feY HTGH MILLEil. 291 



above. But now the summit of the cupola is away; the outer 

 beds of Old Eed Sandstone and Upper Silurians end like books 

 laid sloping against an inverted bowl, and the scars of the outer 

 envelope stand four or five miles apart. The missing arches have 

 been planed off, and according to a moderate computation of Prof. 

 Ramsay's, a thickness of three thousand five hundred feet has 

 been wholly removed. And this is a very small figure compared 

 with others to which his survey of South Wales led him.*' 



Since Eamsay first gave his explanation of these facts the 

 world has become largely familiarized with it, but in order to 

 initiate a few words upon the general subject of denudation, let 

 us ask once more. How came these changes ? 



Denudants. — Eain, frost, and rivers; waves and currents of 

 the sea, and glaciers ; have all at one time or another lent their 

 forces for the waste and destruction of what is now Britain. It 

 is of course quite impossible to define the precise part taken by 

 each in what has been accomplished, but although different geo- 

 logists have often their favourite denudants, no geologist denies 

 that they have all had their influence in geological ages even re- 

 cently by-gone. 



Every thunderstorm or winter ^' spate" gives loud proof in the 

 noisy discoloured torrents that pour towards the sea, of the activity 

 of streams. Eain obviously feeds them with water ; fi^ost and rain 

 less obviously supply them with the solid discolouring materials. 

 Every drop is charged with earthy stuff, and even in the most pel- 

 lucid stream the chemist discovers an invisible burden in solution. 



Taking first the earthy matter borne in mechanical suspension, 

 as distinguished from the substances chemically dissolved, the 

 materials lie to hand for an illustration of the amount by which 

 the British Isles are being denuded through the medium of this 

 kind of river action. It was early seen that as streams bear to 

 the sea all that is worn from the general surface of the land, the 

 aggregate amount of their burdens represents the total results of 

 surface denudation. This method was elaborated first by Dr. 



• Between the Avon and tlio Mcndlp Hills, e.g., he estimated the denudation at nine 

 thousand feet — nciily n mile and tliroe quarters. 



