24 



THE NICKEL ERUPTIVE. 



The most interesting and important feature of the region 

 is the laccolithic sheet forming the synclinal basin and pro- 

 viding the great deposits of nickel and copper ore which 

 have made the district famous. The sheet is 36 miles long 

 and 17 miles broad with a thickness varying from half a 

 mile to two miles and averaging a mile and a quarter. It 

 is estimated to contain 500 cubic miles of rock, and it was 

 once far larger, since it has lost much of its original dimen- 

 sions by erosion. 



The sheet consists of norite on its lower side, passing 

 gradually into micropegmatite on the upper side. Blebs of 

 ore are often scattered thickly through the lower part of 

 the norite, and where there is a depression in the floor 

 beneath this, pyrrhotite-norite merges without any break 

 into ore bodies sometimes containing millions of tons of 

 pyrrhotite and other sulphides. Unquestionably all three 

 substances, ore, norite and micropegmatite, belonged origin- 

 ally to the great flood of molten rock which rose from some 

 hearth beneath and spread out over the old eroded surface 

 of ancient rocks, including the Sudbury series and the Laur- 

 entian gneisses ; and under the flat-lying Animikie sediments 

 just described. As the magma welled up from beneath, the 

 floor of older rocks collapsed into large or small blocks 

 which settled down allowing the sheet itself with the over- 

 lying sediments to assume the synclinal form. 



Thus more than a mile's thickness of molten rock was 

 blanketed by 9,400 feet of sediments, so that the cooling 

 must have gone on extremely slowly, giving time for the 

 heavier materials to settle to the bottom, and also for the 

 upper, more acid part of the magma to metamorphose pro- 

 foundly the conglomerate immediatelv over it and to silicify 

 and harden the lower part of the Onaping tuff, as just men- 

 tioned. 



The coming up and spreading out of the norite-micropeg- 

 matite sheet profoundly shattered all the adjoining rocks^ 

 and almost everywhere beneath the sheet there is a sort 

 of breccia or conglomerate of fragments of the underlying 

 rock sometimes cemented by norite or ore. 



The freshest norite, which often occurs close to large 

 ore bodies and may enclose portions of the ore, consists 

 mainly of labradorite and hypersthene, with some ordinary 



