[keele] MESOZOIC clays AND SANDS 45 



in all probability represents the lower and less altered portion of a 

 mass of more or less completely kaolinized rock. It is this kaolinized 

 part of the rock that has furnished the clay and quartz sand for the 

 Cretaceous deposits farther north. On Mattagami river a wide band 

 of highly kaolinized garnet gneiss occupies a similar position south 

 of the Cretaceous deposits exposed on that river. ^ 



There may be other bands of kaolinized rock similar to the 

 above in this region which have escaped observation, for it must be 

 borne in mind that kaolinized rock surfaces are comparatively soft 

 and therefore wear down quickly while the surrounding harder rocks 

 stand up as ridges. 



The hard rock ridges are the first to be revealed when the river 

 cuts down through the universal covering of glacial drift, and as these 

 ridges control the down-cutting process, the more deeply buried, softer 

 rocks are seldom brought into view. 



It is not known whether the basin in which the clays settled 

 contained salt w^ater or fresh water. A lowering of the land by 

 300 feet would cause the sea from Hudson bay to cover the Palaeozoic 

 coastal plain and change the shore line to the northern margin of the 

 pre-Cambrian rocks, a distance of 70 miles south of the present limit 

 of tidal water. This limit corresponds approximately with the 

 encroachment of the sea in late Pleistocene times. 



Folding of the Palaeozoic rocks with uplift might block the north- 

 ward flowing drainage from the pre-Cambrian upland and cause 

 temporary fresh water lakes to exist in a depression between the 

 upland and the folded zones. The Silurian rocks on Moose river have 

 been gently folded, while the Devonian rocks which lie between them 

 and the pre-Cambrian escarpment are quite fîat and apparently 

 undisturbed, and it is on this area that the Mesozoic clays have been 

 found. 



It is simpler, however, to account for the presence of the sedi- 

 ments as the result of marine submergence in Mesozoic times. 



1 Ontario Bureau of Mines, 1920, Vol. 29, Part 2, p. 17. 



