264' Dr. FiTTON on the Strata below the Chalk. 



tioned in the list subjoined. One of the most remarkable is the Diceras 

 Lonsdalii, PI. XIII. fig. 4, for some very fine casts of which I am indebted 

 to Mr. GoodhalL 



No trace of any member of the Wealden group, nor of the Portland series, 

 has yet been observed in this quarter; and it will be seen from the Section, 

 No. 16., that the Kimmeridge clay and Oxford oolite thin out rapidly in 

 rising towards the west at Sandridge Hill. 



(141.) Swindon. — I am indebted also to Mr. Lonsdale for my first infor- 

 mation as to the existence of the Portland stone and sand at this place, — and 

 the occurrence there of large nodular masses of stone identical with those of 

 Shotover in Oxfordshire, and of the French coast near Boulogne. The Section, 

 PI. X. a, No. 17., explains the succession and general relations of these strata. 

 It will be remarked, that the Lower green-sand is distinguished from the 

 Upper by the interposition of the Gault* : but though the Portland stone 

 forms one of the most prominent features of this Section, no trace either of 

 the Weald-clay or the Hastings-sands has yet been observed above it, and 

 the indications of the Purbeck strata are inconsiderable. 



Chalk. — The remarks above stated on the contrast between the spaces occupied by the Upper 

 and tlie Lower chalk, apply with even greater force to the tract between Calne and Swindon, — or 

 more properly between Cherhill and Liddington, — than to Salisbury Plain. The heights from 

 Hackpen to Barbury Hill, that of Liddington Castle, and the high range thence to Polton Down, 

 forming prominences above the lower chalk on the west and north-west, and bounding an extensive 

 district, distinguished by the absence of streamlets and other characters from the more fertile 

 regions of the lower chalk and chalk marl. 



Between Barbury and Liddington Castle, the Upper chalk retires to the south and east towards 

 Ogbourn St. George, so as to form a sort of amphitheatre. The outline of tlie higher summits 

 when seen from sufficient distance, is in many cases almost rectilinear and horizontal; of which 

 the ridge between Ogbourn St. George and Ogbourn St. Andrew affords a good example. 



The Lower chalk forms a lower and uniform escarpment from Compton Basset on the east of 

 Calne to the village of Liddington, and thence northward nearly to Little Hinton and Bishop- 

 stone. From the lower beds at Liddington a copious spring breaks out, in a position analogous to 

 that of Lydden-spout on the coast near Folkstone (8.), and of the range of springs which every- 

 where appears at the foot of the chalk downs (70.) 



Enormous masses of siliceous grit (Grey wethers), like those which occur in profusion near 

 Marlborough, and in several places over the great chalk district on the south of that place, are 

 sometimes found here beyond the chalk escarpment. One of these, about 12 feet by 8 in its 

 diameters, was about a mile from the chalk-hill at Burdrop Wood ; and with these, in some instances, 

 are masses of siliceous conglomerate, — the Hertfordshire pudding-stone. 



* In the first edition of Mr. Greenough's map, the spaces assigned to the Chalk and Green- 

 sands here, are erroneous : the boundary of the latter on the north-west being really that of the 

 lower chalk ; and that of the Chalk, as expressed in the map, being truly the limit of the upper 

 chalk only. 



