58 GEOLOGY. 



the surface of the land. They seem to show that a large part of the 

 ocean bottom is so nearly flat that, if the water were removed, the 

 eye would hardly detect irregularities in the surface. This statement 

 does not lose sight of the fact that the ocean bottom is, in certain places, 

 markedly irregular. Volcanic peaks and striking irregularities of 

 other sorts abound in some places. Nevertheless if the bottom of 

 the sea could be seen as the land is, its most striking feature, taken as 

 a whole, would be its apparent flatness. 



With the topography of the sea bottom the topography of the land 

 is, in its details, in sharp contrast. In order to get at the history of 

 the latter, we may study the sequence of events which would follow 

 the emergence of a portion of the former. 



Suhaerial Erosion without Valleys. 



For the sake of emphasizing the fundamental principles involved 

 in the work of running water, a hypothetical case will first be studied 

 in some detail, even at the risk of elaborating processes already under- 

 stood. The principles themselves will find application later in relations 

 which are much less simple. 



Let it be assumed that the area of newly emerged land is a circular 

 dome-shaped island. The simplest possible condition is represented 

 by assuming its slope to be the same in all directions from the center, 

 and its materials to be absolutely homogeneous. Such an island 

 would be subject to all the forces ordinarily operating on land surfaces. 

 The chief agency tending to modify land surfaces is atmospheric pre- 

 cipitation. It will be assumed that the rain falls on the surface of 

 the island with absolute equahty at all points, and that all other forces 

 which affect it operate equally everywhere. 



The rain falhng on a land area disappears in various ways; part 

 of it evaporates, part of it sinks, and part of it runs off over the surface. 

 If the island be composed of fine and unconsolidated materials, such 

 as clay, the water which runs off over the surface will carry sediment 

 down to the sea. If the island be composed of solid rock instead, 

 exposure to the air will cause it to decay, and the products of decay, 

 such as sand and mud, will suffer a like fate. 



For the sake of a clear understanding of the processes involved, 

 two cases may be postulated; one in which the waters of the sea re- 

 move the sediment washed down from the hypothetical island as fast 

 as it reaches the shore, and one in which they allow it to accumulate 



