522 



DETRITUS OF SILURIAN RECAPITULATION. 



In concluding this chapter, I may state, that after a patient examination of the older 

 drift and its purely local character throughout this region, its production is best ex- 

 plained by reference to those movements by which the solid strata were successively 

 upheaved from below the sea ; on which occasions, the beds being much shattered, all 

 loose fragments were thrown off upon their slopes. I may, perhaps, venture further, 

 and assert that the following positions have been established. 



1 . That the drifts being of local origin, the agency must also have been local. 



2. That the dry combs and depressions in which no streams flow, but in which large 

 masses of drift are lodged, must have been excavated or modelled by a body of water 

 which once filled them ; and as that water could not have been derived from the drainage 

 of the country, it consequently must have been produced by the combs being below the 

 level of the then existing seas, and having a direct communication with them. 



3. That the action of this water, affected either by tides or periodical freshes, pro- 

 duced in part the detritus, wearing it down into pebbles and sand. 



4. That the volcanic outbursts and local disturbances, of which so many proofs have 

 been adduced, caused, from time to time, tumultuous currents, which accumulated the 

 irregular heaps or hillocks of coarse drift ; the same disturbances probably producing 

 permanent changes in the relative level of sea and land. 



5. That these operations gradually changed the hydrographical characters of the 

 districts, and eventually converted the estuaries into lakes and rivers. The processes 

 by which these bodies of water were filled up or reduced to their present size will be 

 considered in the sequel. 



Whether all these deductions be admitted or not, it must be acknowledged, that as 

 Siluria is exempted from all the far transported detritus so prevalent in the surrounding 

 parts of Great Britain, the region cannot have been visited by a great deluge flowing 

 from other countries after its features had been determined and its present valleys exca- 

 vated. In this respect, indeed, it bears the same relation to the British Isles as Auvergne 

 does to the whole of France ; the surfaces of both these isolated tracts being covered by 

 local detritus only, though environed by countries loaded with foreign debris 1 . 



1 See Lyell and Murchison on the Excavation of Valleys in Central France, Edin. New Phil. Journal, vol. vii. 

 p. 15. I hope not to be charged with egotism when I state, that this memoir, in conjunction with the writings 

 of Mr. Poulett Scrope, was among the first which fairly brought the diluvial question to issue in this country, 

 and gave rise to those discussions which led to the refutation of the belief in a general terrestrial deluge, which 

 had affected simultaneously all the surface of the globe. 



