64 GEOLOGY OF THE ISLE OF WIGHT. 



black materia], the inner whorls j)erraeatecl by a phosphatic 

 substance* 



At Blackgang the numerous sections in the lower part of the 

 Gault have been noticed in the description of the Carstone. 

 Inoceramus sulcatus and T. conceutricus have been found in a gulley 

 west of the hotel. The top of the Gault appears in Gore Cliff, this 

 being the only spot in the Undercliff where it is not concealed by 

 fallen rubbish. The beds are similar to those at Compton Bay, 

 and the thicknesses differ but little. According to Mr. Simmst 

 there are here 43 feet of light-coloured Gault (passage beds), and 

 103 of blue Gault, giving a total of 146 feet. 



The sections in the cliff from Bonchurch to Knock Cliff show 

 the lower beds of the Gault only. The passage downwards into 

 the Carstone may be conveniently examined in the brow of the 

 cliff near Bonchurch (^p. 59). 



In Sandown Bay the position of the Gault is marked by a 

 narrow hollow in the cliffs. The passage beds into the Upper 

 Greensand above and the Carstone below are there exposed, but 

 the rest of the deposit is concealed by vegetation. The top 

 layers consist of alternations of blue sandy clays and sands with 

 Vermicularia, about 15 feet thick, and the lower beds of darker 

 blue micaceous clay. The total thickness of the Gault here is 

 about 120 feet. 



Through the central parts of the Island, the Gault occupies a 

 narrow belt of low ground, separating the Upper and Lower 

 Greensands. When not overspread by a downwash of sand, 

 the soil of this belt is wet and rush-covered, and presents a 

 characteristically different appearance from that of the strata 

 above and below. But as a rule the Gault is entirely masked, 

 and sections are exceedingly rare. 



The passage beds into the Upper Greensand are seen in a lane 

 100 yards south-west of Kill, near Chillerton. At Gossard Hill, 

 near Rookley, where a long shoulder of Gault, capped by an 

 outlier of Upper Greensand, juts out across the Medina, a brick- 

 pit has been opened ; but only the weathered surface of the Gault 

 is worked, a pale-blue or nearly white structureless clay. A 

 better section is provided in the brick-pit at Bierley, near Niton, 

 where the lower beds of the Gault are exposed. 



The brick-pits by the side of the railway between Wroxall and 

 Shanklin are worked in Gault that has slipped down the hill-side 

 below the true outcrop (p. 59). One of the most noticeable featm'es 

 in connection with the outcrop of the Gault, is the copious supply 

 of water which it throws out nearly all round the southern Downs 

 of the Island. The greater part of the strata over-lying this clay 

 being of a permeable nature, the rainfall is absorbed by them, and 

 is thrown out in a line of springs along the top of the first imper- 

 meable bed it encounters. The s])rings are of course most copious 

 along the hill-sides where the Gault is at the lowest level, the 



* Geological Guide to the Isle of Wight, p. 70. 

 f Quart, Journ. Geol, Soc, vol. i. p. 76 (1845). 



