SMITHSONIAN EXPLORATIONS, I93O 47 



occurs as a fine network of delicate veins or small irregular masses in 

 both calcite and cobaltite. 



As simple a statement of the geology as can be given is that when 

 this basic rock, called Nipissing diabase, was intruded into the older 

 rocks, fractures were developed along the contacts and in these frac- 

 tures the ore was undoubtedly deposited by solution originating in the 

 diabase. 



A large irregular mass of silver found at the Keeley Mines, Silver 

 Centre, Ontario, was exactly the type of material needed for our exhi- 

 bition, provided some of the ragged appendages which marred the 

 symmetry of the specimen could be removed. This operation developed 

 into a thrilling experience. Hammering, even with a sledge, made no 

 impression except a small bruise on account of the toughness of the 

 silver. Finally, in despair, the mine captain suggested a shot of dyna- 

 mite. Two sticks were plastered on the side of the specimen under a 

 damp blanket of sand, the fuse was lighted, and we retreated to safe 

 quarters to await the report. And what a report it was ! Strangely 

 enough very little silver was removed. One small fragment made a 

 non-stop flight through the side of the mill house and several windows 

 in nearby buildings were shattered, hut most important, from my point 

 of view at least, was the small crack which had just started through 

 the specimen. Smaller charges of dynamite were carefully placed and 

 exploded and little by little the irregular silver prongs were removed. 

 Perhaps few geologists have had the experience of trimming such a 

 precious sample by so unique and striking a method. 



The next district visited was Sudbury, the most important nickel 

 district in the world. Much of the region surrounding the city of Sud- 

 bury is practically barren of vegetation and perhaps to the tourist 

 appears as a desolate waste, but to the geologist it is an admirable ex- 

 posure of a series of interesting rocks. The igneous rock in which the 

 nickel ore is found is a norite, this intruded rock forming a laccolithic 

 sheet some 36 miles long and approximately 17 miles wide. This 

 nickel eruptive was intruded under a thick blanket of older sediments 

 and the slow cooling permitted the ore minerals to separate out 

 towards the bottom of the laccolith. The ore is monotonously uni- 

 form in character over much of the district. It consists of pyrrhotite, 

 pentlandite, and chalcopyrite, and appreciable quantities of platinum 

 metals are also recovered from the ore body. 



A rather rapid series of visits was made to a number of the different 

 pegmatite dykes in this province, resulting in the acquisition of sev- 

 eral rather recently described minerals which were lacking in the 



