GALT AND UPPER GREEN SAND.— - 



CHALK. 



201 



found in the green sand near Sidmoutb, are composed of opaque 

 chert on the outside, and contain within, mammillated concretions of 

 beautiful chalcedony, and occasionally perfect minute rock crystals. 

 Some of the sandy concretions near Sid mouth have a beautiful green 

 colour, which I found to proceed from green sulphate of iron.* 



The total thickness of the green sand where it is fully developed, 

 is more than 400 feet. The lower sand is generally ferruginous, 

 and has been called iron sand, from the large quantity of oxide of 

 iron disseminated through it; but the lowest beds often contain green 

 particles like those in the upper green sand. The upper and lower 

 green sand are, in many situations, separated by a " bed of stiff 

 marl, varying from a light grey to a dark blue." According to Mr. 

 Mantell, its greatest thickness in the south of Sussex is about 250 feet. 

 This bed has been called the Folkstone Marl, but is more generally 

 known by the provincial name of Gait. The marine shells, in which 

 it abounds, are generally distinguished by their brilliant pearly lustre ; 

 they consist of amtnonites, nautilites, a small species of belemnite, 

 and various other shells. 



The upper green sand is remarkable for the chalcedonic appear- 

 ance of the flint or chert which it contains. This sand has been 

 sometimes called fire stone, to distinguish it from the lower green 

 sand. The green particles are composed chiefly of the protoxide 

 of iron and silex, denominated by M. Berthier a silicate of iron. In 

 some parts of the Savoy Alps, the beds analogous to green sand are 

 of enormous thickness, and are nearly black, but contain many of the 

 same fossils as the English green sand. From these beds I obtained 

 hamites, scaphites, and various species of small echinites. The 

 upper green sand, as before observed, becomes intermixed with an 

 argillaceous and calcareous bed called chalk marl, which may be re- 

 garded as the lowest bed of the under chalk. It is of a darker colour 

 than common chalk, but burns into useful grey lime. 



Chalk. — In England and the northern parts of Europe, this rock 

 is better known by its mineral characters than any other of the 

 secondary strata. Its prevailing colour is nearly white ; it has an 

 earthy texture, and is generally so soft as to yield to the nail. These 

 are, however, not the universal characters of chalk. The lower 

 beds in Yorkshire are red, and the scaglia of the northern Alps, 

 which is a mode of chalk, has also a red colour ; in some parts of 

 the Alps this rock is highly indurated, and resembles more, white 

 statuary marble than English chalk. The greatest thickness of the 

 chalk strata in England may be estimated at from 600 to 800 feet. 

 The upper beds contain numerous nodules and short irregular veins 

 of flint ; the lower chalk contains fewer flints ; it is, generally, hard- 



* On the east of Sidmouth, immediately above the town, I observed green sand, 

 intermixed with black particles which I ascertained to be the black oxide of man- 

 ganese, as they gave a violet colour to glass when fused. 



26 



