270 Davis — Bearing of Physiography upon Suess\ Theories. 



their rocks may have been horizontally crushed in some earlier 

 period of deformation ; for in all three cases the mountainous 

 areas were described as having been worn down to moderate relief 

 after their rocks had been deformed, and their present altitude 

 was explained as having been gained subsequently, by displace- 

 ments from which the accepted characteristics of compression 

 were absent. In none of the examples here adduced was 

 particular attention paid by the investigators above named to 

 the features of asymmetry and curvature of mountain tread, 

 upon which Suess lays importance ; hence that phase of his 

 theory is not here further considered ; but in so far as the pres- 

 ent relief of the ranges mentioned is concerned, it should not, 

 if the explanation by non-compressional displacement is correct, 

 be ascribed to or correlated with tangential movement, however 

 fully the internal structure of the ranges may be thus accounted 

 for. My own observations on the Tian Shan had led to essen- 

 tially the same conclusions, as already stated. 



The nature of the forces by which displaced fragments of 

 peneplains have gained their present altitude is not at once 

 apparent, but the following considerations lead to the belief 

 that forces of uplift may have acted in giving such ranges as 

 the Bural-bas-tau and its fellows their actual elevation. If it 

 be agreed that the highlands of the Tian Shan are parts of a 

 once relatively low-lying peneplain, and that their present 

 attitudes exclude lateral compression as a cause of their present 

 altitude, then two contrasted explanations may be offered to 

 account for the crustal displacement by which the rivers of the 

 region have been excited to deep erosion of the surface that 

 was previously safe from their attack. It may be supposed, 

 on the one hand, that the whole region once had the altitude 

 of the Bural-bas-tau highland and its fellows, and that since 

 then the now lower parts of the region have subsided to the 

 present levels ; or, on the other hand, that the whole region 

 once had an altitude similar to that of the steppes between 

 Yiernyi and Semipalatinsk — with as many and as strong monad- 

 nocks and residual ranges as further observation shall demand 

 — and that since then the higher parts of the region have been 

 uplifted. The first supposition is the view adopted by Suess 

 and by a number of European geologists ; the second is the view 

 generally accepted by American geologists, as well as by some 

 Europeans. I have not been able to devise any means of mak- 

 ing absolute choice between the two views, but it seems to me 

 that many reasons may be adduced for the probable correctness 

 of the second view rather than of the first. 



The theory of subsidence may be called the theory of horsts / 

 horst being a mass of earth-crust which is limited by faults and 

 which stands in relief with respect to its surroundings. The 

 Harz mountains in northern Germany and the central plateau 



