MINERALS AND GEOLOGY OF CANADA. 445 
consolidation of such deposits. Many of our Canadian limestones, it 
may be observed—as those, more especially, which occur at the base 
of the great Trenton group (see Parr V.)—are highly siliceous. 
The enormous pressure exerted upon low-lying sedimentary beds by 
those above them, must likewise have been sufficient in many instances 
to have effected consolidation. Loose materials, as graphite powder 
used in the manufacture of the so-called ‘black lead” pencils, are 
thus rendered solid by artificial pressure. Spongy platinum, again, 
by the same process, is converted into the solid metal. 
The heat transmitted in earlier periods from subterranean depths, 
or generated amongst low-lying sediments by natural causes, 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) Changes to which the Sedimentary Rocks, collectively, have 
been subjected.—These changes comprise, principally : (a) Elevation 
above the sea level, with alternations of upheaval and depression ; 
(6) Denudation; (c) Tilting up and Fracturing; and (e) Metamor- 
phism and Cleavage. It is, of course, to be understood that whilst 
certain strata may have experienced all of these effects, others, on the 
contrary, have been subjected to upheaval, or to upheaval accom- 
panied 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 sedi- 
ments, under water ; and from the marine remains which so many of 
them 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. 
The sinking of the sea would appear at first thought to be the more 
rational explanation of this phenomenon ; but if we look to existing 
Nature, we find no instance of the going down of the sea, whilst we 
have many well-proved examples of the actual rising and sinking of 
the land. In connexion with this inquiry, it must be borne in mind 
