268 Davis — Bearing of Physiography upon Suess* Theories. 



mountain range due to lateral pressure. Its rocks may cer- 

 tainly have suffered compression in some past age, for they not 

 infrequently exhibit deformation of the kind that geologists 

 accept with good reason as evidence of the action of tangential 

 forces. Furthermore, the region may in some past time have 

 risen to mountain heights as a result of such compression, for 

 it seems reasonable to associate superficial disorder and eleva- 

 tion with crustal compression, whether the work of compres- 

 sion be superficial or deep-seated. But the time when 

 compression deformed the rocks and when the region rose in 

 mountain form as a result of such compression must now be 

 long past, because the even surfaces or peneplains of the high- 

 lands and the steppes truncates the disordered structures, and 

 thus proves that a long period of erosion — in effect, a physio- 

 graphic cycle — has elapsed since the compression took place ; 

 and to this period must be added the early part of another 

 cycle, sufficient for the dislocation, elevation and partial dissec- 

 tion of the peneplain and its residuals of the first cycle. The 

 ranges that my path crossed were so largely composed of 

 massive crystalline rocks that it is impossible for me to state 

 what relation generally exists between the trend of the exist- 

 ing ranges and the strike of the deformed strata : in one small 

 range, the trend of the crest and the strike of its steep dipping 

 beds diverged at a strong angle ; in another case, there was 

 rough coincidence between range trend and strike of slaty 

 cleavage ; both these ranges gave good indication of having 

 been worn down to low relief before their present altitude was 

 gained. It is, therefore, not now possible to say what relation 

 exists between the reliefs of the original ranges, due to com- 

 pression, and the existing ranges, due to some other kind of 

 displacement. 



Whether the peneplain fragments seen in the existing ranges 

 and the broad peneplain of the steppes were once parts of a 

 single very extensive peneplain, or whether they represent parts 

 of neighboring but isolated peneplains, need not now be fur- 

 ther considered — particularly as the facts needed to settle this 

 question are not yet in hand. It is sufficient to note that the 

 actual attitudes of the peneplain fragments, large and small, now 

 seen in the Tian Shan ranges are not such as to indicate the direct 

 action of forces of horizontal compression. It is true, as above 

 intimated, that one may suppose the dislocation of the mountain 

 blocks to be the superficial result of a deep-seated compression ; 

 but our ignorance of the processes that go on within the earth 

 is so profound that a speculation of this kind has no compulsory 

 value ; it certainly does not entail the classification of the exist- 

 ing Tian Shan system in the group of mountains due to forces 

 of lateral compression. Hence the Tian Shan should not, in my 



