TRANSACTIONS OF SECTION C. 655 



13. The pre-Glacial Decay of Bocks in Eastern Canada. 

 By Robert Chalmers, F.G.S.A., of the Geological Survey of Canada, 



Although the question of the subaerial decay of rocks has been before geologists 

 for many years, it does not appear to have received much attention in glaciated 

 countries. One reason of this may be the prominence given to the action of 

 Pleistocene ice in the production of the superficial deposits, the origin of the 

 boulder clay, moraines, kames, &c., being apparently quite readily explained by 

 such action, while the sedentary beds beneath, due to rock decay, are often so 

 thin and fragmentary that they seem to have been overlooked. It is, nevertheless,, 

 becoming more and more evident in the detailed study of the superficial deposits 

 that the materials of rock decay, from which all others have been derived, form a 

 very important constituent of the series. 



In Eastern Canada a wide field for tbe study of the products of rock decay 

 exists, in which, so far, but few workers have been found. Sir J. W. Dawson 

 described beds of this kind occurring at Les Eboulements, Quebec, where Utica 

 slates have been changed to a great depth into a sort of clay.' Dr. T. Sterrj' 

 Hunt also observed instances of the similar decomposition of rocks in the vicinity 

 of Montreal, especially at Rigaud Mountain." The writer has been investigating 

 phenomena of this kind since 1884. and has noted beds of decayed rock beneath 

 the boulder clay in New Brunswick, Nova Scotia, Prince Edward Island, and in 

 South-Eastern Quebec, while in the JMagdalen Islands the whole of the superficial 

 deposits consist of rock debris, some portions of which are, however, more or less 

 stratified by marine and atmospheric action, no glaciation having taken place 

 there. 



In the present paper the question of rock decay during the geological ages 

 which preceded the Tertiary is not considered. 



Beds of decomposed rock of variable thickness and more or less modified occur 

 wherever the surface of the rocks has not been abraded by Pleistocene ice, though 

 the evidence of ice action may be present and boulder clay often found overlying 

 them. In South-Eastern Quebec the hilly, broken country along the northern 

 slopes of the Notre Dame range appears to have protected these in some measure 

 from glacial erosion, and hence they occur in thick sheets in certain places,, 

 especially in river valleys. The stratified and indigenous pre-Glacial beds met 

 with in the valley of the Chaudiere, for example, taken together, are not les& 

 than 45 feet thick. In New Brunswick, Nova Scotia, and Prince Edward Island 

 the glaciation has been comparatively light in many districts, and consequently 

 remnants of these materials are found there also, though iu a greatly denuded 

 state. 



A general section of these beds, as recognised in Eastern Canada, may be given, 

 showing briefly in descending order their character and sequence as noted in 

 different places beneath tbe boulder clay : — (1) Transported and stratified water- 

 worn gravel with beds of fine sand and clay. (2) Coarse, stratified gravels, usually 

 yellow and oxidised, the materials wholly local. (3) Sedentary rotted rock, 

 passing into solid rock beneath. 



Certain portions of the region, as, for example, the eastern extremity of the 

 Gaspe Peninsula, the Magdalen Islands, and some localities in Prince Edward 

 Island, exhibit no abrasion from Pleistocene ice, and the surface, therefore, presents 

 nearly the same appearance as it probably did in the later Tertiary period. -VJ 



The mineralogical character and consistency of the decayed rock materials are, 

 of course, different upon each geological formation, varying from coarse and 

 angular, upon the older crystallines, to clay, with scaly fragments in districts- 

 occupied with slates, and changing into sand and gravel where sandstones prevail. 



The products of rock decay as observed beneath the boulder clay are, therefore, 

 of two kinds, indigenous and modified, the latter thickest in the ancient river 



' Notes on the Post-Pliocene Geology of Canada, Canadian Naturalist, vol. vi. 

 1872. 



^ American Journal of Science, vol. xxvi. 1883, pp. 208, 209. 



