﻿1850.] 



TRIMMER ON THE ERRATICS OF KENT. 



33 



of the boulder clay, or lower erratic tertiaries, and these again by the 

 sand and gravel of the upper erratic tertiaries, and if we find on the 

 denuded surface of these erratic tertiaries freshwater beds containing 

 the remains of extinct mammals, — their surface furrowed, and the 

 furrows filled by another unconformable deposit which extends to 

 various heights, and fills similar furrows in the denuded surface of 

 the erratic tertiaries and older strata, — there can be no doubt that 

 the operations which produced these phenomena at higher levels 

 than the freshwater beds, and on the denuded surface of older strata, 

 were subsequent to the interval during which the desiccated bed of the 

 glacial sea was a terrestrial surface, inhabited by the large pachyderms. 

 Again, if tracing the deposits of the glacial sea southwards, from 

 Norfolk to the northern edge of the valley of the Thames, we there 

 find gravel resting on the boulder clay, we may safely identify that 

 gravel with the upper erratics of Norfolk, even although it may occur 

 in disconnected patches, the result of denudation — a form in which 

 it is also frequently found in Norfolk, and notwithstanding some dif- 

 ference of lithological character ; and if, on the south of the valley 

 of the Thames, we find similar gravel to that of its northern edge at 

 levels which prove them to be disconnected portions of a stratum 

 once continuous, we may safely identify the gravel south of the Thames 

 with the upper erratics of Norfolk, even although the lower erratics 

 be absent, and although the gravel be in immediate contact with the 

 eocene tertiaries or the chalk. At any rate the onus of proof lies 

 with those who assign to it a higher antiquity. 



Again, if at lower levels, within the valley of the Thames, at short 

 distances from the present stream, and at certain heights above it, 

 bones of extinct pachyderms and fluviatile shells, — the shells with 

 one exception identical with those now inhabiting the neighbourhood, 

 — are buried beneath gravel composed of the same materials as that 

 at the higher levels, it is a legitimate inference that the gravel in the 

 trough of the Thames has been derived from that of the higher levels, 

 brought down by denuding processes ; and that the mammalian de- 

 posit is more recent than the upper erratic tertiaries, and consequently 

 than the Norwich Crag*. 



This inference is indeed confirmed by granitic detritus being found 

 associated with the mammalian remains, since debris of this rock does 

 not occur in the Crag, or in any tertiary strata older than the glacial 

 submergence. 



Against such evidence we should be wrong in inferring that the 

 mammalian deposits of Clacton and Brentford were of different ages, 

 because the one contains only Rhinoceros leptorhinus and the other 

 only R. tichorhinus, which were co-inhabitants of Britain before the 

 glacial submergence ; and because, at Clacton, in addition to the living 

 species of molluscs of the neighbourhood, common to the two deposits, 



* Substituting denudation during the process of elevation for the " retiring di- 

 luvian waters," this is a repetition of the argument by which Dr. Buckland (Geol. 

 Trans. 1st Series, vol. v. p. 521) maintained that the excavation of the valley of 

 the Thames had taken place after the transport of the Warwickshire gravel con- 

 taining pebbles of the Lickey quartz rock. 



VOL. VII. — PART I. D 



