278 TROF. J. PRESTWICH OX THE RAISED 



At the northern end of the island, where the Eubhle rests against the 

 slope of the scarped edges of Portland strata (400 feet high), it 

 forms a mass of uiistratified rubble some 60 feet thick or more. 

 At both places it consists entirely of local, angular and subangular 

 debris and blocks, some of large size, derived from the Purbeck and 

 Portland strata ; and in both cases it contains land-shells, — with 

 some marsh-shells at Chesilton, where the rubble contained also 

 Cypris Browniana and Candona Candida. There may have been 

 a piece of marshy ground or a pond in the Kimmeridge Clay here, 

 previously to the impact of the Head. 



•O Pupa marginata, Drap. 



O Succinea uhlonga, Drap. 



O HeLix virgata. Da Costa. 



O Lhmax agresfls, Linn, (the shield) 

 • O LimncBa peregy-a, Miill. 



O Found at Portland Bill. • Found at Chesilton 



O LimrifBa truncatida, Miill. 

 (opercula). 



• Bpthinia tenfaculata. Miill. 



• Flanorbis glaber, JefFr. 



I note particularly these structural conditions of the Head, on 

 account of their bearing on the theoretical questions that will after- 

 wards arise. 



Between Portland and the Exe the sea has cut back the Jurassic 

 and Cretaceous strata of the Dorset coast, and formed the deep bay 

 of Lyme Regis, in which there are no traces of the old Beach. It 

 no doubt took a more direct course across the bay than the present 

 beach. The remains of it are, however, to be found in the present 

 beach and in the Chesil Bank,^ both of which are in great part re- 

 constructed out of its materials. 



The valley of the Exe would seem at this time to have formed 

 an estuary. Mr. W. Yicary pointed out to me a slight terrace 

 on either side of the valley, corresponding with what probably 

 would have been, the beach-level, about 15 feet above O.I). ; and 

 Godwin-Austen mentions the occurrence of marine shells at a depth 

 of a few feet from the surface of the valley near Exeter. 



(15) The South Devon and Cornish Coasts. — The cliffs of hard 

 Palaeozoic rocks of these counties have undergone comparatively 

 slight wear since the Kaised Beach period. Consequently, rem- 

 nants of that Beach are common along these coasts, nestling in many 

 nooks and sheltered places. They are most of them well known, 

 having been described by the early Cornish and other geologists.^ 

 A very complete account of them, with additions, has also lately 

 been given by Mr. W. A. E. IJssher.^ I may therefore confine 

 myself to brief notices, except in those cases where there are special 

 points to remark upon. 



^ ' On the Origin of the Chesil Bank,' Proc, Inst. Civ. Eng. vol. xl. (1874-5) 

 part ii. p. 61. 



2 Trans. Roy. Geol. Soc. Cornwall, vols. i. to x. See also De la Beche's 

 * Report on the Geology of Cornwall, Devon, &c.,' and Mr. Pengelly's several 

 papers in Trans. Devon Assoc. I have visited the principal of these Beaches. 



3 Quart. Journ. Geol. Soc. vol. xxxiv. (1878) p. 449, and Geol. Mag. for 1879, 

 p. 27 ; see also his ' Post-Tertiary Geology of Cornwall.' 



