Geology of the Neighbourhood of Weymouth, &;c. 19 



stone. The sudden and total disappearance and termination of the Portland 

 formation at Portisham, seems not to result from its accidental intersection 

 at that place by the great Ridgeway fault ; but rather from the tendency 

 which is common to this, with most other great formations, to terminate 

 abruptly where they are accumulated to their fullest thickness; thus we find 

 the full thickness of the chalk and oolite formations exhibited for many 

 hundred miles along the line of their great escarpments throughout England, 

 and in like manner the full Portland strata terminate abruptly in the Vale of 

 Weymouth : they reappear and again terminate with equal abruptness in the 

 Vale of Tisbury, and exhibit nearly the same features of sudden termination 

 in the hills near Brill and Thame in Oxfordshire, and near Aylesbury and 

 Whitchurch in Bucks. 



The fact of the interrupted deposition of the Portland formation in En- 

 gland, occurring as it does only in the limited districts just mentioned, and 

 in one other small spot at Swindon, is analogous to the interrupted deposits 

 of other strata, particularly the most recent members of the oolite formation. 

 We know not the cause of these irregularities ; the fact may be illustrated by 

 the case of the Kimmeridge clay, which attains at Kimmeridge a thickness 

 of 600 feet, is reduced to 70 feet near Oxford, disappears entirely in some 

 places along its line of bearing, and again resumes its strength in the Vale 

 of Pickering, and near the coast of Yorkshire. The Portland stone has not 

 yet been identified on the opposite coast of Normandy, but M. E'lie de 

 Beaumont states that he has discovered it in Burgundy*, and M. Dufrenoy 

 says that it occurs largely in the vicinity of Angoulesmef- We think it pro- 

 bable that it will soon be recognised among the great calcareous formations 

 of the Alps and Apennines. 



Portland Sand'^. 

 The consolidated and calcareous beds of the Portland stone are separated 

 from the Kimmeridge clay by the interposition of a deposit of sand and marly 

 sandstone at least eighty feet thick, exceeding the total thickness of the 

 Portland stone itself This deposit is coextensive with the Portland stone 

 throughout nearly the whole of the coast of Dorset, and is well exhibited by 

 a vertical section near Black Nore, on the west cliff of the Isle of Portland, 

 and along the west shore from Black Nore to the village of Chesilton, Its pre- 

 vailing character is a siliceous sand, so abundantly mixed with grains of green 

 earth, as to be scarcely distinguishable from the lower strata of the green- 

 sand formation near Lyme and Seaton : it also contains large semicalcareous 



* Ann. des Sciences Nat., July 1829. f Annales des Mines, 1829, vol. ii. p. 434. 



i See PI. III. fig. \.n.o. and fig. 3. 



D 2 



