158 MINERALS AND GEOLOGY 



river- waters) also contain considerable quantities of silica in solution ;. 

 and there is reason to believe that in former periods of the Earth's 

 history, springs of this kind must have prevailed to a very great ex- 

 tent. These, flowing into seas and lakes where sediments were under 

 process of deposition, must also have lent their agency towards the 

 consolidation of such deposits. Many of our Canadian limestones, it 

 may be observed, are more or less siliceous. 



The enormous pressure exerted upon low-lying sedimentary beds 

 by masses of superincumbent strata, must likewise have been sufficient 

 in many instances to have effected consolidation. 



The heat transmitted in earlier periods from subterranean depths, 

 or generated amongst low-lying sediments by chemical action, may 

 also have been concerned in the work of consolidating the originally 

 loose materials of stratified rocks. It may be remarked, likewise, 

 that sediments occasionally become solidified by simple desication. 

 The shell-marl, or calcareous tufa, of our swamps, &c., becomes thus 

 hardened on exposure to the air. 



(3) SUBSEQUENT ACTION OF NATURAL FORCES ON SEDIMENTARY 



ROCKS. 



The more important effects produced on sedimentary rocks, from 

 their first period of aggregation, are as follows : (a) Elevation above 

 the water-level, with Alternations of Upheaval and Depression ; 

 (6) Denudation ; (c) Tilting up and Fracturing ; (d) Metamorphism. 

 It is of course to be understood, that whilst certain strata may have 

 experienced all of these effects, others may have been subjected to 

 upheaval, or to upheaval accompanied by denudation, only. 



(a) Elevation above the Sea Level : The stratified rocks, it has 

 been shown, must have been deposited originally, in the form of 

 sediments, under water ; and from the marine remains which so many 

 of these rocks contain, it is evident, that, as a general rule, they 

 were laid down on the bed of the sea, either in deep or in shallow 

 water. We find these rocks, however, now, at various heights above 

 the sea-level, and frequently far inland. Hence of two things, one : 

 either the sea must have gone down, or the land must have been 

 elevated above the water. 



