1847.] PRESTWICK ON THE BAGSHOT SANDS. 387 



fossilifferous, and then again 50 feet of laminated gree7i sands and 

 hrown clays, making a total thickness of about 360 feet, and including 

 the equivalents of the Bracklesham Bay beds. 



This division I consider to be represented in the Bagshot sa7ids by 

 the fifty feet of central, brown, and liver-coloured clays and green sands. 

 As a question of synchronism, this difference of thickness between the 

 supposed equivalent strata of Hampshire and Bagshot is not essential ; 

 for in a distance of forty to fifty miles, the varying action of the tides 

 and currents of the sea of that period renders conceivable a greater 

 accumulation of sediment in one place than in another *, especially as 

 it is probable that it was derived from some point south-west of the 

 site of the Isle of Wight. But although the accumulation of sediment 

 may in these ancient seas have varied in quantity, from this or any 

 other cause, still, if originating in the degradation of the same land 

 and spread by the same waters, we ought to find in its mineral cha- 

 racters traces of its common origin ; and these more important con- 

 ditions we shall in this case find perfectly fulfilled. Thus we have in 

 both districts the same peculiar brown and liver-coloured clays — in 

 the Bagshot sands usually very finely foliated, and in Hampshire 

 more coarsely laminated with fine sands. The occurrence of green 

 sands is very distinctive, as in no other part of the English tertiaries 

 do they of themselves form entire and persistent strata. 



In the Hampshire series this division is characterized by occasional 

 beds of lignite and by frequent layers of rounded flint pebbles, de- 

 noting stronger temporary drifts ; so in the Bagshot series at Shap- 

 ley, Worplesdon and Goldsworthy, one or two thin, but well-marked 

 and persistent layers of round flint pebbles occur, and at the latter 

 place a compact bed of lignite ten inches thick immediately under- 

 lies the main stratum of green sand (see "1 of a," fig. 3, p. 382). 



There is however one element almost entirely wanting in the Bag- 

 shot sands, and that is the carbonate of lime, which enters, though 

 in small proportion, into the composition of many of the strata of 

 this portion of the Hampshire series f. With this exception, the 

 mineral character of this division in the Hampshire and in the Lon- 

 don districts is identical ; it is the measure only which is different. ' 



At Southampton we have lately had an opportunity of observing 

 the development of these two divisions corresponding with the lower 

 and middle Bagshot sands both in structure and organic remains. 

 With the exception of the section at the artesian well, the super- 

 position of the other strata is not exhibited. From dip and range I 

 however believe it to be as follows : — 



* I am rather inclined to attribute it however chiefly to variations in the rate 

 of subsidence during the formation of these strata. 



t It there frequently forms irregular flattened concretions, sometimes of a very 

 large size, and consisting usually of a hght green-coloured calcareous sandstone 

 slightly argillaceous. At Southampton however many of them have been found 

 bearing a curious resemblance in shape and appearance to large cannon-balls. Un- 

 like the septaria of the London clay, the interior of these concretions is solid and 

 compact. 



