Dr. FiTTON on the Strata below the Chalk. 183 



served also at the top of the higher Downs, on the south of the island, resting on a plateau of the 

 Upper green-sand ; and the lower beds of the chalk are in many places visible in descending from 

 these heights, as from Niton to the undercliff, and above Old Park, on the south-west of Wol- 

 verton. The gradation of hue and texture, from the whitest chalk, — through various shades 

 of grey, and various degrees of softness, to the state of bluish marl, resembles precisely what is 

 observable in the same part of the series at Beachy-head, on the shore near Folkstone, and on 

 the opposite coast of France, under the cliffs of Blanc Nez. 



The central ridge, or anticlinal line of the chalk, it will 'be remarked, is neither continuous nor 

 rectilinear ; but is divided, about the middle of the island, between Carisbrook and Shorwell, by a 

 tract of irregular ground, intervening between two nearly parallel branches of the chalk range about 

 three miles apart. The first, or southernmost, of these branches runs directly from the Needles to 

 Brixton-down ; the more northern from Arreton-down to Bembridge-down and Culver-clifF. The 

 course of the principal streams of the island is transverse to that of the central chalk range : the 

 river Medina, which nearly bisects the island from north to south, rising high upon the sands, about 

 Stroud Green on the south of Kingston, and running almost directly northward to the estuary at 

 Cowes. The other principal stream, the Eastern Yar, springs beneath the southern chalk, about 

 Whitwell, not far from Niton; and after a tortuous course, instead of joining the sea in Sandown 

 Bay, it cuts through tlie ridges of the green-sands and the chalk, between Yaverland and Brading. 

 The egress of the streams, therefore, is here analogous to that already described in Kent and 

 Sussex : and this effect may not improbably be ascribed, in both cases, to fissures, by which the 

 lines of drainage were in the first instance determined. 



Upper Green-sand. — This formation is but partially disclosed along the base of the central ridge 

 of chalk ; but is distinctly seen in the sections at Compton-bay, and on the south of Bembridge- 

 down, and in several places along the escarpment of the undercliff* ; the course of which, it is to 

 be observed, is not exactly parallel to the anticlinal ridge, but oblique to it, running towards a 

 point about 17° south of west. The total thickness here, in some places approaches to 100 

 feet, and on an average is not much less than 70 feet: its step-like projection beyond the 

 chalk, as in Western Sussex, is conspicuous in several places, especially beneath St. Catherine's 

 Down. One of the most accessible sections is in the ground rising above the ravine at Luc- 

 combe, where the road from Shanklin to Bonchurch crosses the strata obliquely. About the 

 middle of Old Park is a continuous section of beds, apparently undisturbed and more than f of a 

 mile in length, rising very gradually towards the south, or west of south, and consisting above, for 

 between 30 and 40 feet, of subcalcareous sand and stone, alternating with chert in concretions, — 

 and below of similar sand, with spongiform masses, like those of the upper part of the sections near 

 Folkstone ; (21.) and (22,). But one of the best sections, perhaps, of the Upper green-sand, is 

 visible above the road from Niton, towards Blackgang-Chine, over the Sand-rock spring. The 

 brow of the chff at that place is about 591 feet above the seaf ; and the beds composing the ver- 

 tical face at top, which is itself more than 100 feet high, may be thus divided : 



Alternations of soft subcalcareous stone, with concretional chert. . . 30 to 40 feet. 



Yellowish grey sand and stone, with some chert 60 — 80 



Alternations of yellowish grey and bluish stone, and sand 15 — 20 



* As above Western Lines ; above St. Lawrence, where the road comes down from the interior, 

 and thence westward ; between Rans and Marables. At the place last mentioned the precipice is 

 about 60 feet high, with gault beneath, and is traversed obliquely by a footpath up the whole face. 



■j- Englefield and Webster, p. 238. 



VOL. IV. — SECOND SERIES. 2 B 



