116 THE ROYAL SOCIETY OF CANADA 



similar to those so common in the pre-Cambrian areas of northern 

 Canada. Granite and granodiorite-gneiss with a distinctly foliated 

 structure of Laurentian ? age intrusive in the greenstone. Diorite- 

 porphyries, quartz-porphyries, etc., intrusive into both the green- 

 stone and the gneiss. As some of these porphyries which are in- 

 trusive in the gneiss have not participated in the diastrophic processes 

 which have affected it, they are evidently considerably younger than 

 it, and in default of other evidence may be considered to be of Algoman 

 age. These are the dykes which would seem to be most directly asso- 

 ciated with the gold-bearing veins. 



The various districts and properties briefly described above in- 

 clude most of the better known occurrence of gold-bearing quartz 

 veins in the Pre-Cambrian Shield of Central Canada. In writing 

 about the properties I have quoted freely from the reports of the geolo- 

 gists who have visited and described them, but nevertheless, I have 

 personally visited most of them during the past four years, and to 

 facilitate their study have had prepared between three and four hun- 

 dred microscopic sections of the ores and of the rocks with which the 

 ores are associated. Many of these sections have been examined for me 

 by Professor T. L. Walker, and Messrs. G. S. Scott and E. Thomson. 



In a general consideration of the veins they may be divided into 

 different groups according to their mineral contents. I have already 

 considered them in two groups according to whether the chief asso- 

 ciated mineral is (1) Arsenopyrite, or 

 (2) Pyrite. 



The Pyrite group may be again divided according to whether the 

 veins are characterized by 



a. Tourmaline and Scheelite 



b. Molybdenite 



c. Zinc blende and Galena 

 d Chalcopyrite, 



etc. 



Or, disregarding their mineral contents, they may be divided into 

 two general classes according to the character of the fissures in which 

 they occur, namely, 



1st. Veins in shear oi fracture zones, formed by pressure. , 

 (a.) Where the rock had previously been rendered schistose 

 by dynamometamorphic processes. In this case the direction of 

 pressure which may have been from any quarter in any of the three 

 dimensions, undoubtedly controlled the movement of the rock. In 

 some cases the layers of the rock moved on each other without frac- 



