448 



Maiv — On Subaerial and Marine Denudation. 



I pointed out the general tendency of the eroding coast line to be 

 straight, in contrast with the sinuous course of lines of equal height 

 on the land surface. Fig. 7 represents part of the coast of South 

 America, on which I have endeavoured to roughly show the disposi- 

 tion of the adjacent land-lines of equal height as indicated by the 

 disposition of mountain chains and river courses. The belt of the 

 land marked A may be taken to represent a range of altitude from 

 the sea level to 200 feet, b from 200 to 400 feet, c from 400 to 600 

 feet, and d land over 600 feet in height. One of the most striking 

 features here indicated is, that whilst the various land lines have a 

 certain parallelism and concentric disposition with each other, they 

 show no kind of relation to the direction of the coast line. If the 

 prevailing land contour has been brought about by the same agency 

 as the coast line, how is it that they do not exhibit some kind of 

 affinity ? Why should the sea have carved a particular kind of out- 

 line on the present coast and not have followed the same principle 

 of erosion (as indicated in Fig. 8) when at a higher level on the 

 present land surface ? A comparison of Fig. 7 representing the 

 actual land contour, and Fig. 8 representing the lines of equal 



Fig. 8. — Hypothetical Land Contour on the principle of Coast Erosion . 



height, had the coast contour been applied to its surface, will readily 

 exhibit the essential difference in structure between the result of 

 subaerial denudation and marine erosion. 



If we for the present pass over the case of irregular coast lines 

 resulting from unequal power of resistance in the rocks, the general 

 tendency to be straight seems easily capable of explanation. In the 

 first place, the ordinary wave-action along a coast is tolerably equal, 

 and there are no prima facie reasons why, under similar circum- 

 stances, it should produce an irregular result, or why it should persist 

 at one particular point in eroding at a greater rate than at another. 

 The action of the waves Avithin an inlet, be it ever so straight and 



