240 GEOLOGY OF THE ISLE OF WIGHT. 



to this favoured spot, for the restoration of health, 

 or the alleviation of mental affliction. 



Geological structure of the Undercliff. — 

 The structure of this district will be readily un- 

 derstood by an examination of the western extre- 

 mity of the Undercliff. The diagram, Ugn. 17, 

 (p. 215), illustrates the position of the several groups 

 of the cretaceous beds at St. Catherine's Down, 

 which is 830 feet in height ; the strata are nearly 

 horizontal. The greensand series reaches to the 

 top of the cliff at Blackgang, upwards of 400 feet 

 high ; the gait lies above it, and forms a bed more 

 than 100 feet thick. The marls, sands, and cal- 

 careous sandstones, comprised under the name 

 of Jirestone, in thickness about 100 feet, next 

 appear, and rise into a vertical inland cliff, half a 

 mile from the shore. Strata of white chalk form 

 the summit of St. Catherine's Down, the total 

 thickness amounting, in some places, to 250 feet ; 

 but on the south-western brow of the hill these 

 beds thin out, and form a mere capping over the 

 firestone.* 



The Gait is seen at the top of the ravine west of 

 Blackgang hotel, resting upon the greensand which 



" -Mr. Lowry, the eminent engraver, informs me that the firestone reaches 

 id within 133 feel of the beacon on St. Catherine's Down, as he ascertained 

 by barometrical observation in 1837. 



