

 of rock which is much denser than the crystallized intrusive and, a fortiori, 

 than the magma that crystallized. The facts of the field thus lead the observer 

 directly to question the statement that the existing batholithic contacts were 

 established at the time of initial intrusion. If the batholiths are due simply to 

 injection, like dikes, sheets, and laccoliths, why should they be so greatly super- 

 cooled, while the much smaller bodies are as often superheated? The whole 

 matter becomes clear if it be assumed that the initial temperature of a batho- 

 lith was as high as that of its hottest satellite, and that during the long period 

 of cooling the magma of the main chamber incorporated masses of the country- 

 rock. In this way new contacts were established in succession, and the last one, 

 with a relatively narrow contact aureole, was established in the feeble, supercooled 

 condition of the magma just before solidification. This theoretical deduction 

 is so patent that it is ranked alongside of the ' facts ' of field relations. 



Among the commonest phenomena associated with the contact zones of 

 plutonic, igneous rock bodies( bosses, stocks and batholiths) is that of extensive 

 shattering and disruption of the invaded formations along the contacts. A 

 host of memoirs on exotic granite, syenite, diorite, gabbro, and other deep-seated 

 rock masses contain references to this particular phenomenon. It consists, in 

 its ideal development, of the appearance of two concentric belts of mixed rock 

 occurring between the homogeneous main body of igneous material and the 

 encircling country-rock unaffected by any serious mechanical disturbance due 



