UPPEE EOCENE (bARTOX AND UPPER BAGSHOT FORMATIONS). 607 



common to both formations, do not help to settle the question, ex- 

 cept that some of the species are more distinctive of the latter than 

 of the former. If we take the 12 to 20 feet of green clayey sand of 

 the London basin, with Nummalites Icevigatus, to represent the 37 feet 

 of green clayey sand with the same fossils, and which is altogether 

 nndistinguishable from it, in the Whitecliff-Bay section, we should 

 have the following thickness to account for in the London basin 

 before reaching the base of the Barton series, supposing the two 

 series to be all uniform in thickness : — 



Hampshire Basin. 

 93 ft. Huutingbridge Beds, sandy clays, various. 



{5 ft. Sandstone. 

 23 ft. Brook Bed, greenish sandy clay. 

 Fecten-corneus zone. 

 66 ft. clay with belts of lignite. 



187 ft. 



Towards this we have in the London Basin : — 



Between 70 and 80 ft. of loamy sand passing into pure sand up to 

 the horizon at which the determinable fossils have been found. 



1 ft. pebble-bed. 



10 to 20 ft. of loamy sand and clays overlying the green sand. 

 So that, even allowing for very considerable thinning, we should have 

 no difficulty in placing the fossiliferous horizon in Tunnel Hill 

 beneath the base of the Barton Series in Hampshire. The palaeon- 

 tological evidence, which has been sifted with care, almost precludes 

 this, however, the list containing nine species which are peculiar to 

 or are not known to pass beneath the Barton in this country. 

 Against this we have to set the four Bracklesham species and two or 

 three undeterminable casts which are more like Cerithia of that age 

 than anything else. But, practically, we are bound to take the 

 pebble-bed as a base, since there is nothing above it which would 

 furnish any recognizable dividing line ; and to put the whole of the 

 200 feet of Upper Bagshot sand into the Bracklesham, against the 

 weight of evidence, such as it is, is out of the question. 



The area occuj)ied by the formation in the London basin is com- 

 prised in sheets 8 and 12 of the Geological Survey Map, and with the 

 exception of a small possible outlier at Highclere, near jSTewbury, 

 it only exists in the main mass of the Bagshot Beds. Easthamp- 

 stead Plain, Finchampstead Eidges, Chobham Eidges, Fox Hills, 

 Hartford Bridge Flats are formed of it. The surface is usually 

 barren heath, or is covered with plantations or woods of self-sown 

 Scotch fir, whilst the more clayey Bracklesham supports beech, 

 alder, birch, &c., and is to a far greater extent under cultivation. 

 In our sketch map (fig. 11, p. 616) the shading represents the 

 Upper Bagshot as mapped by the Geological Survey, and the 

 numbers refer to the various localities mentioned in the present 

 paper. The dividing line between the Upper Bagshot and the 

 Middle Bagshot is drawn at a higher level than that fixed on by 

 Professor Prestwich on the ground that the yellow sands contain 



Q.J.G.S. No. 175. 2p 



