222 PROCEEDINGS OF THE GEOLOGICAL SOCIETY, [Feb. 8, 



between them should be drawn. Mr. Godwin-Austen has remarked 

 on the close connexion between the Upper Greensand and the 

 Gault, and is inclined to consider their differences due rather to 

 the conditions than to the thne of deposition. Probably we should 

 be near the truth in representing the Upper Greensand and Gault 

 as one formation, with arenaceous characters prevailing in its lower 

 part, and inconstant and irregularly developed beds of clay in its 

 midst. In Kent these beds of clay are thick, well defined, and 

 sharply separated from the sands above and below ; but in the 

 Isle of Wight they are much less strikingly characterized ; and as we 

 proceed eastward they thin out and finally disappear in the neigh- 

 bourhood of "White Nore, in Dorsetshire. Thus in the Blackdown 

 beds of Devonshire we have the representatives of the sands above 

 the Gault Clay and those below it brought together, a fact which 

 may account for the anomalous character of the fauna of those beds. 

 It is not, however, improbable that these western portions of the 

 Cretaceous series, like certain similar deposits in the north-west of 

 France, may be of somewhat earlier date than those of the east of 

 England, though continuous with them, and may thus represent the 

 latter part of that great period which must have elapsed between 

 the Neocomian and the Cretaceous. It must be remembered, how- 

 ever, that the Ammonites of the Blackdown beds are nearly all weU- 

 knowji Gaiilt species. My friend Dr. Lycett informs me, too, that 

 the supposed correspondence betwen the Trigonice of the Blackdown 

 beds and those of the Neocomian breaks down on a careful and 

 critical examination of the specimens. It is not improbable that 

 some other of the supposed anomalies of the fauna of the Black- 

 down beds would also disappear on a more careful examination of 

 the fossils. 



3. Thinning-out westward of the Neocomian and Wealden. — 

 Nothing can be more striking than the great difference between the 

 thickness of the beds which lie between the Chalk and the Portland in 

 Swanage Bay, and in the section of Man-of- War's Cove and Durdle 

 Cove. This effect is, as we have already seen, partly due to overlap ; 

 but in a much greater degree it is owing to the tendency which all 

 these beds have to thin away towards the west. From the very 

 detailed sections published by the Geological Survey * (the work of 

 Messrs. Bristow, Whitaker, and the Kev. 0. Fisher), and from my 

 measurements, the accompanying Table has been constructed, which 

 shows in what a very marked manner this attenuation of all the 

 beds towards the west takes place. Of this general tendency the 

 Punfield formation partakes. Mr. Whitaker has recently shown 

 that the lower beds of the Chalk exhibit a similar thinning-out 

 towards the west. 



4. Relation of the Purheck to the Wealden. — At the only points 

 where the junction of the Purbeck and Wealden is clearly seen, 

 namely in Worborrow Bay and Mewps Bay, the marls with 

 Paludince, which form the top of the Purbeck, are seen to pass, by 



* Geological Survey in England and Wales, Horizontal Sections, Sheet 66, 

 and Vertical Sections, Sheet 22. 



