62 GEOLOGY. 



plain the spreading and lowering suggested by Figs. 39 and 40 would 

 go on until the central mass of the island was brought down to a gra- 

 dient in harmony with that of the gently sloping border, as shown in 

 Fig. 41. When this had been accomplished there would be a relatively 

 large land area with low slopes (Fig. 41) in place of the smaller 

 area with steeper ones (compare Figs. 39 and 40). The basal part 

 of the larger island from the center to the original margin would be 

 made up of the original material in its original position (unshaded 

 part of Fig. 41). Its surface would be covered, least deeply near its 

 center and most deeply near the original margin, with debris gradually 

 shifted from higher levels, as shown in Fig. 41. 



Were such an island as that shown in Fig. 41 once formed, the 

 rain falling on it, and flowing off over its surface, would carry off its 

 surface soil and spread it about the shores. Though the surface of 

 the marginal flat of Fig. 40 was as low as running water could bring 

 it at the time it was developed, the conditions of erosion have changed 

 by the time the land reaches the conditions shown in Fig. 41, and the 

 same amount of rainfall may now be effective in erosion. In the first 

 case (Fig. 40) the water descending from the higher part of the land 

 brought down sediment and started across the flat with a load. Its 

 energy was consumed in transporting what it had, not in getting new 

 material. In the second case (Fig. 41) the water flowing over the 

 gently sloping surface has no initial load, and its energy is therefore 

 available for erosion. Under continued rainfall, the area of the land 

 shown in Fig. 41 would be increased as before by successive marginal 

 deposits (see Fig. 42), and at the same time its average height would 



Fig. 42. — Diagram to illustrate the result of the continuation of the processes shown 



in Figs. 39-41. 



be reduced. The lowering and enlarging of the island would continue 

 until the whole surface was brought so nearly to the level of the sea 

 that water would cease to run over it with sufficient velocity to carry 

 away even the fine material of its surface. Such a surface, brought 

 down as low as running water can degrade it, is also (see p. 57) a 

 hase-level. It wifi be seen from the foregoing illustrations that a graded 

 surface may pass into a hase-level, with no sharper line of demarkation 



