172 Transactions of the Canadian Institute. [Vol. VII. 



and are narrow and steep-walled towards the northeast, indicates that 

 the streams which carved them flowed towards the southwest. These 

 streams may have been initially consequent on a plain inclined towards 

 the southwest, but whose inclination has since been altered by secular 

 uplift or depression, so that the present St. Lawrence flows over the 

 lowest portion of the sag. The direction of the streams has undoubtedly 

 also been controlled by the direction of the master joints of the lime- 

 stone, and the valleys may have been developed by headward growth of 

 streams guided by these joints. To the writer this latter alternative 

 seems the more probable, though additional field work is necessary 

 before a definite opinion can be expressed. The outlet to the present 

 St. Lawrence seems to be a complex of several of these valleys in which 

 the water is now flowing in a reversed direction owing to secular changes 

 in elevation. 



Joijited and Fissured Uplands. — Another feature of the rock surface 

 of the limestone uplands, found upon the intervalley ridges, along the 

 Black River escarpment, and upon the many outliers in front of the 

 escarpment, is the joint structure, which has split the surface layers into 

 rhomboidal blocks of various sizes. Subsequent weathering of the 

 upper blocks especially, has widened the fractures and rounded the 

 edges of the blocks more or less. In some cases we find till and pieces 

 of gneiss in these widened fractures, and in others the glacial striae bend 

 obliquely downwards in crossing the curved surfaces near the open 

 fissures. Again, over wide areas of almost bare rock, the joints occur, 

 but the blocks are close together and there is no weathering or rounding 

 of the edges, and the striae cross the joints without deflection. These 

 features occur sometimes within short distances of each other on lime- 

 stones that are identical in texture, and so far as known, identical in 

 composition. They are found both at the edges of the upland and some 

 distance back from them ; unfissured areas are sometimes found close 

 to the edge of the escarpments. 



The jointing which produced the rhomboidal blocks preceded the 

 earliest ice advance. The relation of the ice-scoured surface to the 

 open fissures shows the existence of these fissures before the advent of 

 the ice which planed that surface. The low temperature of the sub- 

 glacial water, and the absence of organic matter in solution, except the 

 small amount derived from the preglacial soils, render it improbable 

 (but not impossible) that the subglacial waters could have materially 

 widened them. During interglacial times, at least portions of the area 

 were below the level of standing water, and were possibly covered with 

 ice, so that it seems very probable that much of the weathering pro- 



