P. Chalmers — Pre- Glacial Decay of Rocks. 275 



have not been removed from their original situation. These 

 have sometimes been called residuary or indigenous, or simply 

 rotten rock in situ. 



The modified pre-glacial beds consist really of the sedentary 

 material removed from their original position, and worn and 

 changed by the agency of rivers, streams, and, perhaps, to some 

 extent by that of the atmosphere before they were deposited 

 in stratified beds. They are best developed in protected val- 

 leys, although oftentimes greatly eroded and in places almost 

 wholly swept away by glacial denudation and river action. In 

 some valleys, owing to the heavy filling of bowlder-clay thrown 

 down upon them during the glacial period, or to the disloca- 

 tion of portions of the valleys from local differential move- 

 ments, they have been partially preserved and form thick strati- 

 fied deposits. 



Physiographic Features of the Region. 



Topographically the region embraced in these observations 

 exhibits highly diversified features and its geologieal forma- 

 tions include a great variety of rocks of different ages and 

 character. It has also been the theater of a remarkable system 

 of mountain-building action, and has undergone great and 

 repeated oscillations of level. The subserial erosion and base- 

 levelling to which it has, from time to time, been subjected, 

 have doubtless largely obliterated the original features, except 

 within a few limited areas, — the only plane surfaces of any 

 extent now existing, properly attributable to these causes, being 

 the St. Lawrence valley and the Carboniferous area of JS r ew 

 Brunswick. The northeast Appalachian Mountain system con- 

 stitutes the principal topographical feature, extending through- 

 out the entire length of the region and terminating at Gaspe. 

 This system has subsidiary and parallel ranges or spurs with 

 intervening valleys. As a rule these valleys are floored with 

 the latest rocks, and thick deposits of Pleistocene age usually 

 occupy them. Although the existing mountain ranges and 

 valleys were originally due to erogenic movements, the minor 

 topographical features are largely the result of the processes 

 of disintegration referred to slowly and unceasingly in progress. 

 The slates and sandstones seem to have been more deeply de- 

 composed than the crystalline rocks. In the valleys and espe- 

 cially beneath the bowlder-clay, the decomposed products under 

 consideration are found in the thickest beds, although usually 

 occurring also upon the slopes and ridges. Thus while glacial 

 erosion and other forms of denudation have been active and 

 long-continued, these sedentary and other pre-glacial stratified 

 beds have been preserved in sheets of greater or less extent 

 and thickness in every part of the country. 



