GOLD-BEARING VEINS OP BOG BAY 497 



The latter show well defined walls with brecciated extraneous matter frequently 

 inclosed in the quartz or sparry matrix, while the adjacent rocks are but slightly, 

 if at all, laminated or metamorphosed. The newer or open-fissure veins of the 

 later formations are sometimes continued with the same characters into the under- 

 lying Archean rocks. On the other hand the veins in the Archean rocks of more 

 ancient origin rarely show two well defined walls, they seldom contain extraneous 

 brecciated matter, and the adjacent country rock is generally laminated and highly 

 metamorphosed. The laminated portions are, in naany instances, an important 

 factor in the gold veins of the latter class, as shown by those of Bog bay. 



The author believes that the auriferous veins of this western Ontario district 

 were all formed in Archean time, because after many years of exploration he 

 knows of no place around lake Superior where such veins penetrate the later for- 

 mations. Such veins occur in the Archean areas on both sides of this lake, as at 

 Jackfish bay, at Schreiber and around Shebandowan lake on the ncfi'th side, and 

 again at the Ropes gold mine near Ishpeming on the south side, but nowhere in 

 the later rocks which lie between the last mentioned locality and the others. 



The metalliferous veins formed during the Archean period have, therefore, a 

 different set of characters from those which originated in subsequent times, when 

 the crust of the earth had become thicker and colder. In the early age of the 

 globe to which the Archean rocks belong, the crust must have been thinner and 

 weaker, the heat greater, the gaseous elements more powerful, and the shrinkage 

 of the crust more rapid and intense than in later times. Therefore we might 

 expect to find the rock formations greatly fractured and the sides of the rents 

 ground and laminated to a greater extent than in veins of later date. Although 

 veins of the open spaced fissure kind do occur in some places in these ancient 

 rocks, still the majority of their veins are of the other variety, in which the space 

 for the reception of the gangue was created by solvents circulating through the 

 crushed material along the fractures. In the latter case, the size of the vein proper 

 is not a fair measure for comparing the strength, value, or continuity of the ore- 

 body with that of an ordinary fissure vein occurring in newer formations. The 

 lamination of the walls of the veins formed in Archean time may be taken as a 

 guide for distinguishing them from those of the other class. The author is of the 

 opinion that mining men and experts in general will be mistaken if they look for 

 the general characteristics of open-fissure veins in judging of those of the western 

 gold fields of Ontario. 



Gold-bearing veins of the class here described are not confined to the Bog Bay 

 district. Similar ones, accompanied by mikadoite, cut granite rocks in the vicinity 

 of Sawbill lake on the Seine river. It is supposed that the granite of this locality 

 was erupted about the same time as that of Bog bay and from an analogous magma. 

 Dikes of felsite and diorite, like those of the latter locality, intersect this granite. 

 The Hammond Reef mine of the Sawbill region is situated upon a belt from 100 to 

 500 feet in width of parallel quartz and mikadoite veins having characters similar 

 to those of Bog bay. In fact, these characteristics, to a more or less extent, pre- 

 vail throughout these extensive Ontario western gold fields. 



LXX— Bull. Geol. Soc. Am., Vol. 10, 1898 



