THE WORK OF RUNNING WATER. 57 



RAIN AND RIVER EROSION. 



Rain and river erosion began when the first rains fell on land sur- 

 faces. Neither the location nor the nature of the first land surface 

 is kno^Mi. There is little reason to beheve that the ocean was ever 

 universal, but there is reason to beUeve that most land areas have 

 at some time or other been covered by the sea. The prevalent con- 

 ception that land areas which were once submerged came into exist- 

 ence by being elevated above sea-level, should be supplemented by 

 the alternative conception that submerged areas may have become 

 land by the depression of the ocean basins, thus drawing off the water 

 from the areas where it was shallow. Thus in Fig. 36 the sinking of 



Fig. 36. — Diagram to illustrate the origin of lands by the lowering of the sea-level due 

 to depression of the sea bottom. If the bottom is depressed from a to 6 the sur- 

 face will be drawn down from cc' to dd', and the surfaces cd and c^d' will become 

 land. 



the sea-bottom from a to h would lower the surface of the water from 

 cc^, to dd^, and draw off the water from the surfaces cd and (/d\ 



Without attempting to picture the character of the original land 

 our study of subaerial erosion may begin T\ith an area which has just 

 been changed from sea bottom to land. ^Tiat is the nature of such a 

 land surface? Of what material is it composed, and what is the char- 

 acter of its topography? Concerning its constitution something 

 may be inferred from the nature of the deposits now found at the bottom 

 of the sea. Near the shore and in shallow water they often consist 

 of gravel and sand, though other materials are not wanting. Far 

 from shore and in deep water they consist for the most part of fine 

 sediments, some of which were washed or blown from the land, some 

 of which came from the shells and other secretions of marine animals, 

 some from volcanoes, and some from various other sources. The 

 topography of the newly emerged land may have had some likeness 

 to the topography of the sea bottom. The mmierous soundings which 

 have been made over large areas of the sea have sho^m that its bottom 

 is. as a rule, free from the numerous small irregularities which affect 



