MINERALS AND GEOLOGY OF CANADA. 451 
been levelled down by the agency of denudation. In mountainous 
districts, the fracturing of strata has sometimes given rise to narrow 
valleys or gorges, called “valleys of dislocation,’’most of which have 
been subsequently widened by the atmospheric disintegration of the 
surrounding rocks, and by the streams or torrents of which they 
usually form the channels. 
(e) Metamorphism and Cleavage.—The subject of metamorphism 
has already been sufficiently explained, under the head of Metamor- 
phic Rocks, above. It is merely alluded to here as one of the 
changes to which strata of various geological ages have been sub- 
jected. The term “cleavage” is applied to a peculiarity affecting 
many clay-slates, and occasionally other strata. The rocks thus 
affected, are rendered eminently fissile or slaty by numerous cleavage 
planes which run through them in a direction generally inclined to 
that of the lines of bedding. The latter, in inclined strata especially, 
are sometimes distinguished with difficulty from the planes of cleavage, 
but they may be discovered by tracing out lines of fossils, or imterca- 
lated bands of a slightly different mineral composition, colour, &c., 
which mark, of course, the planes of deposition, and across which the 
cleavage lines usually pass without interruption. That cleavage is a 
superinduced effect, is shewn by this latter circumstance, and more 
particularly by the fact that imbedded fossils and stones are frequently 
elongated in the direction of the cleavage planes. The cause of the 
phenomenon is still exceedingly obscure ; but it is now very generally 
regarded as due to loug continued pressure acting at right angles, to 
the lines of cleavage, whilst the rock was permeated by water or 
steam, or whilst it still retained its sedimentary condition. Many of 
the slates of the Eastern Townships, as those of Richmond, Kingsey, 
Melbourne, Westbury, &c., owe their fissility to superinduced 
cleavage. 
CLASSIFICATION OF ROCKS IN ACCORDANCE WITH 
THEIR RELATIVE AGES. 
Our preceding illustrations have shewn us the distribution of rocks 
into three great groups—Eruptive, Metamorphic, and Sedimentary 
rocks—in accordance with their modes of derivation or general forma- 
tive processes. But these rocks admit of another and far more in- 
teresting classification: one based on their relative ages or periods of 
formation. 
