84 GEOLOGY OF THE ISLE OF WIGHT. 



not be clearly distinguished. Inoceramus mytiloides is abundant 

 in the Middle Chalk. In the same neighbourhood a deep cutting 

 for the coach-road shows in the upper part : — 



Ft. In. 



Alternations of chalk and marl, top not seen - - - 120 



Rocky chalk, very impure, with glauconite ; passing down 

 into the - - - - - - -58 



Chloritic marl (see p. 70) - - - - - 11 6 



Large specimens of Ammonites rhotomagensis, Defr., occur here, 

 and A. varians, Sow., is common but badly preserved. 



The Middle and Lower Chalk are both seen in a pit north of 

 Mottistone, where the latter has been worked near its base ; but 

 the junction between the two subdivisions is obscured. This 

 seems to have been the pit alluded to by Mr. Whitaker,* and 

 the layer of hard yellowish nodules seen by him may have been 

 the Melbourn Rock. The pits do not reach up to the horizon 

 of the Chalk Rock. 



At the west end of Brixton Down a fine series of pits extends 

 from the Upper Chalk to the Malm Rock of the Upper 

 Greensand. The Upper and Middle Chalk are seen in the 

 uppermost pit on the north side of the Calbourne and Brixton 

 road, the section, which was measured in company with Mr. 

 Whitaker, being as below : — 



West end of Brixton Down. 



Ft. In. 



r Chalk with flints, seen up to about - - 20 



TT ni IV J Rough nodular chalk, without flints - 20 



upper unaik -^ N^^^iar ^halk (Chalk Rock), the nodules 



L green-coated - - - - 1 3-6 



Rough nodular chalk - - - 1 6 



Middle chalk - Smooth massive chalk - - . fi 0-)- 



The lower beds of the Middle Chalk are seen in a pit a few 

 yards further south, but the Melbourn Rock is not now exposed. 

 Another and larger pit in the Middle Chalk has been opened 

 about one third of a mile further west in Mottistone Down, and 

 seems from the character and curvilinear jointing in the lower 

 part to have reached the Lower Chalk, but the Melbourn Rock 

 is not distinguishable. Holaster suhglobosus occurs in these lower 

 beds. 



The Brixton Down pit was visited in 1865 by Mr. Whitaker, 

 and figured on p. 403 of the paper already quoted. At that time 

 a line of clay was visible, which seemed to shew an unconformity 

 (or perhaps false bedding) in the Chalk ; for southwards it was 

 further from the nodular bed, whilst northwards the latter was 

 not seen, but seemed to be cut off. This line of clay, however, 

 runs persistently through the Island at a scarcely varying distance 

 above the Chalk Rock ; it was in fact selected by M. Barrois as 

 the base of his Chalk-with-flints. Moreover, it was figured by him 



* Quart. Journ. Geol. Soc, vol, xxi. p. 402. 1865. 



