UPPER GREENSAND. 67 



Rock (excludiDg a few feet at the top) was grouped with the 

 passage-beds into the Gault as the zone of Ammonites mflatus*' 



The most important bed commercially is the band of freestone, 

 from 3 to 5 feet thick, above alluded to as occurring a short dis- 

 tance below the base of the Chert Beds. This freestone is not 

 recognisable in the east or west ends of the Island, but has been 

 largely worked as a building-stone in the southern hills, being 

 especially conspicuous in the clifl: between Blackgang and Bon- 

 church. Between it and the Chert Beds lie one or two bands of 

 " firestone " and " rubstone." 



The Chert Beds attain their fullest development near Ventnor. 

 In Sandown Bay they can scarcely be recognised. The chert, 

 though used for road-metal, is not much worked, except in gaining 

 access to the freestone below. Some of the beds of chert are 

 crowded with the spicules of sponges. 



Dr. Hindef remarked of the Chert Beds of the quarry at 

 Ventnor Station that they " so abound with spicules that they 



may be considered as a continuous sponge-bed The 



chert is usually of a light brown tint, and in thin sections under 

 the microscope it is seen to be filled with spicules and spicular 

 casts imbedded in a translucent matrix of chalcedonic silica. The 

 spicules are likewise of chalcedony, and their canals are infilled 

 with glauconite. Another variety of chert, also very abundant, is 

 of a grayish or greenish-white tint ; it differs from the former in 

 that the matrix is of amorphous silica, while the inclosed spicules 

 are of chalcedony. The chert bands .... are enveloped 

 in an outer crust, of varying thickness, of white or yellow 

 siliceous porous rock, which is interspersed with the empty moulds 

 of spicules. 



" In some of the thicker masses of chert there are cavities or 

 pockets filled with spicules, loosely mingled in a grayish siliceo- 

 calcarous powder, in which there are also numerous well-preserved 

 foraminifera, chiefly of the genus Textularia. The spicules in 

 these cavities have undergone a remarkable alteration in structure ; 

 they appear to have lost their original silica, which has been 

 replaced by glauconite and some other silicate of a greenish-white 

 aspect. The replacing material has only partially filled the form 

 of the original spicules, and thus they look like mere shadowy 

 casts of complete spicules. These in many cases are peculiarly 

 distorted and contracted." Spicules occmTed in the lower beds 

 in the quarry also, but not so abundantly. 



By Dr. Barrels the Chert Beds and the freestone below them 

 were correlated with the Warminster Beds. A specimen of 

 Clathraria Lyellii, a cycadeous plant, which it will be remem- 

 bered occurs in the Wealden Beds, has been obtained from the 

 Upper Greensand by Capt. Ibbetson in bastard freestone at the 



* Recherches sur le Terrain Cretace Superieur de I'Angleterre et de I'lrlande, 

 p. 107. 



t Phil. Trans., vol. 176, p. 418. 1886. 



£ 2 



