318 Dr. FiTTON on the Strata below the Chalk. 



SUMMARY AND GENERAL REMARKS. 



(163.) The order of the strata mentioned in the preceding pages is repre- 

 sented generally in the abstract Section, PI. X. b. fig. 13. 



Thickness of Strata. — I have not inserted in the engraved section the thickness of the 

 groups ; both because it varies much in different places, and that in general my estimates are so 

 loose as not to be relied upon. Accurate measurements of the thickness of the English strata 

 have very seldom been made. The following are approximate numbers, derived however, princi- 

 pally, from estimates by the eye. 



Chalk. — The entire thickness of the chalk is exposed in many of the sections on the south- 

 eastern coast; but westward of the Isle of Wight the beds are convergent and much reduced in 

 bulk. Between Deal and Folkstone, Mr. W. Phillips has estimated the total thickness at 820 feet'. 

 The two sections at the extremities of the Isle of Wight, between Whitecliff and Sandown Bays 

 on the east, — and between Alum Bay and Broadbench, at the western extremity of the central 

 ridge, furnish good evidence ; the strata being so nearly vertical, that the horizontal line across 

 their direction, does not very much exceed the perpendicular to their surfaces. Mr. Greenough 

 has accordingly estimated the thickness at Culver at 1300 feet^; and the interval on the Ord- 

 nance map, which is very nearly the same at the two extremities of the island, (being in both 

 places about 2 furlongs, or 1320 feet,) may probably be considered as somewhat exceeding the 

 maximum thickness of the chalk in England. 



Mr. Conybeare estimates the chalk as ranging between 600 and 1000 feet' ; and Mr.Dela 

 Beche at 700 "i. The height of the cliff at Beachy Head, which includes at top part of the flinty 

 chalk, and goes down very nearly to the Upper green-sand, is only 535 feet* ; but if 250 feet be 

 added, for the remainder of the flinty chalk, (350 being the thickness of that division near Dover^) 

 the aggregate thickness on the Sussex coast will be about 800 feet. 



At Wendover Hill, the summit employed as the Ordnance Station is 905 feet above the sea? ; 

 and the canal at Wendover being 404| feet, and still within the chalk, the difference, 500 feet, is 

 certainly less than the total thickness at that place. 



But the whole of the chalk passed through in the boring above mentioned, at Diss in Norfolk^ 

 (which includes at the top 100 feet without flints,) was only 510 feet. 



The wide range of variation in thickness above stated (supposing the measures to be correct), 

 arises in a great measure from the unequal abrasion and removal of the upper strata ; but must 

 in part also be ascribed to original inequality in the thickness of the chalk itself. 



Upper green-sand. — The thickness of this group near Folkstone does not exceed 30 feet, and 

 is probably much less''; but it swells out considerably near Godstone, and Merstham, and at the 

 latter place is upwards of 30 feet thick '". The depth of the wells sunk through it, in the 

 Malm rock strata of Hampshire, varies from 60 to 100 feet". In Western Sussex the thickness 



' Geol. Trans., 1st Ser. vol. v. p. 18. - Conybeare, Outlines, &c. p. 85. 



' Ibid. ^ Tabular View, 2nd edit. ^ Section, No. 6. Plate X. a. 



" Phillips, Geol. Trans., 1st Ser. vol. v. p. 18. ' Supra (149.), p. 284 ; and Sect. 20, PI. X. a. 



^ (160.), p. 311. 9(9.), p. 107. "> (49.), p. 140. " Murchison, 



Geol. Trans., 2nd Series, vol. ii, p. 99. 



