1922] Whitman: Genesis of the Ores of the Cobalt District 273 



that at least a parallel drainage existed on the pre-Cobalt surface. 

 If this can be taken as evidence that the major line now represented 

 by the ruptured Cobalt Lake syncline existed during the carving of 

 the pre-Cobalt surface, then on this line we have evidence correspond- 

 ing to that along the supposed Lake Timiskaming fault to show that 

 such structural lines have persisted in activity through long periods 

 of time, the Cobalt Lake fault probably having been active from pre- 

 Cobalt to post-Keewenawan time. 



Injection of the diabase. — Perhaps it is permissible to discuss in 

 this place the injection of the sheet of Nipissing diabase, since I regard 

 it as an important diastrophic agency; and although interest attaches 

 to other aspects of its advent, its participation in the production of 

 significant structures is sufficient reason for its special mention. 



Attention was called in the first reference to the diabase to the 

 fact that the margins are finegrained, indicating that at first the losses 

 of heat to the adjacent rocks were more rapid than the accessions of 

 heat from fresh arrivals of magma, and that the interior of the sill 

 for 100 feet is of fairly even grain, and unsegregated. The mass must 

 have slowly wedged its way into place, bodily lifting the roof an aver- 

 age distance of 1000 feet. 



In the roof rocks of the sheet are certain faults and dykes of dia- 

 base, which seem to have had their origin in the uneven lifting of the 

 superjacent mass as the sheet slowly entered its berth. Also in the 

 configuration of the mass itself, there are abrupt slopes which indi- 

 cate that it was injected along a warped surface. It must be admitted 

 that the roof rock was lifted irregularly, and suffered jostling first 

 from one direction and then from another. The structural effects of 

 this process are nowhere determinable with exactness, because the 

 overlying sediments have been eroded away, and the heterogeneous 

 character of the basement complex has been responsible for such a 

 differential propagation of stresses that the deformations defy analysis. 



At the time of injection the Cobalt Series lay perfectly flat and 

 undisturbed. The entering magma probably wedged open its own 

 channel, since the form of it is quite arbitrary and undulating, in 

 some cases following, and in others ignoring contacts and bedding 

 planes. When the invasion was completed the sheet had a roughly 

 uniform thickness over the entire area, but topographic relief was most 

 likely reflected through isostatic adjustments as an influence tending 

 to produce unevenness in the sheet's thickness, which varies from 600 

 to 1100 feet, and more. 



