666 ALFRED W. G. WILSON 



is aware, no case has been reported in which the actual contact 

 between the two was located in or near the bottom of one of 

 the characteristic pre-sedimentary basins. In fact, it unfor- 

 tunately happens t"hat most of these contacts must be below the 

 present local water level. Hence at present we are only justified 

 in stating that the summits and sides of the crystalline ridges 

 beneath the sediments are as bare and as little decayed as are 

 the similar ridges away from the sediments. 



The condition of the surface beneath the sediments shows 

 that processes which could produce a denuded fresh rock sur- 

 face were in operation long before the Pleistocene. As to what 

 these processes may have been there is considerable room for 

 divided opinion. 



Glacial erosion would seem to be competent to remove soft 

 material from localities where disintegration had penetrated 

 deeper than elsewhere. In the process it would scour the less 

 disintegrated residuals and would thus produce the characteristic 

 topography of the upland everywhere. The depth to which it 

 would scour the bed rock itself is, of course, at present indeter- 

 minate away from the regions underlain by sediments, but it has 

 produced no noticeable discordance between the parts of the 

 buried crystalline ridges protected by the sediments and closely 

 adjacent parts not thus protected. Again, if the bare, fresh sur- 

 face has been produced by glacial scouring, it wovld seem that 

 we must infer a pre-Paleozoic glacial period during which the 

 sub-Paleozoic peneplain was scoured off. Of such a period, 

 beyond the existence of the scoured plain itself, there is, in the 

 regions under discussion, no known recorded evidence. 



In this connection it may be well to recall that Dr. Geikie has 

 described a somewhat similar surface occurring over a small area 

 of Laurentian gueiss in northwest Scotland (Ross and Suther- 

 land counties). He drew attention to the fact that this mam- 

 millated surface passed beneath the younger sediments, and 

 notes that it is strongly suggestive of presedimentary glacial 

 erosion. 



The deep soils of the earlier peneplain might have been 

 removed from the ridges, leaving them comparatively fresh, by 



