'90 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



solution, many are only divided into grains by a partial solution, 

 whilst fragmental rocks only lose their cementing materials. 



Gravity as a factor in weathering is not a ver}' great one. In cases 

 where the vai'ious beds of rock are of different textures and hardness, 

 and the harder bed may be overlying the softer, such as may be seen 

 in the case of the Niagara limestones overlying the softer shales of 

 the Clinton, the weathering of the soft underlying beds gradually 

 undermines the hard overlying bed, and in time the action of gravity 

 causes the projecting i)iece to break off and fall down to the bottom 

 of the cliff. In this way also gravity acts as a transporting agent. 



Vegetation. — It is not so easy to determine to what extent weather- 

 ing is due to vegetation. The plants have a direct tendency to aid, 

 -while indirectly plant life has a tendency to retard, erosion. Directly, 

 plants aid erosion by the penetration of the soil by their roots, thereby 

 allowing the admission of moisture to the underlying I'ocks, and also 

 by the penetration of the harder rocks and thereby in a mechanical 

 way owing to the enlargement of their roots, as well as the admission 

 of water causing fracture. On the other hand, a profuse vegetation 

 has a tendency generally to retard erosion. It is true, disintegration 

 by solution is, owing to the increased power of percolating water, in- 

 creased, but the soil is protected from the mechanical erosion of the 

 rain drops and rills ; transportation, owing to the grasping conserva- 

 tive action of the roots, is retarded and brought almost to a stand- 

 still ; the action of frost upon the underlying rocks is destroyed by 

 the soil reaching beneath the limit of frost action and the effects of 

 rain drops are spent upon the foliage. The power of the vegetation 

 in any district of retarding erosion lies in the fact that it interferes 

 with the transporting agent just at the place where that agent is 

 weakest. 



The power of erosion due to running water is called corrasion. 

 'Olear pure water corrades by solution, and mixed muddy water partly 

 by solution but chiefly by mechanical action of attrition. Wherever 

 the declivity is steep and the motion of the water rapid, the corrasion 

 by solution is reduced to a minimum. The increased velocity of tlie 

 .stream due to the increased steepness of the bottom increases the 

 transporting power of the water, and in that manner increases its 



