202 



University of Califoi-iiia. 



[Vol. 2. 



the maximum width of the submarine platform is at either end of 

 the block, while the minimum width is along the fault scarp. 

 The 600-foot contour runs in a northwesterly and southeasterly 

 direction. 



As shown by the soundings in figure 1 and Plate 5, Cortes, Tanner, 

 and Osborn banks exhibit characters identical with those of the 

 upper submarine platform. They are more or less platform-like in 

 character, and at an average depth corresponding to that of the 

 submarine platform, and the descent beyond the 600-foot contour is 

 generally rapid. There can be little doubt, therefore, that these 

 banks were islands, only a short time ago, geologically speaking, 

 and that they have been completely truncated by wave action 

 during the most recent stages of marine abrasion. 



If the coastal platform is wave-cut and -built, as the writer 

 believes and as the above details would go to show, then the follow- 

 ing conclusions with regard to the later history of the coast may 

 be drawn from its general character. The broad surface with gen- 

 tle gradient between the 200- and 400-foot submarine contours would 

 indicate a considerable period during which the land stood near 

 one level. This evidence is strengthened by the fact that this in- 

 terval is comparatively wide even where the rocks are hard and the 

 cliffs high and abrupt; for, as will be shown later, cutting along 

 such a coast line is comparatively very slow. The generally 

 steeper slope between the 200-foot contour and the shore line would 

 indicate a much shorter period during which the land was sinking. 

 Whether or not the depression is now in progress can not be 

 stated. These conclusions are in accordance with the evidence 

 furnished by drowned valleys in the case of those islands whose 

 topography admits of such features. 



The exceptional width of the submarine platform as compared 

 with that of the elevated terraces may be accounted for in part by 

 a prolonged period of wave action at or near one level, as already 

 indicated; in part, also, by the fact that the last formed of a series 

 of terraces is apt to be broader than those previously formed, at 

 higher levels, because each succeeding terrace in cutting back its 

 cliff tends to cut away and obliterate the platform of the terrace 

 next above. 



