THE SOUTH-EASTERN COALFIELD 403 



only a purely local feature, an arenaceous facies of the basement 

 bed of the Chalk, known as the sub-zone of Stauronema carteri, 

 a fine sponge which is not rare. Though nearly 16 ft. thick 

 at its outcrop at Folkestone, this bed is not persistent inland. 

 It is scarcely represented in the Tilmanstone pit, but is 4 ft. 

 thick at the Woodnesboro' boring. 



The total thickness of the zone of Ammonites varians is 

 58 ft. 



The Gault. 



Where the Glauconitic Marl is poorly represented, the passage 

 from the Chalk to the Gault is gradual. The base of the Chalk 

 is richer in lime than in marl : the top of the Gault is richer in 

 marl than in lime. 



The Upper Gault, or zone of Ammonites rostratus, thickens 

 inland, for while the total thickness of the Gault at Folkestone 

 barely exceeds 100 ft., it approaches nearer to 200 ft. as we 

 proceed inland towards the north-east. The following are the 

 figures for the various borings : 



Dover 



121 ft. 



Ellinge 164 „ 



Ropersole 119 „ 



Waldershare ....... 163 „ 



Fredville 147 „ 



Goodnestone . . . . . . . 181 „ 



Barfrestone (upper limit doubtful : probably over 1 50 ft. thick). 



Tilmanstone Pit 129 „ 



Woodnesboro' . . . . . . 150 ,, 



In this connection it is worth noting that borings at Chatham 

 and at Frindsbury, north of Maidstone, proved a thickness of 

 192 and 193 ft. respectively, and at Trottescliff the bottom was not 

 reached after 183 ft. had been passed through (cp. Jukes-Brown, 

 Cret. Rocks Brit., i. p. 86, Mem. Geol. Surv. 1900, and Whittaker, 

 Q. J. G. S., xlii. p. 38, 1886). It will be seen that these thick- 

 nesses are by no means exceptional when we note that 181 ft. 

 of Gault was proved at Goodnestone. This thickening to the 

 north is balanced by an attenuation to the south, for the 112 ft. 

 at Dover, and 100 ft. (approximately) at Folkestone is reduced 

 to only 42 ft. at Wissant in the Boulonnais. It seems to be the 

 Upper Gault which suffers'; the Lower Gault, that is the zone 

 of Ammonites lautus and Ammonites interruptits, the part beneath 

 the bed with crowds of crushed Inoceranius sulcatus, remaining 



