4 Prof. BucKLAND and Mr. De la Beche on the 



Conybeare has stated*, into the form of an arch or saddle, on each side of 

 which the superior strata repose in corresponding order of succession, having 

 their dips respectively conformable to the north and south dips of the sides of 

 the arch, whilst their direction or line of bearing is nearly east and west, and 

 parallel to the axis of this central arch. Thus, on each side of our axis or 

 anticlinal line of forest marble, the Oxford clay reposes conformably, and 

 constitutes two parallel valleys, which extend from the sea in Weymouth Bay 

 to the Chesil Bank. To these succeed two similar parallel ridges of the 

 Oxford oolite formation, which are again overlaid by two parallel belts of 

 Kimmeridge clay, on which are still further superimposed two distinctly cha- 

 racterized deposits of Portland stone ; and on the north side only, above the 

 Portland stone we find strata belonging to the Purbeck series, to the green 

 sand, the chalk, and plastic clay. 



The superficial breadth of the belts on each side of the anticlinal line is in 

 the inverse ratio of their dip ; and as this dip is most rapid on the north side, 

 the surface of the northern belts is less broad than that of the corresponding 

 belts on the south side. 



Having taken this general view of the structure of the coast of Dorset, we 

 will again examine its component parts in a descending order, and point out 

 in regular succession the history and peculiar circumstances of each forma- 

 tion of which it is composed, beginning from the eastern extremity at White 

 Nore, and thus connecting it immediately with the observations of Mr. 

 Webster. 



Tertiary Deposits. — Plastic Clay, S^c. 



It is well known from the maps of Mr. Webster f and Mr. Greenough;|;, that 

 the tertiary strata which fill the chalk basin of Hants and Dorset find their 

 western termination about three miles east of Dorchester. Between this ter- 

 mination and the great south-western escarpment of the chalk, there occur in- 

 sulated patches of the same tertiary deposits, and many large blocks of pudding- 

 stone and of gray-wether sandstone, which show that the original limits of the 

 tertiary formations extended far beyond their present outlines, and were pro- 

 bably almost coextensive with the chalk. Thus near Came Down on the Ridge- 

 way about four miles south of Dorchester, we have a deposit of rounded chalk- 

 flint pebbles, resembling the round tertiary gravel of Blackheath, near Lon- 



* Outlines of the Geology of England and Wales, by the Rev. W. D. Conybeare, F.G.S. &c., 

 and William Phillips, F.G.S. &c., pp. 182, 192. 



•j- Geological Transactions, First Series, vol. ii. Plate IX. 



^ Geological Map of England and Wales, by G. B. Greenough, Esq. P. G. S. 1819. 



