Wynne — Denudation and its Causes. 9 



Plains. — The observations of Professor Jukes/ that tlie lesser eleva- 

 tions around a summit decline in altitude as they recede from it, so as 

 to suggest the former existence of a wide, gently-sloping plain, is in 

 uniformity with the fact that the superficial inclinations of ancient 

 ground subsequently became sea bottom, covered by unconformable 

 deposits, often sloping at very low angles ; although in many instances 

 there is reason to assiune that peaks and valleys like those of our 

 own time were not absent. We can, however, form but a vague 

 idea as to the shape of the original surface of the ground before the 

 denudation of any particular locality began to take place, because 

 that surface has been removed, and so few land surfaces of any 

 period are known which have not suffered, more or less, from 

 denudation. 



The origin of plains is perhaps the greatest difficulty in the 

 question of production of forms by denudation. We can conceive 

 of sea-cliffs being forced into existence by waves, or inland pre- 

 cipices being the result of slow atmospheric agencies ; of gaps being 

 cut in the land and portions isolated by marine action ; and of deep 

 valleys being gradually eroded by ice or rain ; bixt the production of 

 large plains, irrespective of the kind or stratification of their rocks, 

 is not so easy to realise, perhaps because we do not absolutely see 

 them in process of formation. 



Knowing that the sea continually eats away the land, we can 

 imagine that its action, being continued indefinitely upon land at 

 rest, would produce some sort of plain ; but it is hard to conceive of 

 this horizontal action upon rock, insoluble in salt water, having no 

 limit ; while if the land were changing its position, slopes would be 

 more likely to result than either cliffs or plains. 



Most plains are formed of ground sloping at various low angles (in 

 the case of alluvial flats, even these low angles are concealed by a 

 surface more nearly horizontal), but although the sea seems to be the 

 most likely cause for such an undulating surface, we do not generally 

 find that plains are bordered by old sea-cUffs. It may be said that 

 these could only be expected where an interval of rest took place 

 in the elevation of the land ; yet while this seems reasonable, we 

 must bear in mind that something approaching to such an interval 

 appears to have been requisite for the production of a plain. 



We may suppose, if the space between England and Ireland were 

 elevated above sea level, a large but not quite horizontal plain would 

 exist, with marked lines of cliff along each side : the ground would 

 be covered with recent marine deposits — rivers would drain the 

 water off, and yet it would be difficult to say that none of the con- 

 figuration of the land was due to old subserial denudation, as we 

 know that land beyond the limits of the coast line of Ireland has 

 been submerged. 



The old sea-cliffs would mark the passage from lower to steeper 



near the latter of the above localities, and the ancient graveyard of Mahim, partly- 

 washed away at the northern extremity of Bombay Island. 



1 In his paper " On Eiver Valleys of the South of Ireland." Quart. Journ. Geol. 

 Soc, London, Nov., 1862 (p. 402). 



