Dr. FiTTON on the Strata below the Chalk. 263 



to the east of north, between Gore Cross and Bushton Down, but turns suddenly eastward im- 

 mediately on the south of Urchfont. On the east of the Avon, the range of the upper chalk is 

 less distinct ; but it is directed towards the parting of the streams on the south of Buvbage. 



The difference between the upper and the lower chalk is very well expressed in the Ordnance 

 Maps, especially in the more recent sheets, which are engraved with more lightness and deli- 

 cacy of expression than some of their precursors. The great natural district of Salisbury 

 Plain, more than twenty-five miles in extent from west to east, and twelve miles from north to 

 Routh, though diversified by numerous irregularities, is almost wholly destitute of streamlets ; 

 and would be without running water, but for the presence of the more considerable rivers, which, 

 in many cases, do no more traverse the surface of this region, the greater part of their supplies 

 being derived from a different soil, and remote situation. This point of structure is known to 

 be so characteristic of the Upper chalk, both in England and France, that good maps alone, — such 

 as that of the Ordnance, or even of Cassini's map of France, would be sufficient to indicate the 

 space occupied by this stratum. In the tracts, on the contrary, occupied by the Lower chalk — 

 and still more decisively in the Chalk marl, few, even of the smaller valleys, are without streams; 

 and hence, as well as from the difference in soil, the vegetation also is different, and the fertility 

 of the lower regions forcibly contrasted with the naked and barren aspect of the higher downs. 

 Were it not that the colour is so nearly alike, these two groups would no doubt have been 

 distinguished in geological maps by different hues ; and they ought to be so, wherever the scale 

 is sufficient to express the distinctive features above mentioned. 



(140.) Vale of Pewsey. — The general structure of this tract is fully de- 

 scribed in the excellent paper of Dr. Buckland to which I have already re- 

 ferred. It is, in fact, a valley of elevation: its eastern and narrower portion 

 being- a continuation of the anticlinal ridge, which Dr. Buckland has described 

 as extending from the south-east of Inkpen in Berkshire to the west of Shal- 

 bourne and Ham; where the Upper green-sand has been brought to the sur- 

 face by subterraneous force, but is separated by a sort of bridge of the Lower 

 chalk, about Burbage, from the portion of the former stratum which has been 

 elevated in the same manner within the \ale of Pew^ey itself. At the open- 

 ing of the Vale, however, between Great Cheverell, Urchfont, and Devizes, 

 no derangement is apparent, though it is very probable that the surface is 

 disjointed by less perceptible fissures; — the Lower green-sand, continuously, 

 occupying the whole of the entrance of the vale; as is expressed on the map, 

 and more distinctly shown in fig. 4. of Mr. Lonsdale's sections. It will be 

 observed that the beds immediately below the chalk are here more fully de- 

 veloped than in the Vale of W^ardour, the Lower green-sand being per- 

 fectly distinguishable; as might have been expected, from the proximity of 

 this place to the great expanse of that stratum in Surrey and Hampshire : 

 and these subdivisions continue to be distinct through the northern counties 

 as far as the coast of Norfolk. The small number of fossils which I could 

 obtain, in addition to those enumerated in Mr. Lonsdale's paper*, are men- 



* Geol. Trans., 2nd Series, vol. iii. p. 266, &c. 

 VOL. IV. SECOND SF.RIES. 2 M 



