Geological Society. 4^55 



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 bells 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 Wealden formation terminates a little W. of Lulworth 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- 

 trate trunks of coniferous trees and stems of Cycadeoideae. These 

 trunks lie, partly sunk into the black earth, like fallen trees on the 

 surface of a peat bog, and partly covered by the incumbent lime- 

 stone. Many stumps of trees also remain erect, with their roots 

 attached to the black soil in which they grew, and tiieir upper part 

 in the limestone; and show that the surface of the subjacent Port- 

 land stone was for some time dry land, and covered with a forest, 

 and probably in a climate such as admits the growth of the modern 

 Zamia and Cycas. This forest has been submerged ; first beneath 

 the fresh waters of a lake or estuary, in which were deposited the 

 Purbeck beds and sands and clays of the Wealden formation, 

 (amounting together to nearly 1000 feet), and subsequently beneath 

 the salt water of an ocean of sufficient depth to accumulate all the 

 great marine formations of greensand and chalk. 



• Cicoi. Trans., .Second Series, vol. ii. o. 42. 



7. Below 



