DYNAMICAL GEOLOGY. 213 



formed. In the case of a subsiding coast, the effect of 

 wave-action would be to destroy resisting cliffs and obstacles 

 as the sea advanced inland, and thus to give origin to a 

 submarine plain. Sir Andrew Ramsay and Mr. Uavison had 

 described in general terms the "abrasive" work of the breakers, 

 and shown how as the level of land became degraded by 

 subaerial forces of denudation, the margin next the sea 

 arrived at its base-level of erosion, and sank as a denuded 

 plain below the advancing sea. Such a plain was called by 

 Ramsay a plain of submarine denudation. Von Richthofen 

 adopted the term "abrasion," and used the expression a "plain 

 of abrasion" to signify more particularly a submarine platform 

 whose surface had been abraded during subsidence of the 

 land by the destructive action of marine breakers and currents. 

 Sir Archibald Geikie, on the other hand, thinks that submarine 

 platforms have owed their degradation of level essentially to 

 subaerial agents of erosion, and that they represent land 

 surfaces which had arrived at the base level of erosion before 

 they were submerged, the action of the waves merely 

 completing the process of levelling. De Lapparent, Penck, 

 and many other geologists similarly explain the origin of 

 plains of denudation by subaerial erosion. 



Recent maps of Oceanography show at a glance that sub- 

 marine platforms sometimes extend for many square miles as 

 a marginal belt around continents or islands, and geographers 

 find it very difficult to determine the precise conditions to 

 which these " peneplains " owe their existence in the various 

 regions. Several German and Austrian geographers, following 

 Richthofen's methods, have conducted special investigations 

 on this subject during recent years (Fischer in 1885 and 

 1887, Kriimmel 1889, Philippson 1892, Penck 1894). 



The old idea, favoured by De Maillet, Buffon, Cuvier, and 

 others, that marine currents played an important part in the 

 configuration of the globe, has been proved fallacious. Marine 

 currents lose their strength as they come into the shallow areas 

 near the coast; they increase in strength where they pass 

 through narrow channels, especially where, as in the Straits of 

 Gibraltar and the Bosphorus, they sweep between two seas. 

 The origin of the deeper furrows and basins in the floor of the 

 ocean can in very few cases be explained by submarine erosion. 

 As a rule, they represent either continental valleys that have 

 been submerged or troughs formed by crust-movements. 



