CHAPTEE XV. 



PUB-BECK BEDS AND IKON-SAND OF SHOTOVEB. 



NOT without change of level in the solid land, but without 

 angular disturbance of the sea-bed, as far as is yet known, the 

 marine series of Portland was succeeded in the same area by 

 beds of estuarine, lacustrine, and fluviatile origin. In the Isle of 

 Purbeck the earliest consist of fresh-water and estuarine shelly 

 limestones and marls, a little varied by shallow marine deposits; 

 and these are followed by sands and sandstones interstratified with 

 marls and clays and fresh-water limestones, to the extent of a 

 thousand feet in depth, called ' Wealden.' In the Vale of Wardour 

 a more limited series appears, which consists mostly of fresh- water 

 marls and thin limestones; at Swindon the marine top of the 

 Portland is covered by a slight addition of fresh-water 'Purbeck' 

 beds ; while at Shotover, and near Aylesbury and Brill, such beds 

 are themselves capped by a variety of sandstones, sands, and clays, 

 which contain fresh-water shells, and constitute truly a ' Wealden' 

 formation. 



From the earliest days of geological inquiry in England, the 

 range of sand-hills on the north side of the cretaceous basin of 

 London, rich in ochre, fullerVearth, and sands of many colours, 

 has been the subject of frequent examination. As early as 1 7 23, 

 Holloway a , writing to Woodward, traces the range, and describes 

 its geographical relations and principal products ; Smith, in 1800-5, 

 mapped its course, identified it, as he thought, with the iron- 

 sand of Wilts, Kent, and Sussex, and placed it in his map (1815) 

 between the gault and the Portland rocks. Conybeare b took a 

 lively interest in the same rocks, and, from personal research, de- 

 scribed them in Shotover Hill, and through a considerable tract 



* Phil. Trans, xxxii. p. 419. b Geol. Engl. and Wales, pp. 136-143. 



