and on the Basins of London and Hants. 1 23 



position of their component strata to a force acting- from below, and elevating 

 the strata along- their central line of fracture, I shall venture so far to involve 

 this theory of their origin with the facts which they display, as to designate 

 them by the appellation of Valleys of elevation : of course due allowance 

 must be made for their subsequent modification by diluvial denudation. For 

 further examples of inclosed valleys, affording similar axes or centres of eleva- 

 tion, I refer to the map and section of the Bristol coal-basin, accompanying the 

 memoir by Mr. Conybeare and myself on the S.W. coal district of England, 

 in the last volume of the Geological Transactions, [Vol. i. Part II. 2d series;] 

 where, in following the anticlinal lines that traverse that district, it will be seen 

 that the strata on either side of them are in some places elevated into lofty 

 ridges, and in others pass along the lowest points of the valleys. An example 

 of the latter kind may be seen in the Vale of Westbury near Bristol ; and also 

 near Thornbury, Berkeley, and Newnham on Severn. In the section of the 

 South-Welch mineral basin, (PI. XXXIV, of the same volume,) a similar 

 example occurs in the Vale of the Ely to the west of Llandaff; and in following 

 the great anticlinal line which extends across the Severn from the west extre- 

 mity of the Mendip Hills through Glamorganshire and Pembrokeshire to 

 Milford Haven, there are several examples of valleys similarly constructed, 

 and composed of a basis of old red-sandstone, inclosed by hills of lime- 

 stone, Avhich face each other with an opposite escarpment, and have an 

 opposite dip. 



It will be seen, if we follow on Mr. Greenough's map the south-western 

 escarpment of the chalk in the counties of Wilts and Dorset, that, at no great 

 distance from these small elliptical valleys of elevation, there occur several 

 longer and larger valleys, forming deep notches, as it were, in the lofty edge 

 of the chalk. These are of similar structure to the smaller valleys we have 

 been considering, and consist of green-sand, inclosed by chalk at one extremity, 

 and flanked by two escarpments of the same, facing each other with an oppo- 

 site dip ; but they differ in the circumstance of their other and broader extre- 

 mity being without any such inclosure, and gradually widening till it is lost in 

 the expanse of the adjacent country. 



The cases I now allude to, are the Vale of Pewsey to the east of Devizes, 

 that of the Wily to the east of Warminster, and the valley of the Nadder ex- 

 tending from Shaftsbury to Barford near Salisbury; in which last, not only the 

 strata of green-sand are brought to the surface, but also the still lower forma- 

 tions of Purbeck and Portland beds and of Kimmeridge clay. 



It might at first sight appear that these valleys are nothing more than simple 

 valleys of denudation ; but the fact of the strata composing their escarpments 



r2 



