THEIR GEOLOGICAL AGE. 209 



his opinion that ' it belongs doubtfully to the same period 

 as the Kingston, Brentford, Kew, and London beds, which I 

 am inclined to consider as among the most recent of our 

 drift-deposits, and as posterior to the period of the great 

 boulder-clay of Norfolk and Suffolk ; but the relation of these 

 beds is nowhere clearly seen : ' and he adds that this ques- 

 tion of relative age depends on a variety of collateral evi- 

 dence which he hopes to lay before the Society at a future 

 period. 



It would be presumptuous in me to question the stratigra- 

 phical residts of a distinguished practical geologist, so emi- 

 nent for his profound knowledge of the tertiary formations as 

 Mr. Prestwich. But if by the passage I have cited it is 

 implied that the Brentford fluviatile strata containing the 

 mammalian remains described by Mr. W. Kirby Trimmer and 

 other beds of the same age, in the Valley of the Thames, are of 

 a later age than the boulder-clay, the conclusion is entirely 

 at variance with the mammalian fossil evidence, and would 

 involve the startling corollary that, after the Norfolk lacus- 

 trine beds had been submerged for a long term of the glacial 

 period, during which enormous deposits of boulder-clay and 

 till were accumulated above them, the Pliocene mammalian 

 fauna again reappeared on the emerged siirface with a 

 restoration of all the warm climatal conditions. I believe 

 that it will be found that, while the Thames Valley fluviatiles 

 at Grays Thurrock and elsewhere form a terrace on either 

 side of the valley on a higher level than the 'valley' or mam- 

 moth gravel spread out below at Brentford, or in its imme- 

 diate vicinity, the gravel-beds are in direct superposition to 

 the fluviatile strata, and that the mammalian remains found 

 there belong to two distinct geological ages, according to the 

 depth below the surface of the bed from which they are ex- 

 humed. These upper gravel-beds I believe, with Mr. Prest- 

 wich, to belong to a date posterior to the boulder-clay and 

 till. The enumeration of the Brentford fossil mammalia 

 given by Mr. Morris includes both the Pliocene and the 

 Post-glacial fauna. I have seen grinders both of Euelephas 

 antiquus and of the true Mammoth from the Brentford 

 section. 



Mr. Joshua Trimmer, in his memoir entitled, ' Generali- 

 zations on the Erratic Tertiaries of Norfolk,' among other 

 results gives the following important conclusion :— That with 

 regard to mammalian remains, ' We have two elephantine 

 periods, one preceding the submergence of the erratic period, 

 and the other inhabiting the country at the close of the period 

 of elevation. To the former are to be referred the mammalian 

 Crag, and the remains of the bone caverns in general ; to the 



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