E. Chalmers — Pre- Glacial Decay of Rocks. 281 



ducing expansion and contraction of the rocks, thus causing 

 numerous superficial cracks, crevices and joints which open 

 them to the disintegrating agents referred to. Exfoliation, 

 which is often seen in granite districts, and sometimes on 

 diorite bosses, is chiefly due to this expansion and contraction. 

 Although at first glance decomposition would seem not to have 

 been as deep in this country as to the south of the glaciated 

 zone, yet such an inference may not be correct, owing princi- 

 pally to the fact that denudation has been so much greater ; 

 and the existing remnants of these pre-glacial beds do not by 

 any means show the amount of rock-decay which took place. 

 The removal of so much of the decayed materials by atmos- 

 pheric and glacial action must have exposed some portions of 

 the rock surfaces at least to a renewed attack* of the disinte- 

 grating processes time after time — a condition of things 

 which would not occur so generally in regions upon which ice 

 never gathered. It is true that the mantle of snow which 

 envelopes the earth's surface here for five or six months every 

 year and the freezing of the superficial beds during the same 

 period have a conservative effect, and tend to check and prac- 

 tically arrest decomposition for a certain time ; but rock-decay, 

 nevertheless, may have been more effective and rapid at other 

 seasons than in those parts of the earth where the decayed 

 materials lie and accumulate as residuary beds to a great thick- 

 ness. Thus while the process referred to does not exhibit the 

 accumulated beds here that are found in southern latitudes, it 

 does not necessarily follow that the depth of such action may 

 not have been as great. The fact just stated has to be borne 

 in mind, namely, that the decayed materials have undergone 

 great denudation, thus giving the disintegrating forces renewed 

 power at intervals to act directly upon the rocks. 



The principal oxidation and decay of the rocks would seem 

 to be from chemical agencies, whether on the surface as the 

 rotten rock was denuded, or deep-seated beneath a load of 

 sedentary material, or along joints, cracks, etc. These pro- 

 cesses have been in incessant operation since the region became 

 dry land. At present the great mantle of bowlder-clay acts as 

 a preserver of the rocks from the decaying processes in many 

 districts, though even under this impervious capping the agents 

 of decomposition, doubtless, to some extent, carry on their 

 work. 



It thus appears that if the climate and the seasons of Eastern 

 Canada in past ages were at all similar to those prevailing at 

 the present day, the conditions of rock-decay would necessarily 

 be somewhat unlike those of tropical and non-glaciated regions, 

 though probably the results may not have been very far differ- 

 ent. The general character of the beds might possibly be nearly 



