366 k. bell pre-paleozoic decay of crystalline rocks. 



Absence of Paleozoic Deposition and its Significance. 



The greater part of the Archean area referred to probably never re- 

 ceived any deposit of Paleozoic rocks upon it, so that although numerous 

 long and deep valleys, such as those which have been referred to, existed 

 since an earlier period, no evidence of such rocks may ever be found in 

 the larger number of them. 



The depressions already mentioned as filled with outliers and project- 

 ing tongues of Potsdam sandstone and Black river limestone along the 

 northern boundary of the Paleozoic formations between Georgian bay 

 and the foot of lake Ontario also go to prove, as we have seen, mat the 

 surface of the Archean rocks had been reduced to something like its 

 present level and aspect before these beds were laid upon them. 



Effects of glacial Denudation. 



In a former paper * the writer described the effects of glacial denuda- 

 tion in forming valleys and water channels along the courses of ancient 

 lines of crushing and subsequent decay in the crystalline rocks north of 

 lake Huron. A similar effect has been produced in this region, where 

 groups of parallel joints run close together, with a considerable breadth 

 of more solid rock on either side, and where the comparatively thin 

 walls between them have become softened by the long continued pene- 

 tration of surface water. It is probable that the deep decay along these 

 joints and lines of fracture took place in pre-Paleozoic times. The dis- 

 integrated rock along them Avas removed by glacial erosion with almost 

 equal facility, no matter what their course may have been, even where 

 the resulting channels are narrow and lie transversely to the direction of 

 the excavating force. A good example of this, on a small scale, is shown 

 in figure 2 of plate 16, which is taken from a photograph of a gap in 

 the granite on George island, at the northwestern extremity of Georgian 



bay. 



» 



*See Report of the Ontario Bureau of Mines, 1892. 



