272 Dams — Bearing of Physiography upon Suess'' Theories. 



only standard of comparison. It is at best a movable standard, 

 yet in the present problem it will fairly serve onr purpose. 



So far as I have been able to learn, the horsts described 

 by Suess are not examined to discover how far the form of 

 their surface may give indication of the altitude that they 

 possessed with respect to sea-level before the occurrence of the 

 faults by which they have been placed in relief : yet such an 

 examination is well worth while. In the case of the Schieferge- 

 birge of the middle Rhine, for example, the form of the upland 

 is for the most part that of a well-developed peneplain, more 

 or less warped, here and there surmounted by residual monad- 

 nocks, and now dissected by valleys. This district, therefore, 

 stood at a moderate altitude above sea-level, before its present 

 altitude was gained. Hence, if the present altitude were gained 

 by the subsidence of the surrounding lower lands, we are con- 

 strained to believe that not only the surrounding lower lands, 

 but all the oceans went down as well, and all the continents 

 with them — the Schiefergebirge and the neighboring uplands 

 alone standing still. The same is true of the plateau of north- 

 ern Arizona, across which the Colorado has cut its precocious 

 canyon : the plateau is a surface of denudation, and according 

 to the most reasonable interpretation was once a lowland of 

 small height above sea-level : if its present altitude has been 

 passively gained by the subsidence of the surrounding lower 

 districts, then in this case again all the oceans must also have 

 gone down, and the rest of America aud the other continents 

 with them. Many more examples of the same kind might be 

 mentioned, but the most impressive one that has come under 

 my own observation is the Bural-bas-tau. It seems extrava- 

 gant to suppose that all the rest of the lands and seas had to 

 sink by some 10,000 feet in order to leave that range and its 

 fellows in strong relief. 



It is not intended, however, to imply that every horst was 

 formed by a separate subsidence of all the rest of the world, 

 for in so far as peneplained horsts are of the same date, of pro- 

 duction and of the same altitude, one subsidence of the rest of 

 the world will suffice for their production ; but many of the 

 known examples of such horsts are in very different stages of 

 dissection, and they stand at different altitudes ; hence various 

 widespread subsidences must, according to this theory, be 

 admitted in order to account for them. It is not clear to me 

 whether this world-wide extension of subsidence has been con- 

 sidered by Suess : it does not seem to be explicitly excluded in 

 his writings; it may be that it is tacitly accepted, to be more 

 fully treated in later volumes : he says indeed, in discussing 

 changes of level in the Rocky mountains, "we shall have to 

 return to questions of this kind, and I hope then to be able to 

 show that there is need of correcting more than one wide- 



