COMPARISON OF ENGLISH AND AMERICAN SCHOOLS. 393 



valleys as a matter already proved, and goes on from this to the possible 

 ultimate result of the valley-making processes. Again, the English school 

 denies, tacitly or directly, the probability or even the possibility of a 

 period of still-stand long enough for essentially complete subaerial denu- 

 dation close to sealevel, but assumes the possibility of a period of still- 

 stand or of slight depression continuous and long enough to allow the 

 sea waves to plane off the sinking lands. The American school tacitly 

 questions the occurrence of great erosive transgressions of the sea during 

 either a still-stand or a slow depression of the land, but admits the possi- 

 bility of essentially complete subaerial denudation to an average sealevel, 

 above and below which the land long hovers in many minor oscillations 

 before a new attitude is assumed by great depression, elevation or de- 

 formation. It should be borne in mind that the depressed and buried 

 or the uplifted and dissected plains of denudation whose origin is in 

 question are in no cases geometrical planes ; they nearly always possess 

 perceptible inequalities, amounting frequently to two or three hundred 

 feet; but these measures are small*compared to the inferred constructional 

 relief of earlier date, or compared to the deep valleys often eroded be- 

 neath the plain if it has been uplifted. B}^ whatever process the so-called 

 " plain of denudation " was produced, an explanation that will account 

 for a peneplain of moderate or slight relief is all that is necessary. Abso- 

 lute planation is so rare as hardly to need consideration here. 



In no respect is the contrast between the two schools more strikingly 

 shown than in the beliefs concerning the cover of unconformable strata 

 that lie or are supposed to have lain upon an oldland. The continental 

 members of the English school generally regard these strata as an essen- 

 tial result of the process of marine denudation during slow depression ; 

 if such strata are absent from a dissected plateau, their absence is ex- 

 plained by denudation after uplift. The American school does not give 

 the cover of unconformable strata an essential place in the problem ; if 

 present, it is generally ascribed to deposition following the submergence 

 of a region already for the most part baseleveled by subaerial agencies. 



Review of the a priori Argument. 



It may be noted that the value of marine agencies gained a high repu- 

 tation for effective work before subaerial agencies w^ere recognized as 

 significantly affecting the form of the land, and that from that time to 

 the present the importance of the latter agencies has been steadily in- 

 creasing in the minds of geologists. The manifest work of waves on a 

 bold coast was perceived at a time when the production of valleys by 

 rain and rivers was scouted. Today it is not so much that the absolute 

 strength of marine erosion is given a smaller value than heretofore, but 



XLVI— Bull Gkol. Soc. Am., Vol. 7, 1895. 



