Corre!ipondence — Rev. 0. Fisher. ^-99 



hood to be able to contribute many observations to the elucidation of 

 the question at issue regarding the age of the Thames valley. But, 

 as I have already stated in your pages, I cannot admit the presence 

 of the re-arranged material, which I call " trail," to be any proof of 

 geological antiquity ; regarding it, as I do, as an accompaniment of 

 the last general denudation of the surface. When Mr. Wood says 

 that he does not admit the existence of this deposit (though it is not 

 strictly a deposit) as ''a formation,"^ I understand liim to mean that 

 he thinks that peculiar condition of the sub- soil to be of various 

 geological ages, from the glacial drift upwards, instead of referring 

 it to our period, as I do. Thus, although we disagree upon the age 

 of the trail, we are in accord as to its existence, and also as to its 

 having no bearing on the question in hand, viz., the antiquity of the 

 mammaliferous deposits of the Thames valley. 



It was asked during the discussion on Mr. Dawkins' paper, why 

 the Boulder-clay did not cross the valley of the Thames. I then 

 offered the suggestion that the cause might be found in the elevation 

 of the Weald. Denudation is a function of altitude. In a given dis- 

 trict it requires a certain amount of coherence in the constitution 

 of a deposit to enable it to resist destructive influences of altitude. 

 Hence, if the Boulder-clay was once spread over the North Downs — 

 and they have been raised higher since — we need seek no other rea- 

 son for its disappearance in that area. There is, I believe, a parallel 

 case in Hants and Dorset. The elevation of the southern part of the 

 Isles of Wight and Purbeck, and of the Weymouth district, south of 

 the uplifted chalk, is probably of the same date as that of the Weald. 

 Now, in the south-western counties, the Boulder-clay, as I believe, 

 is represented by the thick bed of coarse flint gravel which forms 

 the capping of most of the tabular hills of the New Forest and of the 

 Tertiary country of South-east Dorset. But this bed of gravel does 

 not cross the Chalk Downs. It appears to have been lifted up and 

 carried away together with all the other deposits which once lay 

 upon the Chalk ; and, in passing, I may mention that the Tertiary 

 strata which cap Eidgway Hill near Weymouth are vertical, being 

 just as much affected by the disturbance as those of Alum Bay. 



Now the Thames valley is so near the northern boundary of the 

 Weald that we may well conceive the local disturbance to have been 

 felt in it. And, indeed, the occurrence of a fault bringing up the Chalk 

 against the London clay near Purfleet is probably part of the same 

 movement. Again, the altitude attained by the Middle Drift along 

 the hills south of Chelmsford and Witham is almost in itself suffi- 

 cient proof that the disappearance of the Boulder-clay in that direc- 

 tion is due to denudation. 



My impression is, that the mammaliferous bed of Grays Thurrock 

 is of the same age as that of Clacton. It is possible that species may 

 be present in the Clacton deposit which have not been collected, for 

 it is most difficult to obtain specimens there. The late Mr. John 

 Brown, by a combination of assidiiity and good fortune, obtained a 

 good many ; but although I watched the place for nine years I never 

 1 Geol. Mag., Vol. V. p. 43. 



