1922] Whitman: Genesis of the Ores of the Cobalt District 255 



conglomerates and finer sediments. These formations were, in turn, 

 folded into nearly vertical attitudes, and were intruded by dikes of 

 lamprophyre and batliolithic masses of pink Lorrain granite. 



The complex was then eroded until the Lorrain granite was ex- 

 posed and the Cobalt Series of conglomerates and greywackes was 

 deposited in horizontal or nearly horizontal beds upon a land surface 

 which, according to Miller, was hilly and rough. 



Miller refers to the Nipissing diabase as a sill between 600 and 

 1100 feet thick, probably of Keweenawan age, which was injected in 

 an approximately horizontal position concordant with the bedding 

 or with the contact between the Cobalt Series and the Keewatin for- 

 mation, the amount of overburden not being estimated. The diabase 

 is supposed to have come from a local vent, though the possibility 

 is admitted that there might have been more than one feeding 

 channel. 



After intrusion this formation was in turn intruded by small dikes 

 of granophyre and basalt, presumably representing segregation pro- 

 ducts from the plutonic source of the diabase. 



At the beginning of the Silurian period the overburden and much 

 of the diabase itself had been eroded away, and the region was sub- 

 merged beneath the sea, where it remained until the Devonian, receiv- 

 ing a thick mantle of richly fossiliferous limestone. By the close 

 of the Pleistocene the limestone had been largely removed, and the 

 underlying formations exposed over the greater part of the region. 



Structures. — Two systems of deep-seated structures were recog- 

 nized as being in some way connected with the silver-cobalt deposits. 

 They find topographic expression as long lines of depressions, occupied 

 for the most part by streams and lakes. They trend respectively 

 NW-SE and NE-SW; and their age was held by Miller to be post- 

 diabase. In regard to them Miller wrote : 



From the geological maps and the plan showing the distribution of the veins 

 at Cobalt, which accompany this report, it will be seen that while belts of frag- 

 mentary rocks strike approximately northeast and southwest, as for example, the 

 belt along the railway at Cobalt, and the Glenn and Kerr Lake belt, the majority 

 of veins have a strike different from this. It would also appear that the strike 

 of the veins in this area has little connection with the disturbance which caused 

 the great majority of the great rivers and chains of lakes in the district to follow 

 one or the other of the well defined directions. 



The water courses and lake axes which lie in a northwest and southeast line 

 are not so prominent on the maps as are the northwest-southeast ones just de- 

 scribed. Still they form a not indistinct' system, and as is indicated by Fig. 51, 

 they seem to have an important, but as yet little understood relationship to the 



