594 TRANSACTIONS OP SECTION 0. 



of Rainy Lake was not made by a single sudden up-welling of granite, but by 

 a long succession of slow inflows from various quarters. Meantime the rocks 

 above must have been stretched and fractured during the long ages of elevation, 

 and must have been exposed to the usual destructive forces, which may even 

 have kept pace with the elevation during its late stages when differences of level 

 became pronounced. 



The coarse-textured granitoid gneiss making up the batholith must have cooled 

 at great depths and exceedingly slowly. 



The Praising of the Domes. 



Some curious dynamical problems are involved in the raising of the domed 

 mountains. It is conceivable that fluid lava could be forced by the unequal 

 pressure of shifting mountain blocks through a suitable system of pipes into 

 cisterns, so as to form laccolithic domes, but no such mechanism seems possible 

 with batholiths. The granite of the batholiths was plastic rather than fluid, as 

 shown by its having been dragged into the gneissoid structure. The areas 

 affected covered sometimes 1,000 square miles. We know of no system of dykes 

 to serve as pipes or passages, of no solid floor beneath, of no faulted blocks to 

 provide the pressure. It is generally assumed that the protaxial granites and 

 gneisses in great mountain ranges have risen because of the relief from pressure 

 beneath anticlines due to lateral thrust. It is doubtful if these irregularly 

 scattered ovals, sometimes 30 miles across, can be adjusted to any system of 

 anticlines. 



Some years ago I ventured another explanation. Granite is specifically 

 lighter than most of the greenstones and schists of the. Keewatin; and molten 

 granite, even if not at a very high temperature, is lighter than the relatively 

 cold rocks above it. If the rocks above were unequally thick, so that some 

 areas were less burdened than others, it is conceivable that these differences in 

 gravity might cause the granite to creep slowly up beneath the parts with the 

 lightest loads, while the overlying rocks sagged into synclines in the heavily 

 loaded parts. 1 



Whatever their cause, these oval batholiths enclosed by meshes of schist are 

 the most constant feature of the Canadian Archaean, though in many places 

 erosion has cut so deeply that the meshes have all but disappeared, leaving only 

 straight or curving bands of hornblende schist enclosed in the Laurentian gneiss. 

 Very similar batholithic relations of the Laurentian with the Grenville series of 

 Eastern Ontario are described by Drs. Adams and Barlow, though the batholiths 

 are generally much smaller. Batholithic mountains were typical of the Archaean 

 in North America, and, at least in some cases, also of Archaean regions in other 

 parts of the world. 



Subdivisions of the Canadian Pre-Cambrian. 



Until recently the rocks of the Canadian Shield were usually divided into 

 three parts — the Laurentian, the Huronian, and the Animikie and Keweenawan 

 — the last two being only doubtfully included in the pre-Cambrian. These three 

 divisions are still the only ones shown on the latest general map prepared by 

 the Geological Survey. Lawson's separation of the Keewatin as a lower group 

 than the Huronian was generally recognised as valid, but in practice the sub- 

 division of the two in mapping was difficult, and was only carried out in detailed 

 surveys. His proof that the Laurentian was eruptive and later than the Keewatin 

 was accepted. 



As the classification adopted by the American geologists in the Lake Superior 

 region differed from that used in Canada, a Correlation Committee was appointed 

 five or six years ago to draft a compromise, which runs as follows : — ■ 



1 Bull, Geol. Soc. Am., vol. ix. pp. 223-238. 



