1;§47J PRESTWICH ON THE BAGSHOT SANDS. 385 



sounding the sands and gravel with iron rods. Some of these con- 

 cretions attam a size of ten to twelve feet across, and three to four 

 feet thick. Flint pebbles, sometimes only slightly rolled and angular, 

 at other times perfectly rounded, occur in them. The sandstone is 

 friable when first excavated, but hardens by exposure. This upper 

 division of the Bagshot sands is best exhibited in the ridges of Frimley 

 and Chobham, which are formed entirely of it, and in the heaths of 

 Bagshot, Hartford Bridge and Sandhurst. Its thickness in the first 

 of these localities appears to be as much as 250 to 300 feet. These 

 sands are usually very barren, but the lower beds near the central 

 green sand become more fertile, and the cultivation of them is gradually 

 extending. 



Position of the Bagshot Sands. 



Such are the subdivisions of the Bagshot sands, and such their 



general Hthological structure. As a mass they are almost entirely 



''- siliceous, and rarely exhibit even a trace of carbonate of lime. The 



contrast with the preceding nearly pure argillaceous series of the 



London clay is marked. 



As it will be necessary to bring forward all the organic remains as 

 evidence of the age of the Bagshot sands in another part of this paper, 

 we will for the present pass them over, and proceed to the discussion 

 of the main question, viz. with which of the Hampshire and French 

 tertiary strata are the Bagshot sands synchronous * ? 



From a careful examination of all the phsenomena, I am convinced 

 that they cannot be associated with the Hampshire Freshwater strata, 

 or with the sands above or immediately below them, nor yet with the 

 Gres de Fontainbteau or the Gres de Beauchamp ; but that, however 

 anomalous it may appear at first sight, they must be considered as 

 the equivalent of that group to which the rich fossiliferous beds of 

 Bracklesham belong ; that they are synchronous with the central 

 vertical strata at White-Cliff Bay, and with the great central mass of 

 variegated and light-coloured sands of Alum Bay (see PI. XIV. Comp. 

 Sec. fig. 3 to 1 1), including possibly the Barton clays. At all events 

 these Bagshot sands are older than the Freshwater or Fluvio-marine 

 series. ^ 



Such being their position in England, they would I think be repre- 

 sented in the French tertiaries by the lower part of the Calcaire gros- 

 sier and the Glauconie grossiere. They would thus occupy a low 

 position in the Eocene series, following in uninterrupted sequence the 

 formation of the London clay, and being in perfect conformity with 

 the progressive development of the marine strata of the Hampshire 

 tertiaries. 



In instituting a comparison of superposition, mineral structure and 

 fauna between the Bagshot sands and the Bracklesham series, we 

 must take as our Hampshire type that section which best exhibits 

 the sequence of the strata and the range of organic remains. For 



* I am aware that I am here deviating from the usual plan, but I am following 

 the course of my own investigation, in which I was chiefly guided by a study of 

 superposition and physical conditions. It was only at a very late period that I 

 obtained any decided corroborative evidence by organic remains. '' 



