88 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



enormous floods or anything of greater force than tlie powers at work 

 during the present time. We will find in comparison with other 

 districts that many of them have accomplished a much greater 

 amount of work within a mucli shortei' pei-iod of time, and with 

 apparently more inadequate instruments. 



The great factor, or at least one of the great foctors, and probal)ly 

 the greatest, in the formation of the contour of the surface of the 

 earth is erosion, or denudation, or degrailation, or land sculpture. To 

 complete the operation under whatever denomination we may place^ 

 the w^ork, two agents are required — the disintegrating agent and the 

 transporting or removing agent — without the one the other would be 

 comparatively haimle.ss and have little or no effect. Without the 

 transporting agent the disintegrating of the material could only be 

 carried on to a small extent — the mass of disintegrated material would 

 form a protective barrier to the underlying rock and thus prevent 

 any further destruction, and the work of erosion would be retarded 

 until the transportation of the already destroyed material. The 

 removal of this is the work of the transporting agent. The protect" 

 ing barrier of destioyed or disintegrated matter being removed, an- 

 other surface is laid bai-e to the attack of the disintegrating agent. 

 That again is removed, and another destroyed and removed, until the 

 whole of the material has been worn away. 



All this must be done by the assistance of the transporting agent,, 

 as the disintegrating or destroying agent has no power of removal, or 

 at best an exceedingly limited power due to the action of gravity ; its. 

 action is wholly confined to destruction. 



The transj)orting agent has only a veiy limited power of destruction 

 due altogetlier to mechanical cau.ses chiefly cor rasion. Transportation 

 is chiefly carried on by running watei-. A small proportion is also 

 performed by the wind, and a still smaller by ice. 



The chief factors in erosion are rain, snow, ice, rivers, waves, and 

 tides, and ocean currents. The air in the shape of wind or as a 

 medium through which chemical action attacks the rock, is also to a 

 slight degree a factor in the problem of erosion. 



It is, however, with only two of these agents that we have to deal 

 with regard to the formation of the central basin. The atmospheric 

 effects doubtless had something to do with it, but to what extent we- 



