200 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



Wegener, or one or other of the modifications of it. It is for us to 

 watch and test all the data under our own observation, feeling 

 sure that we shall have to adapt to our own case Galileo's words 

 "e pur si muove." 



Ever since it was realized that the inclination and folding of 

 rocks must be attributed to lateral or tangential stress and not solely 

 to uplift, shrinkage of the interior of the earth from its crust has 

 been accepted as the prime mover, and whichever of the current 

 theories we adopt we cannot deny the efficacy of so powerful a cause. 



The general course of events in the formation of a mountain range 

 is fairly well known : the slow sinking of a downf old in the crust 

 during long ages; the filling of this with sediment pari passu witli 

 the sinking and associated softening of the subcrust due to accumu- 

 lated heat; the oncoming of lateral pressure causing wavelike folds 

 in the sediments and the base on which they rest; the crushing of 

 folds together till, like water waves, they bend over and break by 

 overdriving from above or, it may be, underdriving from below; 

 fracture of the compressed folds and the traveling forward for great 

 distances of slivers or "nappes" of rock, generally of small relative 

 thickness but of great length and breadth, and sliding upon floors 

 of crushed rock; the outpouring and intrusion of igneous rocks, 

 lubricating contacts and complicating the loading of the sediments; 

 metamorphism of many of the rocks by crystallization at elevated 

 temperatures and under stress, with the development of a new and 

 elaborate system of planes of reorientation and movement; and 

 elevation of the Avhole, either independently or by thickening with 

 compression and piling up to bring about a fresh equilibrium. 



Such a course of events would be brought about by lateral pressure 

 developed during the consolidation phase of each of the thermal or 

 magmatic cycles. At each period of their building, mountains have 

 arisen along lines of M-eakness in the crust, especially coast lines and 

 the steep slopes marking the limits between continents and ocean 

 basins. This is consistent with Joly's theory that the thrust of 

 ocean beds against land margins is the cause. 



But the advocates of continental drift point to the siting of ranges 

 across the paths along which the drifting movement is supposed 

 to have occurred, and they consider that the moving masses are 

 i-esponsible ; and indeed that the ridging and packing of the crust 

 has in the end checked and stopped the movement. They note that 

 the great western ranges of America occur in the path of any western 

 drift of that continent, the Himalayas in the course of the postulated 

 movement of India, the East Indies in front of Australia ; and that 

 the Alpine ranges of Europe may be linked with the crushing of 

 Africa toward the north. 



