On the Silica Strata of the Lower Chalk. 



231 



way ' on the Ordnance map, the strata dipping not much more than 5" to the 

 north. The rock, which is there called ' Marlstone,' does not precisely agree 

 with any I have seen between this place and the coast, but is very like some 

 of the strata in the corresponding place at East Knoyle, on the north of the 

 vale of Wardour. It is a sub-calcareous sandstone, or variety of ' firestone,' 

 very soft, uniform, of a yellowish-green, or cream-colour, scarcely effervescent, 

 and remarkable for its lightness ; and it includes concretions of a hard 

 splintery limestone, approaching to chert, and of much greater density than the 

 stone which surrounds them." 



The " chert " here alluded to consists of masses of the blue 

 building limestone which occupy the upper part of the series. 

 The chert of the Isle of Wight and elsewhere has a more flinty 

 appearance than this rock, which is indeed a true limestone. 



From Farnham to Petersfield there occurs the greatest expanse 

 of the gault and the firestone rock anywhere to be found in this 

 kingdom : and this silica rock occupies, as above stated, a 

 surface of several square miles. To the readers of this Journal 

 it will be unnecessary to recapitulate the outcropping of these 

 strata below the ranges of the Chalk Hills, as they are fully 

 detailed in our paper on the Phosphoric Strata of the Chalk 

 Formation, vol. ix., page 56 to 84, and in the publications therein 

 referred to. The chalk, marl, and underlying firestone, or silica 

 rock, are there generally described as follows. Then our object 

 was to describe the j^hosphatic beds of this remarkable deposit ; 

 now it is to supply more definite information than has hitherto 

 existed of its silica beds : — 



" Immediately below the soft marl bed the first division of the greensand 

 formation commences, viz. the upper greensand and firestone rock. It com- 

 monly comprises three distinctive subdivisions. The first is a thin green 

 band of marl, more or less silicious, abounding in organised fossil remains ; it 

 lies below, and is in contact with, the soft dirty white marl above mentioned. 

 In thickness it varies from a few inches to 10 or 15 feet. To this division the 

 attention of agriculturists is particularly invited, it being most remarkably rich 

 in phosphate of lime. It rests upon a rubbly mass of broken-up rock, from 10 

 to 20 feet thick, which is also impregnated with a notable quantity of phos- 

 phitic matter. In this lower mass are nodules of a purely white substance, 

 thickly interspersed, which, with the peculiar green colour of the bed above 

 them, maybe serviceable as a means of identi lying the stratum in different 

 situations. 



" The second subdivision is the firestone rock, or building-stone. Its 

 thickness is extremely uncertain ; in some places consisting merely of one bed 

 of stone, while in others it forms a series of layers, the aggregate thickness of 

 which reaches to nearly 100 feet, as, for example, at the Underclift'of the Isle 

 of Wight. This rock becomes softer in its lowest position, and gradually 

 merges into a soft clayey marl, which constitutes the third subdivision. This 

 again, in its inferior parts, becomes more and more argillaceous, until it is 

 finally lost in the gault or blue marl stratum." 



The valleys of Wardour, Warminster, and Pewsey, in Wilt- 

 shire, have considerable outcroppings of the firestone rock. 

 These tracts all possess a high reputation for their agricultural 

 fertility, but we have not had an opportunity of examining the 

 constituents of the rock from these localities. 



