Ch. XIII.] THE INCLINED POSITIONS OF STRATA. 287 



to the influence of known agents : the materials of each of 

 these secondary rocks are such as atmospheric and aqueous 

 causes would produce by their operation on older rocks ; the 

 position of these strata depends perhaps on the superficial 

 form of the primary rocks on which they repose, and the 

 amount of their angle of inclination may also depend on the 

 same cause, or on the form of the concretionary layers of 

 which these rocks are composed ; and, lastly, the alteration 

 which they have experienced at various times, as regards 

 their situation above or below the sea level, may be attributed 

 to the expansive power of the central fire, which, even at the 

 present day, produces elevations and depressions of the land 

 over extensive districts, without affecting the previous position 

 of the rocks. 



This view of the subject is at variance with the fundamental 

 data on which M. de Beaumont's theory of the relative an- 

 tiquity of mountain chains is founded : for, if inclined and 

 horizontal strata may be continuous; and, if the degree of 

 inclination may have arisen, either from the manner of de- 

 position, or of consolidation, being in some measure regu- 

 lated by the surface of the inferior rock on which it rests, 

 but independent of the direction and dip of its strata ; how 

 is the period of elevation to be determined? Boue, Cony- 

 beare, Lyell, and others, have, on various grounds, advanced 

 objections to this theory; and it may, therefore, be presumed 

 that, without being modified, it cannot be maintained. It is 

 not intended, at present, to enter on this topic> but to confine 

 our remarks to the question whether horizontal strata, re- 

 posing on others which are inclined, furnish sufficient evidence 

 that the latter have been upheaved before the deposition of 

 the former. 



" To determine the general unconformable position of two 

 rocks, sometimes requires very great care," says De la Beche*; 



* Geological Manual, 8vo, p. 481, 



