ON SUB-WEALBEN EXPLORATION. 493 



mined the Director of the Survey to separate these beds. The clays of the 

 coast are now called " Fairlight Clays '' (Mr. Gould's term), while the lowest 

 limestone scries of the inland district are coloured as Purbecks. 



The difference in the strata rendered such a change in the classification 

 desirable, and it has been confirmed by the discovery of thick beds of gypsum 

 in the Sub-Wealden boring. Although found in detached blocks in the 

 Purbeck beds of Dorsetshire, gypsum was unknown before in the Weald. It 

 should also be noticed that the Ashdown Sand, which comes between the 

 Fairlight Clays and the Wadhurst Clay, is only about 150 feet thick at 

 Hastings ; but the Ashdown Sand of the Brightling district (including under 

 that term all the strata intervening between the Wadhurst Clay and the 

 Purbecks) is 300 or 400 feet thick. We may then fairly assume that the 

 lower part of these sands (which is much more clayey than the upper part) 

 represents the Fairlight Clays of the coast. 



In classing these beds with the Purbecks we are only repeating the 

 opinion of Mr. Conybeare (the earliest geological writer on the district). 

 Subsequently they have been referred to the Purbecks by Sir H. De la Beche, 

 Prof. Edward Forbes, Dr. Fitton, Mr. Godwin- Austen, and others ; in some 

 cases even by Dr. Mantell himself. 



The Purbeck beds of Sussex consist chiefly of clays and shales. The lime- 

 stones are chiefly found upon two horizons — an upper one, called " the 

 greys " or " the vein-greys," and a lower one, called " the blues." These are 

 separated by about 100 or 140 feet of shales, interspersed with only a few 

 thin beds of limestone and a little sandstone. Below the "blues" are impure 

 limestones (bastard blues). The lowest strata known previous to the boring 

 were the " dunk shaws," thin flaggy limestones found in Eounden Wood, 

 crowded with Ctipridea valdensis. 



The total thickness hitherto known may be estimated at a little over 

 300 feet. 



The boring at Netherfield began at a point about 250 feet down in the 

 Purbecks, just below the "blues." Mr. Willett and Mr. AV. Boyd Dawkins 

 proposed the site ultimately chosen ; and no other spot in the district would, 

 all things considered, have presented equal advantages. 



The strata passed through up to the present time (September 1873) are as 

 follows : — 



Strata, Thickness. Depth from surface. 



ft. in. ft. hi. 



Shales ]G G 



Bhie Ihnestone 2 6 19 



Shale r, 24 



Blue limestone 2 26 



Shale 4 30 



Limestone 16 31 6 



Shale 4 35 6 



Limestone 3 38 6 



Shale 4 42 6 



Limestone 4 46 6 



Hard blue shale 15 6 62 



Hard grey shale 3 65 



Hard shale 14 6 79 6 



Shales, with crystals of carbonate of lime 9 i?8 6 



Grey shale 13 101 6 



Greenish shales, with gypsum veins 20 121 6 



Impure gypsum 8 6 130 



Pure white gypsum 4 1 .34 



Tmpuro gypsum 5 6 139 



