GREY CHALK MARL. 101 



those above enumerated, and the turrilites, hamites, and scaphites, but very 

 rarely occur. 



On the surface, a narrow belt of this deposit appears to encircle Lewes 

 Levels, separating the latter from the edge of the chalk hiUs ; this want 

 of continuity, however, does not extend beneath the surface: the marl 

 is invariably found upon sinking through the alluvial clay, of which the 

 Levels are composed. Protrusions of the marl through the clay occur in 

 some situations, and these form islands when the levels are inundated, a 

 circumstance that, previously to the improved state of the navigation of 

 the Ouse, was of very frequent occurrence. The principal elevations of 

 this kind are the two extended ridges called the Rhies, which have been 

 already described; these, in all probabiUty, owe their form to the action of 

 eddies, or opposing currents. 



Minerals. 

 The mineralogical productions of the grey marl are few, and offer but 

 little variety; they consist of various modifications of sulphuret of iron, 

 and crystallized carbonate of Hme. 



1. Crystallized carbonate of lime. 



This mineral is frequently semi-diaphanous, varying in colour from a 

 lightish grey to a gallstone yellow. It occurs in inconsiderable veins, and 

 occasionally in groups of crystals, hning the cavities of the marl; the usual 

 form of the crystal is that of an acute rhomboid ; of this kind some in- 

 teresting specimens have lately been discovered at Hamsey. 



2. Sulphuret of iron, or iron pyrites. 



This substance, from the decomposition of its surface, is generally of a 

 yellowish rusty brown colour externally. It occurs in a variety of irre- 

 gular fantastic shapes, and oftentimes bears the impression of organic 

 bodies, forming casts of terehratul(£, pectenites, madreporites, and the inner 

 volutions of scaphites. SmaU spherical masses with an elongated stem, 

 their surface beset with obscure pyramidal crystals, and exposing a bril- 

 liant radiated structure internally, are not uncommon. One specimen in 

 my possession contains within a cavity, small crystals of sulphate of lime. 



