CHAPTER XIV 



THE RECORD OF THE ROCKS 



341. Looking Backward. Two opposed agencies now 

 at work on the Earth's surface internal energy ridging up 

 the crust, and solar energy cutting down the heights are 

 sufficient, if they have been long enough in action, to 

 account for all the features of the land. The Uniformi- 

 tarian School of geologists holds that the Earth has attained 

 its present condition after passing through vast ages of 

 change so slow as to be almost imperceptible. The other 

 school, sometimes called that of the Catastrophists, affirms 

 that the processes at work in past time were quite different 

 from those of the present, being much more violent and 

 uncertain in their action. They look on valleys as rent in 

 the solid rock by Earth movements of titanic strength, and 

 on mountain ranges as elevated to their full height in a 

 single stupendous heave of the strata. Erosion is con- 

 sidered only to trim off the broken edges, as a plane 

 smooths down the signs of the rough rending of a saw. 

 Modern research shows that the truth lies between the two 

 extremes. The Earth, like any other cooling body, must be 

 cooling less and less rapidly as time goes on. When the 

 crust was first formed its high temperature must have con- 

 siderably increased the erosive power of water. So, too, 

 tidal friction, now insignificant, must once have been a 

 tremendously powerful agent in shaping the surface ( 104). 

 Thus, while the processes at work have been the same in 

 kind as the Uniformitarians prove, the energy available for 



