§ 9. SUBDIVISIONS OF THE WEALDEN. 375 



Such is the assemblage of deposits to which the term 

 Wealden, first employed in this acceptation by Mr. Martin,* 

 is geologically applied. Alternating layers of clays, sands, 

 and limestones, almost wholly composed of fresh-water 

 univalves, and of small bivalves with minute crustaceans, 

 form the upper series. Sand and sandstone, with bands of 

 arenaceous limestones or calciferous grits, shells, and irre- 

 gular interspersions of lignite, compose the middle group ; 

 while in the lowermost, sands, clays, and argillaceous shelly 

 limestones, with small bivalves, again appear. Another 

 series of clays and limestones, characterized by extensive 

 beds of very small univalves, generally in the state of 

 marble (Purbeck marble), succeeds ; and buried beneath the 

 whole, is a petrified pine-forest, with cycadeous plants ; the 

 trees still erect in the soil in which they grew ! And in all 

 these deposits, bones of colossal terrestrial reptiles are more 

 or less abundant. 



The upper clays and limestones occupy the valleys of the 

 Wealden districts that skirt the inner escarpments of the 

 chalk downs, in Surrey, Kent, and Sussex ; the middle 

 group of sands and sandstones, constitutes the Forest ridge 

 of those counties ; and the lower series appears in the 

 deep valleys in the east of Sussex, around Battle, Bright- 

 ling, Bur wash, and Ashburnham. 



The Purbeck strata, which are distinguished by thick 

 beds of shelly limestone, principally formed of one small 

 species of paludina, appear on the coast of Dorsetshire, in 

 the Island, or more properly the peninsula, whose name 

 they bear. The lowermost deposits of the Wealden, range 

 along the southern shore of the Isle of Purbeck, and crest 

 the northern brow of the Isle of Portland ; overlying the 

 oolitic limestones and clays, of which that insular mass of 

 strata is chiefly composed. 



* Geology of Western Sussex, by LP. Martin, Esq. 



