116 THE WONDERFUL CENTURY. CHAP. xn. 



of a general uniformity, with, of course, considerable 

 local variability. 



It is interesting to note the very different explanations 

 of the commonest features of the earth's surface given 

 by the old and by the new theories. In every mountain 

 region of the globe deep valleys, narrow ravines, and lofty 

 precipices are of common occurrence, and these were, by 

 the old school, almost always explained as being due to 

 convulsions of nature. In ravines, we were taught that 

 the rocks had been " torn asunder," while the mountains 

 and the precipices were indications of " sudden fractures 

 and upheavals of the earth's crust." On the new theory, 

 these phenomena are found to be almost wholly due to 

 the slow action of the most familiar every-day causes, 

 such as rain, snow, frost, and wind, with rivers, streams, 

 and every form of running water, acting upon rocks of 

 varying hardness, permeability, and solubility. Every 

 shower of rain falling upon steep hillsides or gentle 

 slopes, while partially absorbed, to a large extent runs 

 over the surface, carrying solid matter from higher to 

 lower levels. Every muddy stream or flooded river shows 

 the effect of this action. Day and night, month after 

 month, year after year, this denudation goes on, and its 

 cumulative effects are enormous. The material is sup- 

 plied from the solid rocks, fractured and decomposed by 

 the agency of snow and frost or by mere variations of 

 temperature, and primarily by those interior earth- 

 movements which are continually cleaving, fissuring, and 

 faulting the solid strata, and thus giving the superficial 

 causes of denudation facilities for action. The amount 

 and rate of this superficial erosion and denudation of the 

 earth's surface can be determined by the quantity of 



