Reviews — Geological Survey of England. 133 



The geology, originally surveyed and pviUished in 1861, included 

 the Gault, Upper Greensand, Chalk, Eeading-heds, London-clay, 

 Lower Bagshot-beds, the Thames-valley gravel and Alluvium. 

 These were mapped by Messrs. H. Bauerman, W. Whitaker, T. E. 

 Polwhele, and E. Trench. Some revision of this work has now 

 been made by Messrs. H. W. Bristow and W. Whitaker. 



The Drift deposits, which are the important features of this new 

 map, are as follows : 



Dry valley flint Gravel. Eainwash. 



Eiver Gravel and Brick-earth. 



Clay-with-flints. 



Brick-earth. 



Boulder-clay ) r\^ • ^ ^ •*. 



r^ 1 n"^ CI 1 f Glacial deposits. 

 Gravel and band ) ^ 



Pebble Gravel. 



These deposits have been mapped by Messrs. H. W. Bristow, W. 

 Whitaker, H. B. Woodward, F. J. Bennett, W. A. E. Ussher, J. H. 

 Blake, and C. E. Hawkins. 



The general characters and positions of these deposits were indi- 

 cated by Mr. Whitaker in his Memoir on the Geology of the country, 

 published in 1864; but since this much has been done, and particularly 

 by Mr. Searles Wood, jun., in the classification of the various drift 

 deposits of the East of England. 



Although in the list of deposits shown in Sheet 7 no very precise 

 chronological system has been adopted, this could hardly be expected 

 in the earlier drift maps, as, until a very large area has been gone 

 over, such a system could not be arrived at. 



It is not quite clear at present whether the '' pebble gravel " be of 

 a distinct age from the gravel and sand of the Glacial period, as Mr. 

 Hughes would have it, in the neighbourhood of Hertford, or whether 

 we may not regard it, as Mr. Wood does, as a portion of the Mid- 

 Glacial series, owing its pebbly nature to local derivation from 

 Eocene pebble-beds. 



The brick-earths which cap the high ground near High Wycombe, 

 Hampden, Chesham, and St. Albans, may be of different ages, some 

 portions belonging apparently to Mid-Glacial series being associated 

 with the gravel and sand, some associated with the clay-with-flints, 

 being probably due to local denudation of the Tertiaries ; some por- 

 tions appear to overlie the Boulder-clay, and therefore to be of Post- 

 Glacial age. 



These theoretical questions, however, do not affect the value of 

 the map, for whatever age be assigned to the several deposits, 

 the boundary lines will not be affected, and these it is which give 

 the map a permanent value. 



We understand that a large geological map with London as a 

 centre will shortly be published by the Geological Survey. This 

 has long been a desideratum, and we doubt not that it will be appre- 

 ciated, not only for the economic uses to which it may serve, but also 

 as a guide to geological students and local observers, of which latter 

 unfortunately there are but few in the country included in Sheet 7. 



