TERRACES AND RIVER VALLEYS OF WESTERN EUROPE. 273 



Tmt the soundings are here insufficient to show clearly the 

 physical structure of the ocean-bed. All along this coast 

 the margin of the platform very nearly coincides with the 

 200-fathom isobathic line. 



(b.) Composition of the Floor. — Throughout the tract above 

 described from the English Channel to the entrance of the 

 Straits of Gibraltar, the floor of the platform is composed of 

 fragmental matter, such as gravel, banks and sheets of sand, 

 clay and occasionally boulders of rock. These materials 

 have been carried down into the sea by the rivers or dis- 

 lodged from the coast-cliffs, and spread over the floor by 

 tidal currents. Though enclosing molluscs and other animal 

 forms, these materials are essentially different from those 

 which are spread over the floor of the abyssal regions of the 

 ocean consisting mainly of calcareous marl ; and thus the 

 great declivity along Avhich the platform breaks off seawards 

 becomes the physical line of separation between the es- 

 sentially oceanic and essentially littoral deposits. 



''^ Plane of Marine Denndation" ;* and '■'•Base-level of Ero- 

 ^!.on.'"t — In the platform above described we have an illus- 

 tration of the former, and in the cliffs which form its bound- 

 ary along the coast, we have an illustration of the latter. 

 The platform owes its nearly level surface to the action of 

 the sea during periods of emergence and subsequent gradual 

 submergence. The term "plane of marine denudation" is 

 one applied by the late Sir Andrew Ramsay to describe 

 theoretical plane-surfaces of extensive tracts in AVales and 

 clseAvhere, which have been subsequently eroded by river- 

 valleys.J The British-Continental Platform is an admirable 

 example of one of these, and is still in course of formation 

 along the " base-level of erosion " of the coast cliffs by 

 means of wave and current-action. 



II. The Great Declivity. § — I apply this term as it is, in 

 my view, the only fitting one to describe the abrupt descent 

 along which the Continental Platform breaks off and is 

 connected with the vast plain which forms the floor of the 



* Either " plane " or " plain " accoi'ding as we regard it a mechanical 

 or a physical surface. t Professor Emmons of U.S.A. 



{ Physical Geology of Great Britain, 5 Edit. (1878). 



§ The term I had originally emjoloyed for this feature was "Grand 

 Escarpment," but in deference to the objections of some geologists I have 

 substituted the above, although " Escarpment " is allowed in this sense in 

 America. Mr. Hudleston calls it " The Sub-oceanic Continental Slope," 

 Geological Magazine, March, 1899. 



