THE WORK OF RUNNING WATER, 63 



than that which separates a mature man from an old one. In this 

 case, as in the preceding, the island has been base-leveled, but still 

 without the formation of valleys or hills. 



Both the preceding hypothetical cases make it clear that, from the 

 point of view of erosion, every drop of water which runs off over the 

 surface of the land has for its mission the getting of the land into the 

 sea. Under ordinary conditions surface drainage must fail to bring 

 a land area altogether to sea-level, the absolute base-level of subaerial 

 forces; but it is not simply the water which runs off over the surface 

 which degrades the land. That which sinks beneath the surface con- 

 tributes to the same end by slowly dissolving mineral matter below 

 the surface, and finally carrying it to the sea. In this way the reduc- 

 tion of land areas to sea-level may be completed. 



The rain-water which evaporates from the surface without sinking 

 beneath it does not effect much wear; but the water thus evaporated 

 is subject to re-precipitation, so that, in the long run, it may assist in 

 the work which has been sketched. Thus it is not simply the waters 

 which run off over the surface of the land, but all which fall upon it, 

 which unite to compass its destruction. 



The Development of Valleys. 



By the growth of gullies. — Had the slopes of the hypothetical 

 island not been absolutely uniform the processes of erosion would 

 have been different. Let the departure from uniformity be supposed 

 to consist of a single slight meridional depression near the base of 

 the island (Fig. 43). As the rain falls it will no longer run off equally 



Fig 43. — Diagram showing a slight meridional depression in the surface of an other- 

 wise even-sloped island. 



in all directions. A greater volume will flow through the depression 

 than over other parts of the surface having the same altitude, and 

 the greater volume of water along this line will give greater velocity, 

 greater velocity will occasion greater erosion, and greater erosion will 

 deepen the depression. The immediate result is a gully or wash (Fig. 

 44). So soon as the gully is started it tends still further to concentrate 

 drainage in itself, and is thereby enlarged. The water which enters it 



