Ch. V.] ORIGIN" OF GREAT FAULTS. 65 



some faults, on sound mechanical principles, by a succession of move- 

 ments, is far preferable to any theory which assumes each fault to have 

 been accomplished by a single upcast or downthrow of several thousand 

 feet. For we know that there are operations now in progress, at great 

 depths in the interior of the earth, by which both large and small tracts 

 of ground are made to rise above and sink below their former level, some 

 slowly and insensibly, others suddenly and by starts, a few feet or yards 

 at a time ; whereas there are no grounds for believing that, during the 

 last 3000 years at least, any regions have been either upheaved or de- 

 pressed, at a single stroke, to the amount of several hundred, much less 

 several thousand feet. When some of the ancient marine formations are 

 described in the sequel, it will appear that their structure and organic 

 contents point to the conclusion, that the floor of the ocean was slowly 

 sinking at the time of their origin. The downward movement was very 

 gradual, and in Wales and the contiguous parts of England a maximum 

 thickness of 32,000 feet (more than six miles) of Carboniferous, Devonian, 

 and Silurian rock was formed, whilst the bed of the sea was all the time 

 continuously and tranquilly subsiding.* Whatever may have been the 

 changes which the solid foundation underwent, whether accompanied by 

 the melting, consolidation, crystallization, or desiccation of subjacent min- 

 eral matter, it is clear from the fact of the sea having remained shallow 

 all the while that the bottom never sank down suddenly to the depth of 

 many hundred feet at once. 



It is by assuming such reiterated variations of level, each separately of 

 small vertical amount, but multiplied by time till they acquire importance 

 in the aggregate, that we are able to explain the phenomena of denuda- 

 tion, which will be treated of in the next chapter. By such movements 

 every portion of the surface of the land becomes in its turn a line of coast, 

 and is exposed to the action of the waves and tides. A country which is 

 undergoing such movement is never allowed to settle into a state of equi- 

 librium, therefore the force of rivers and torrents to remove or excavate > 

 soil and rocky masses is sustained in undiminished energy. 



* See the results of the " Geological Survey of Great Britain ;" Memoirs, yols... 

 i. and ii. by Sir H. de la Beche, Sir. A. C. Ramsay, and Mr. John Phillips. 



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