292 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [JuilC 1, 



of re-elevation. If these views are correct, we may look for similar 

 outliers of the upper erratics at high levels, where a surface of the 

 upper eocene beds remains unbroken ; but we cannot expect them 

 where the lower beds or the chalk have been exposed by the denuding 

 process. Beds of gravel at low levels will consist of the materials 

 of the upper erratics re-arranged. 



The Shooter's Hill gravel having been thus formed towards the 

 close of the period of submergence, the absence of pebbles, indicating 

 transportation from the north and west, may be thus explained : the 

 Warwickshire gravel had not then passed the Cotswold range ; or, if 

 it had, it was only spreading itself over the high grounds bordering 

 the Cherwell and Thames near Oxford. 



The Dartford gravel, on the other hand, at a lower level, contains 

 those pebbles, because it was formed at a later period, during the 

 process of re-elevation, when the Warwickshire gravel had been 

 brought into the trough of the Thames by the denuding action which 

 formed that trough, and which left outliers of erratic tertiaries on the 

 summits of Bagley Wood, Cumnor Hurst, and other neighbouring 

 hills, as described by Dr. Buckland*. 



At a still more recent portion of the re-elevation period, these 

 pebbles were removed from the Dartford gravel to lower levels, and 

 formed part of the gravel of the ancient river, associated with mam- 

 malian remains at Erith, where I have found them, and at Brentford, 

 where they have been found by Mr. Morris f . The absence of the 

 pebbles of Lickey quartz-rock and other fragments of northern origin 

 from the Wilmington pits and from the Rochester gravel, within the 

 valleys of the Darent and Medway respectively, and the presence in 

 both those deposits of pebbles which appear to have been derived 

 from strata below the chalk which are exposed within the Wealden 

 area, indicate transportation outwards about the middle of the period 

 of re-elevation through the gorges now occupied by those rivers. 

 If the denudation of the Weald and the separation of the eocene ter- 

 tiaries of the London and Hampshire districts were in progress, as I 

 believe they were, during the latter portion of the period of submer- 

 gence, and while the Shooter's Hill gravel was in the course of for- 

 mation, it will explain why that gravel consists only of materials 

 derived from the chalk and eocene tertiaries, and why northern de- 

 tritus, so abundant in Norfolk, Suffolk, and Essex, is absent, as well 

 as the Lickey quartzose pebbles. The advance of these northern 

 erratics was repelled by movements of disturbance, along an east and 

 west line, which gave the currents an easterly direction. 



The mammalian deposits of the valley of the Thames must be re- 

 ferred to a subsequent period of quiescence, and may be considered 

 contemporaneous with the ancient beach at Brighton beneath the 

 bone-bed ; the materials of that beach, and of the old alluvial deposits 

 of the Thames, which had previously been transported, having been 

 re-arranged during that period of quiescence. 



* Trans. Geol. Soc. 1st Series, vol. v. p. 521. 

 t Quart. Journ. Geol. Soc. vol. vi. p. 202. 



