Mackintosh — The Sea v. Rain and Frost. 63 



cast down so as to form an intercepted drainage, giving rise to a swamp 

 which, engendered a peat growth; by the agency of which the 

 portion of the forest so thrown down has been preserved from the 

 destruction that by atmospheric or else human agency has overtaken 

 the rest of it. Lastly, we have the introduction of the river over 

 the previous land-surface and peat-swamp, bringing sediment which 

 by its deposit has formed the marsh-clay of its former shallow bed, 

 before that bed was reduced to its present narrow limits, and 

 deepened by embankment. 



In all this, save to the extent indicated in the original valley by 

 the gravels {x 1, a; 2, and a; 3), we find nothing analogous to terrace- 

 formation, or to the modification of an estuary by the successive 

 elevation of the land and cutting down of .its bed, until the estuary 

 has become a river, as is described to us to be the case with the 

 valleys of the Somme and the Seine. 



[To he continued in our next.) 



rV. — The Sea against Eain and Fkost ; ok the Origin of 



ESCAKPMENTS. 

 By D. Mackintosh, F.G.S. 

 S every part of the crust of the earth has at one time been 

 the surface, it follows that all questions connected with the 

 origin of the present "form of the ground" must be very important, 

 and that on their issue the progress of Geology must in a great 

 measure depend. But on this subject a very wide difi"erence of 

 opinion at present exists. According to one party, consisting of 

 Professor Eamsay, Mr. Jukes, Mr. Geikie, Colonel Greenwood, 

 Dr. Foster, and others, the more abrupt inequalities of the earth's 

 surface have been produced by subaerial or atmospheric causes. 

 According to the other school, embracing Sir Charles Lyell, Sir 

 Eoderick Murchison, Professors Sedgwick and Phillips, Mr. Edward 

 Hull, etc., the sea has been the principal denuding or excavating 

 agent. 



The subaerialists start with the assumption that the sea tends to 

 planei down the land, and that its newly-elevated bed presents a 

 surface divested of abrupt heights or hollows. But this assumption 

 is at variance with the generally-received principle of physical 

 geography, that the bottom of the sea, at any given time, is as 

 uneven as the dry land ; ^ and it is equally opposed to the established 

 geological fact that mountains have been islands and islands moun- 



^ There is a very extensive series of table-shaped mountains under the North 

 Atlantic Ocean. Their comparatively level surface may be partly owing to their 

 being covered with sand. Along their western and north-western edges the depth 

 suddenly descends to 600 feet, and along the southern edge to 900 feet. The descent 

 is nearly perpendicular. Besides these inequalities in the Atlantic there are deep 

 furrows which run north and south (see article "Sea" in Penny Cyclopsedia, and 

 "Section of the Bed of the Atlantic" in Maury's Physical Geography of the Sea). 

 The bed of the German Ocean, though very favourably situated for becoming a level 

 surface, would likewise appear to be far from uniform in depth (see Lyell's Principles 

 of Geology). 



