VIEWS OF ENGLISH WRITERS. 383 



marine denudation is called a 'plain of marine denudation.'"* No 

 appreciable wearing takes place below the level of the lowest tides. No 

 mention is made of a cover of sediments as a characteristic accompani- 

 ment of the plain of denudation, and no consideration is given to the 

 plains of subaerial denudation ; only the lesser inequalities of land form 

 are ascribed to subaerial agencies. 



The edition of " Phillips' Manual of Geology," by Etheridge and Seeley 

 (1885), briefly describes plains of marine denudation (page 131), and 

 under subaerial denudation goes no further than to explain the origin of 

 valleys. 



Woodward, in his valuable summary of the " Geology of England and 

 Wales," t follows his predecessors in adopting the idea of marine denuda- 

 tion for the production of plains. 



Jukes-Brown writes : 



"Plains of erosion are those which have been formed by marine erosion across 

 the edges and outcrops of strata without reference to their inclination, flexures, or 

 fractures. They are surfaces of planation formed by the march of the sea across 

 the country. The limestone plains of central Ireland may be cited as an instance." t 



Subaerial agencies are not considered beyond the formation of valleys. 

 For example : 



"As soon as this surface produced by marine erosion is elevated into dry land, it 

 is subjected to the detritive action of the subaerial agencies already described, and 

 is ultimately carved out into new forms of hill and valley " (page 565). 



Detritive and erosive agencies are grouped under two heads : 



"1. Marine agencies, which act along the margin of the land, and tend to produce 

 an approximate level surface or plain. 2. Subaerial agencies, which act over the 

 whole surface of the land, and tend to produce a system of valleys and watersheds? 

 hollows and relative eminences" (page 564). 



In discussing breaches in the escarpments and hill ranges of the 

 Wealden district, the same author says : 



" The only explanation of these facts is . . . marine erosion first produced 

 a surface of planation across the whole district while it was being slowly elevated, 

 so that this original surface sloped gently from a central line toward the north and 

 south. The primary streams naturally followed these slopes, . . . forming the 

 transverse valleys" (page 581). 



Richthofen is the leading advocate of marine erosion among conti- 

 nental geologists. He treated the origin of plains of denudation, inde- 



* Physical Geology, ls82, p. 577. 



t Second edition, 1887. 



t Handbook of Physical Geology, 1892, p. 620. 



