Dr. FiTTON on the Strata below the Chalk. 319 



is between 70 and 80 feet' ; in the Isle of Wiglit, about 70 feet '2. At Blackdown the thickness 

 of the sand is about 100 feet^ ; in the Vale of Wardour, probably from 60 to 80* ; near Swindon, 

 30 to 50 feef": but at the Castle Hill, Cambridge, it is not more than 18 inches^. Thence 

 through West Norfolk, the stratum is not anywhere distinctly seen ; and at Hunstanton, the 

 only beds which can be supposed to represent it, are not more than 2 feet thick 7, 



Gault. — The thickness of this stratum near Copt Point, derived from barometric measurement, 

 is about 130 feet". In the interior it is difficult to obtain good estimates, and accurate measures 

 can be expected only in wells. At Merstham it is about 150 feet thick ^; in the Isle of Wight 

 probably about 70 feet '" : at Ridge, in the Vale of Wardour, it is about 75 feet " ; at Cotmore 

 Wells near Thame, 90 feet '2. In Cambridgeshire, the entire stratum has been cut through re- 

 peatedly in wells and borings, which give an average thickness of 150 feet'^ ; but at Mildenhall in 

 Suffolk, the blue clay, which seems to represent it, is only 9 feet '*. In West Norfolk Mr. Rose 

 considers that the utmost thickness cannot be more than 15 feet'*; and at Hunstanton the red 

 marly beds which are supposed to contain Gault fossils, are only 4 feet thick'®. 



Lower green-sand. — The measured thickness of this formation can be most easily obtained on 

 the shore between Copt Point and Hythe, where the uppermost subdivision is about 70 feet 

 thick"; the middle group, 70 to 100 feet'** ; and the lowest, 60 to 80 feet'S; the total thickness 

 consequently about 250 feet. In Western Sussex, Mr. Murchison states that nearly 400 feet of 

 sand were passed through in a boring, at Petworth Summerhouse^"; and Mr. Martin conceives 

 that the two lower members may be together about 150 feet thick 2'. In the Isle of Wight the 

 thickness, between Bonchurch Cove and Sandown, Rocken End and Atherfield Rocks, cannot be 

 less than that of the formation near Folkstone, and seems to be much greater. At Brill, in Buck- 

 inghamshire, about 26 feet remain 2'^; in West Norfolk, Mr. Rose considers the thickness of the 

 whole formation to be about 80 feet ^5. 



Wealden. — No measures, on good estimates, of the thickness of the strata in this group have yet 

 been obtained. Mr. Martin assigns 28 1 feet to the Weald-clay cut through in boring, at Petworth 

 in Western Sussex^* ; but in the section at Tiepit and Cowleaze Chine in the Isle of Wight, the 

 clay seems to be no more than 140 feet thick-*. 



The section of that part of the Hastings- sands which is visible between St. Leonard's Church 

 and the top of the great sand rock bed, may be about 200 to 250 feet ; the sand rock bed 

 itself, under the castle, about 80 to 120 feet; and thence to the lowest point upon the coast, 

 east of Hastings, about 200 or 250. — Total thickness, between 400 and 500 feet-s. 



It is difficult, from their contortions, to estimate the thickness of the Purbeck strata on the 

 coast. Mr, Webster has given in detail the stratification of a portion of them, which amounts to 

 124 feet, 8 inches ^^ ; to which may be added about 150 feet, — making a total of about 275 ^s. In 

 the Vale of Wardour, the total remaining thickness of the group appears to be from 40 to 60 



' Martin, p. 20, — who states, however that 100 feet would probably be no exaggeration of the 

 thickness, where the stratum first emerges from beneath the chalk. - Webster, Letters, &c., 



p. 140. : and supra (9S.), p. 183. » Supra (120.), p. 236. ; and (93.) p. 183. * (128.), 



p. 216.; (129.), p. 247. * (141.), p. 265. « (158.), p. 305. ' (161.), p. 314. 



« (11.), p. 109. '-' (49.), p. 140. '0 (93.), p. 184. " (129.), p. 247. '^ (148.), 



p. 279. '3 (158.), p. 306. '•• (160.), p. 311, '* Rose, p. 181. '^ gupra (161.), p. 314. 



'^ (17.), p. 116. '8 (25.) p. 122. '^ (32.) p. 126. ^ Geol. Trans., 2nd Ser. vol. ii. 



p. 102. 21 Martin, p. 36— 40. "-a (143.), A. p. 280. «Rose, p. 176. «" Martin, 



p.36.and40. « Supra(100.), p. 198. "-s (sO.) to (84.), p. 163— 172. «' Geol. Trans., 

 2nd Ser. vol. ii. p. 39. ^^ Supra (104.), p. 209. 



VOL. IV. SECOND SERIES. 2 T 



