[miller-knight] PRE-CAMBRIAN OF ONTARIO 247 



While it is not possible to determine the total amount of erosion 

 to which the pre-Cambrian surface has been subjected during various 

 epochs, since the Keewatin-Grenville rocks appeared above the surface 

 of the primeval ocean, it can be proved to be enormous. 



In pre-Timiskamian times the Keewatin-Grenville rocks were 

 eroded to a great depth, as is shown by the thickness of theTimiskamian 

 sediments, and the deep-seated Laurentian was exposed at the surface. 



Again, in the epoch that gave rise to the Animikean sediments, 

 erosion of all the older series was excessive and long continued. The 

 folded Timiskamian fragmentai rocks were cut down until they were 

 represented merely by comparatively narrow belts in certain localities, 

 while over large areas no remnant of them remained. During this 

 epoch the Keewatin, Grenville and Laurentian again were subjected 

 to great erosion. 



In post- Animikean and pre- Palaeozoic times there was a great ero- 

 sion epoch. This is shown, for instance, by the outliers of the Cobalt 

 series, and the intrusives that penetrated it. 



Since Palaeozoic times there has been again much erosion, and 

 Cambrian, Silurian and Devonian strata have been removed over 

 vast areas, exposing the underlying pre-Cambrian. 



That many ore deposits of various ages have been destroyed 

 during the several epochs of erosion is evident. Two or three hundred 

 feet more of erosion would have left comparatively little, for example, 

 of the Cobalt silver deposits, of which doubtless more has been eroded 

 than has been mined, or of the great Mesabi iron deposits of Minnesota. 



In considering the relation of ore deposits to erosion in the pre- 

 Cambrian of Ontario it appears that workable ore deposits are confined 

 comparatively near the surface of the earth. 



Complete erosion of certain series of rocks, it would seem, may 

 account for the absence of ore deposits in some countries where the 

 pre-Cambrian, although occurring in considerable volume, is barren. 

 For instance, in the northwest highlands of Scotland there is evidence 

 that fragmentai series, probably corresponding to the Timiskamian 

 and Animikean of Ontario, were completely eroded before the de- 

 position of the Torridonian which is considered to represent the 

 Ontario Keweenawan. If we except the Moine or Eastern schists, 

 the age relations of which are doubtful, the rocks now remaining 

 in the Scottish pre-Cambrian appear to represent the Keewatin and 

 Grenville, with the deeply eroded roots of intrusives, and the Keween- 

 awan. If all the sedimentary series that lie between the Keewatin- 

 Grenville and the Keweenawan had been removed by erosion in Ontario 

 there would be nothing left of the ore bodies that are now found 

 enclosed in Timiskamian and Animikean rocks or in the intrusives 



