30 CAUSES OF DIFFERENCE [ch. III. 
and that, in the course of ages the same area may become, again 
and again, the receptacle of such dissimilar sets of strata. 
During intervening periods, the space may either remain un- 
altered, or suffer what is termed denudation, in which case a 
superior set of strata are removed by the power of running 
water, and subjacent beds are laid bare, as happens wherever a 
sea encroaches upon a line of coast. By such means, it is ob- 
vious that the discordance in age of rocks in contact must 
often be greatly increased. 
The frequent unconformability in the stratification of the 
inferior and overlying formation is another phenomenon in 
their arrangement, which may be considered as a natural con- 
sequence of those movements that accompany the gradual con- 
version of part of an ocean into land ; for by such convulsions 
the older set of strata may become rent, shattered, inclined, 
and contorted to any amount. If the movement entirely cease 
before a new deposit is formed in the same tract, the superior 
strata may repose horizontally upon the dislocated series. But 
even if the subterranean convulsions continue with increasing 
violence, the more recent formations must remain comparatively 
undisturbed, because they cannot share in the immense de- 
rangement previously produced in the older beds, while the 
latter, on the contrary, cannot fail to participate in all the 
movements subsequently communicated to the newer. 
Change of Species everywhere in progress. — If, then, it be 
conceded, that the combined action of the volcanic and the 
aqueous forces would give rise to a succession of distinct for- 
mations, and that these would be sometimes unconformable, let 
us next inquire in what manner these groups might become 
characterized by different assemblages of fossil remains. 
We endeavoured to show, in the last volume, that the hypo- 
thesis of the gradual extinction of certain animals and plants, 
and the successive introduction of new species, was quite con- 
sistent with all that is known of the existing economy of the 
animate world ; and if it be found the only hypothesis which 
is reconcilable with geological phenomena, we shall have 
