82 ON EROSIONS OF THE EARTH'S SURFACE. 



They thus prove an amount of erosion previous to the alluvial period, immensely 

 greater than during the deposition of drift and alluvium. 



According to the views of many eminent geologists, I might consider nearly all 

 the crystalline stratified rocks among those originally detrital. But as this is 

 debatable ground, I leave them out, although all geologists, I believe, admit that 

 there are metamorphic rocks of considerable thickness having such an origin. 



4. By the rounded, smoothed, and striated appearance of most of our hills and 

 mountains in the northern portions of continents. These phenomena indicate an 

 erosive agency that has operated long and powerfully, especially on the northern 

 slopes of mountains, to wear them down. To this force we might add that of 

 glaciers, which produce similar effects; and as all know, the two agencies are by 

 some regarded as identical. 



5. The marks of erosions in gorges and on the steep sides of valleys, teach the 

 same lesson. These, I apprehend, are more common than has been supposed ; and 

 it is the chief object of this paper to describe and elucidate them. It is an agency 

 distinct from that producing drift, being referable to two sources, viz : rivers, and 

 the ocean acting upon its shores. 



6. But perhaps the vast amount of materials that must be supplied to fill up 

 deficiencies in the strata, shows most strikingly the enormous erosions that have 

 taken place. Facts on this subject have not, indeed, been accurately determined 

 in many countries. Yet we know enough to be satisfied that miles in depth have 

 often been taken away ; as indeed we might presume must have been the case to 

 supply materials enough for the sedimentary rocks. 



All these facts speak the same language, and impress the careful observer with 

 the magnitude of the work of erosion that has been going on from the earliest 

 times. Yet it is only the careful observer who will be impressed with these proofs. 

 Those who take only general views of the rocks and the surface geology, can easily 

 persuade themselves that even the fragmentary rocks were created just as we now 

 find them ; and some extend such an hypothesis even to the water-worn pebbles 

 and banks of sand. 



Agents of Erosion. 



It may be well briefly to enumerate the agents of erosion upon the earth's sur- 

 face before detailing their effects. They may all be grouped under Atmospheric 

 Air and Water. 



1. Atmospheric Air. 



The four constituents of atmospheric air, oxygen, nitrogen, carbonic acid, and 

 aqueous vapor, are all concerned in the disintegration of the solid rocks, which 

 thus become prepared to be easily acted on by mechanical agencies. Probably 

 oxygen, of which so great a quantity exists in the air, is the most efficient of 

 these agencies, since it has so strong an affinity for most other substances that 

 they will quit their weaker combinations to unite with it. Peroxidation, also, 



