6Q GEOLOGY OF THE ISLE OF WIGHT. 



Boncliiirch to Blackgang, and which reappears inland in the bold 

 brows of St. Catherine's Down, Head Down, Gat ClifF, and 

 St. Martin's Down (Cook's Castle Crag). In the central range 

 the same rock forms the bold ridge of Rams Down, which is 

 scarcely less conspicuous than the Chalk Downs themselves. 



The existence of these striking features is due to the hard- 

 ness of a bed composed of alternations of chert and sand, and 

 underlain throughout the central parts of the Island by a band of 

 freestone. The position of the base of the Chert Beds has been 

 indicated on the map by a broken line in the central and southern 

 parts of the Island, principally on account of their topographical 

 importance. 



Above the Chert Beds a variable thickness of glauconitic sands 

 passing up into the Chalk Marl is known as the Chloritic Marl. 

 Below the Chert Beds there lie from 70 to 90 feet of sands^ 

 called " malm," with bands or lenticular masses of chert and 

 cherty limestone or "rag." Other local names of less common 

 occurrence are " hassock " for the sands, " whills " for sandstone, 

 " shotter-wick " for chert, " firestone " for a stone formerly em- 

 ployed for lining hearths, and " rubstone " for a stone once used 

 for whitening hearths or doorsteps. 



The thicknesses of the Malm Rock and Chert Beds are given 

 for different localities in the Isle of Wight, and for Punfield, in the 

 following table, the thickness of Gault at the same spots being 

 appended to show that the Upper Greensand and Gault thicken 

 and thin together, and not one at the expense of the other. 



The Malm Rock passes downwards into the strata which have- 

 been above referred to as "passage beds" into the Gault. A 

 convenient base for this subdivision has been selected near Ventnor 

 by Mr. Parkinson"^ in a band of chert nodules from which the 

 carapace and rib-bones of a fresh- water tortoise {Plastremys lata, 

 Owen) were obtained by Mr. Norman, and the remains oUIoploparia 

 Saxbyi, M'Coy, by Mr. Saxby.t In other parts of the Island the 

 base has been drawn where the clayey bands begin to pre- 

 dominate over sandy beds. 



The zone of Ammonites inflatus occurs, according to Mr. 

 Parkinson, rather more than 20 feet from the base, while Ammonites 

 rostratus attains its greatest development about 11 feet from the 

 top of the Malm Rock. By Dr. Barrois, however, the Malm 



* Quart. Jmirn. Geol. Soc, vol. xxxvii. p. 370 (1881). 

 f A7m. Mag. Nat. Hist , vol. xiv. p. 116 (1854). 



