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 



