382 Dr. Fitton on the Strata [Nov. 



which seemed to render this part of the series deserving of fur- 

 ther examination. 



Coast of Dorsetshire. 



V. The progressive condensation, and thinning out of the 

 strata towards the west,* is such, that the space occupied by the 

 beds between the chalk and the lowest visible part of the Hast- 

 ings sands, — which is in Sussex (from Folkestone toWinchelsea, 

 fjo-. 1), — more than twenty miles in extent, — is reduced success- 

 ively • — in the Isle of Wight (between Ilocken-End and South- 

 more), to about eight miles ; — at Svvanage Bay, fig. 3, to less 

 than one mile and a half; — at Worbarrovv, fig. 5, to less than 

 three-quarters of a mile ; and finally at JDurdle Cove, fig. 6, 

 where these beds appear, for the last time, on the coast, to less 

 than a furlong : — the horizontal distance in a direct line between 

 Folkestone and Durdle Cove, the extreme points of this series, 

 beino- about 170 miles. This convergence, it is true, appears 

 much greater than it actually is, in consequence of the high in- 

 clination of the strata on the coast to the west of Purbeck, 

 where at last they become very nearly vertical ;f but the con- 

 densation is really sufficient to make it more extraordinary that 

 so many members of the series have been retained, than that 

 some beds should be wanting. 



At Worbarrow, and in the coves to the west of that place, 

 I could not detect any trace of the weald clay, between the sands 

 below the gault ; the'sections (fig. 5 and 6) affording only one 

 continuous series of sandy beds, from the gault to the commence- 

 ment of the Purbeck strata. But at Swanage (fig. 4 and 5) I was 

 more successful, having found there distinctly the equivalent of 

 the weald clay ; — beds of bluish slaty clay containing the cypris 

 faba, and other shells of the same species with those of the Isle 

 of Wight ; and limestone in thin strata, composed of bivalves, 

 with small paludinae, and of oysters, and in some cases en- 

 crusted with obscurely fibrous carbonate of lime. Beds, also com- 

 posed of mottled greenish-grey sand and grey clay, like those of 

 Sandown and Cowleaze chines, occur in this part of the section. — ■ 

 But the sands interposed between the weald clay and the Gault are 

 not in themselves distinguishable, at Swanage Bay, from the in- 

 ferior (Hastings) beds ; the green particles being wanting, and 

 the sands differing only in colour, fineness of grain, and a vari- 

 able admixture of clay. There are among them some remark- 

 able courses of a very fine grained calcareous grit used by the 

 Swanage quarry-men for sharpening their tools : but I have not 



* See Webster, Letters, p. 194, and Plates. 



+ The difference as to the impression produced on the observer, by beds of the same 

 thickness when nearly horizontal, and when highly inclined, — which arises from our habit 

 of estimating heights and horizontal space by very different scales, deserves the attention 

 of those who are not much accustomed to geological observation. Thus a bed, or group, 

 200 feet thick, if horizontal, forms a very striking cliff"; but as part of an highly in- 

 clined series, it may be passed by with comparatively little notice. 



