516 Reviews — Whitakers Geological Model of London, 



Memoir on the Drifts of the district for a careful classification of the 

 gravels. All that we could look for in the Model is the distinction 

 between the gravel-beds proper and the pebble-beds. Of the latter 

 Shooter's Hill is given us as an example. Like the pebble-beds at 

 Hampstead Heath, Barnet, Totteridge, and elsewhere, it is formed of 

 local materials, and may be, as Mr. Searles Wood, jun., once sug- 

 gested, a second-hand accumulation of the Bagshot beds. This 

 would agree with the prior date which is assigned to the pebble- 

 beds as against the Glacial gravels. Although they may have been 

 disturbed during the Glacial Period, the pebble-beds have not been 

 adulterated with rocks foreign to the district. 



The Thames Valley, as seen in the Model, is in its way a sensa- 

 tion. Whatever may have been the condition of the earlier valley, 

 and whether or not it may have been more symmetrical than now, — 

 as, for instance, when the river was cutting down its bed from the 

 higher to the lower terraces which may be seen at Acton and Ealing 

 to-day, — certainly a very feebly-marked main valley is all that 

 remains for the study of Quaternary geographers. We see before 

 us an extremely wide, irregular, and shallow depression, with no 

 traces of the sides of a containing trough, and a generally ruinous 

 condition of the rocks on either side — a good contrast to those dis- 

 tricts of our island where harder rocks narrow and deepen the action 

 of the streams. Nothing but a model can give a true idea of the 

 so-called Thames Valley. 



It would greatly add to the value of this Model if a card were 

 affixed to it bearing a few notes on the characteristic landscape con- 

 tours of the district and their geological causes. Such notes have 

 been already embodied by Mr. Whitaker in the Memoir of the Chalk 

 and Eocene Beds of the London Basin, already referred to, and might 

 well be reproduced in elucidation of the Model. Let the visitor, 

 for instance, refer to the south side of the Model, where Shooter's 

 Hill is shown in vertical section. He will observe that this eminence 

 is an example of the fact that the stratigraphical arrangement of the 

 beds is often the reverse of the contours, and that a geological hill is 

 often in a geographical trough. Let him compare this fact with the 

 following note : " When the beds have been thrown into the form of 

 a trough or basin, be it ever so gentle, there they oppose greater 

 resistance to denudation from their inward dip." (Whitaker, Memoir, 

 pp. 355, 356.) 



Elsewhere Mr. Whitaker has pointed out how Blackheath and 

 similar shingly areas assert the tendency of the pebble-beds and 

 gravel-deposits to form flats. 



The gently rising hills of the London Clay are thus explained ; 

 " FoiTnations which are of uniform structure throughout, and com- 

 paratively thick and wide-spreading, generally give rise to a more or 

 less undulating country. The London Clay, though tenacious, is 

 prone to slip, and cannot support a high angle of slope." — Memoir, 

 p. 356. 



The form of the ground around London has a distinctive and ex- 

 pressive physiognomy of its own, and a selection of such 



