230 On the Silica Strata of the Lower Chalk. 



much difference will be observed in the chemical components of 

 the rock, illustrations of which will be seen in the analyses which 

 follow. Any good geological map will indicate the outcrop- 

 pings of this rock under the generic appellation of ''fire-stone." 



The escarpments of the north and south downs of Kent, 

 Surrey, Hampshire, and Sussex, and those of the chalk hills of the 

 Isle of Wight, give the best display of this stratum. Through- 

 out these districts there are two beds which receive from agri- 

 culturists the local name of " malm " or marl ; viz. the " grey 

 marl " described in vol. xii. of this Journal, which lies above 

 the phosphatic bed of green marl, containing a high per-centage 

 of carbonate of lime ; and the soft brown rubbly rock, lying 

 between the gault and the harder portion of rock. This harder 

 rock constitutes the true fire-stone. In some localities the lower 

 malm contains very little, in others, no carbonate of lime. 

 Confining ourselves for the present to the above districts, we will 

 give as brief an abstract as possible of the observations of our- 

 selves and others relative to the fire-stone rock' as a whole, 

 commencing at Folkstone in Kent and following the escarpment 

 of the north and south downs, till we meet the sea again at 

 Southbourne in Sussex ; and thence afterwards to the Isle of 

 Wight. But we think it not amiss to remind our readers that 

 the boundaries of this stratum are unmistakeable, viz. the 

 gault, or black-land, as it is sometimes termed, and the bed of 

 green fossiliferous marl below the lower chalk. 



At Folkstone the fire-stone rock, exclusive of the chloritic 

 bed of marl, is not more than from 10 to 20 feet in thickness. 

 It follows the escarpment of the chalk hills through Holling- 

 bourne, Maidstone, Wrotham, and Godstone, and thence to 

 Merstham in Surrey. At the latter place, according to Dr. Fitton, 

 the thickness ranges from 60 to 80 feet. From Merstham to 

 Farnham the upthrow is nearly vertical, consequently the de- 

 velopment of the several strata is greatly obscured by detritus. 

 But from the centre of the parish of Farnham through Bentley, 

 Binsted, and the intervening parishes, through Selborne to Peters- 

 field in Hampshire (the dip of the strata being small), there is 

 a widening expanse of this rock in the main agreeing with the 

 inclination of the present surface soil ; so that from Farnham to 

 Petersfield there is a superficies of several square miles of this 

 silica rock, with a depth of from 50 to 200 feet. 



Perhaps it may be as well here to notice the description 

 Dr. Fitton gives of the upper greensand at Farnham, as he 

 there describes a section given by the present writers, which is 

 in fact a portion of Mr. Paine's land whence our lot of specimens 

 was taken : — 



" The upper greensand forms a slight prominence beneath the chalk, on 

 the south of Dippen Hall House, between the words ' Dean's farm' and ' Ridg- 



