218 



The authors take up the history of the geology of the coast of 

 Dorset at the point where Mr. Webster terminates, viz. at the 

 chalky promontory of White Nore, about eight miles E.N.E. of 

 Weymouth, and continue their account of the coast thence 

 westwards to the lias at Charmouth. The Memoir is accom- 

 panied by a map and many sections both of the cliffs and of the 

 adjacent inland district, including the space intermediate between 

 the escarpment of the chalk downs of Dorsetshire and the sea. The 

 authors divide this district into two compartments, viz. the Vale of 

 Weymouth and the Vale of Bredy. 



The structure of the Vale of Bredy is comparatively simple, being 

 chiefly composed of chalk, greensand, Kimmeridge clay, Oxford 

 oolite, forest mai'ble, and inferior oolite, dipping for the most part 

 to the E. and N.E. and divided by thick beds of clay. 



The Valley of Weymouth is more complicated, comprehending 

 tertiary strata, chalk, greensand, Purbeck and Portland beds, Kim- 

 meridge sand and clay, Oxford oolite, Oxford clay, cornbrash and 

 forest marble. To the forest marble belong the lowest strata that 

 form the axis of this district. Nearly all these strata are highly 

 inclined, and dip respectively in two opposite directions from an 

 anticlinal line which runs through a saddle of forest marble from E. 

 to W. 



The uppermost of these strata on the N. side constitute the chalk 

 escarpment of the ridgeway, capped with patches of plastic clay ; 

 whilst on the S. no strata appear above the sea more recent than 

 those which form the Isle of Portland. 



Between the ridgeway-chalk- escarpment and the Isle of Portland, 

 the strata are disposed in a succession of long and narrow belts 

 of clay and stone, the clay constituting valleys, and the stone 

 rising into ridges between the valleys ; all these belts are terminated 

 eastward by the bay of Weymouth, and westward by the Chesil 

 Bank. 



The formations composing this district are described in the fol- 

 lowing order. 



1. Plastic clay and sands, with blocks of puddingstone, and beds 

 of angular flints forming a breccia in place, occur on the surface of 

 the chalk. 



2. Chalk presenting no remarkable peculiarities. 



3. Greensand formation exhibiting no distinct traces of gault. 

 The Weal den formation terminates a little W. of Lul worth Cove. 



4. Purbeck beds appearing in two long insulated patches at Os- 

 mington and Upway. 



5. Portland stone occurring not only throughout the island of that 

 name, but forming a high and narrow ridge parallel and immedi- 

 ately subjacent to the escarpment of the chalk along nearly the 

 whole north frontier of the Vale of Weymouth. 



6. Between the Purbeck and Portland formations there is a very 

 remarkable bed of black earth called the " Dirt Bed," already de- 

 scribed by Mr. Webster as being mixed with slightly rolled pebbles 

 of Portland stone *, and containing, in a silicified state, long pros- 



* Geol. Trans., Second Series, vol. ii. p. 42. 



