CHAPTER XXXVII. 



SUPERFICIAL DETRITUS. 



Introduction. — Division of the subject of superficial detritus. — Drifts comprised 

 within the hydrographical region of Siluria. — Ancient or submarine condition 

 of this region. — All its ancient drift shown to be of local origin, and 

 caused by local agency of tides and currents, influenced by elevations of the 

 bottom of the sea. — Such drifted matter found on slopes descending from ele- 

 vated ranges, occupying both lofty and low positions. — Enormous extent of 

 ancient denudation in the transverse valleys now watered by the tributaries of 

 the Severn. — Distinctions between marine drift and fluviatile accumulations. 

 — Recapitulation. 



Local Drifts within the region of Siluria. 



The description of all the rocks of aqueous or igneous origin, and the account of their 

 various dislocations being completed, I shall terminate the history of the physical 

 geology of the region, by pointing out the nature of the various loose materials which 

 encumber the surface of extensive tracts, but which could not be intelligibly treated of, 

 without a previous explanation of the nature of the rocks from which they were derived, 

 and of the operations which led to their distribution. 



This subject is naturally divisible into two classes. The first includes all those coarse 

 and sometimes far transported fragments, to which some geologists apply the word 

 " diluvium," but which to avoid misconstruction I designate drift 1 . 



1 " Diluvium," as used byElie de Beaumont and the modern foreign geologists, means precisely what I term 

 " drift." In England, however, where the results of distinct physical operations or various mutations of the 

 relative levels of sea and land, were formerly merged under one head, and in some instances forced into accord- 

 ance with the effects of the deluge which destroyed the human race, most geologists have abandoned the use of 

 a word which has been so misapplied. He who connects diluvium with the Deluge of Holy Writ must contend, 

 that all such detritus was produced in one short period. But geologists having now completely ascertained, 

 that each region of the earth has its own superficial diluvia, produced by distinct and separate action, the 

 unambiguous word drift is proposed, which when preceded by the name of the tract whence the materials were 

 derived, expresses at once the intended meaning. Hence, "Silurian drift," "Cumbrian," (or as respects 

 England) "Northern drift," "Scandinavian drift," &c. &c. 



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