1922] Whitman: Genesis of the Ores of the Cobalt District 261 



Lorrain granite was found imbedded in the upper portion of the sill 

 many hundred feet away from any known parent mass. 



In the South Lorrain district I found that the Nipissing diabase 

 exposure is in no sense sill-like in form, but is a hollow arching shell 

 plunging into the Keewatin in all directions around the enclosed area, 

 having apparently once met overhead before erosion carried the higher 

 portion away. In the Gowgauda area also, it appears that the dia- 

 base is more commonly dike-like than sill-like in form. This points 

 to the origin of these masses as many independent offshoots from a 

 deep-seated mother magma, attempting to rise through devious chan- 

 nels through the Keewatin, and stopped or deflected beneath a great 

 thickness of flat-lying sediments, perhaps themselves overlain by a 

 lava flood-sheet of the same magma. However hypothetical the 

 assumption of such an overburden may be, the evidence nevertheless 

 points to the existence of a heavy flat barrier beneath which the dia- 

 base was forced to spread out, and assume irregular forms. 



The diabase is characteristically fairly coarse-grained, that is to 

 say, it exhibits phenocrysts of plagioclase averaging from two to three- 

 sixteenths of an inch in length. Its margins are characteristically fine- 

 grained, usually aphanitic, visible phenocrysts not being discernible 

 within five to ten feet of the contacts. In the interior of the sheet, 

 however, the evenness of the texture disappears, and patches of 

 coarse hornblende porphyry and pegmatitic material become more or 

 less abundant. There are a few anomalous occurrences of diabasic 

 impregnation of the adjacent Keewatin rocks in the form of small 

 reticulate and ill-defined veinlets of diabase penetrating into the walls 

 of the sheet, these veinlets or dikelets varying in width from one-half 

 to less than one-sixteenth of an inch. One of these cases occurs in the 

 south workings of the Kerr Lake Mine on the under side of the sheet ; 

 and a careful study indicated that the space occupied by the dikelets 

 was not produced by distension but by assimilation. Examples of this 

 phenomenon, however, are rare. 



Sufficiently close study was not devoted to the post-diabase intru- 

 sives to enable me either to add to or detract from what has already 

 been written on the subject. However, as the matter bears upon a 

 study of the diabase, and indirectly upon the problem of ore genesis, 

 it received enough consideration to warrant the conclusion that these 

 differentiation products, which form dikes cutting the sheet, did not 

 come from the sill itself, since it has undergone only incipient segre- 

 gation ; but that they probably came from its mother-magma lying far 

 below the surface. 



