294 N. S. SHALER — BROAD VALLEYS OF THE CORDILLERAS 



there has not been sufficient time for the differentiation of new species 

 adapted to the environment. 



If the presumption just above noted as to the age of the broad valleys 

 be affirmed it will serve to throw much light on the duration of the 

 Cenozoic age. The tendenc}^ of geologists has, in my o})inion, been to 

 under-reckon this section of time. It is generall}^ computed, from the 

 thickness of the sediments and other evidences, as far less long than the 

 Paleozoic or even the Mesozoic ages. I have elsewhere* undertaken to 

 show that the geological results of that age must have required many 

 million years for their development. If the further study of the Cordil- 

 leran valleys bear out the preliminar}^ judgment above set forth we shall 

 have yet stronger reasons for revising our conceptions concerning the 

 duration of the last great geologic age. 



Effects of Deposition 



There are certain secondar}^ effects arising from the infilling of the 

 broad valle3'S which deserve attention. In the first })lace we note that 

 the transfer of a vast body of debris from the mountain ranges to tlie 

 center of the troughs must naturally have led to the subsidence of the 

 bases on which the weight was im[)osed and a corresponding rise in the 

 areas whence the materials were deported. If we reckon the average 

 depth of the detrital materials laid down in these valleys at 3,000 feet, 

 then the consequent subsidence should be some large portion of that 

 amount. In all probability the deposits of the greater vales much ex- 

 ceed the amount here assumed. In proportion to the subsidence, but 

 probabl}' somewhat less in amount, there would l)e a rise in the areas 

 of erosion — that is, the bordering mountain ranges. The axis or neutral 

 point of this movement would naturally lie near the junction of the 

 valley deposits with the high countr3^ Its position would be unstable, 

 as it would tend to become extended farther and farther from the cen- 

 tral line of the depression as those dej^osits encroached upon the steeps. 

 Assuming that this effect arising from the isostatic action occurred — 

 that it in some measure occurred is almost a necessary supposition — 

 there are several interesting consequences that require consideration. In 

 the first place we shall be able thereb}^ to account for the great depth 

 of these vallej'S without having to suppose that continuously inclined 

 drainage channels cut in the l)ed rocks still exist. There is, indeed, no 

 reason why the floors of the troughs may not have been borne down for 



♦Report on the Geology of the Cape Cod District. Ann. Rept. Director U. S. Geo!. Survey, 

 1 S9G- 97, p. 588. 



