PFHBECK BEDS OF THE YALE OP W ARDOUR. 



249 



Mr. Andrews says, " the Purbeck strata occupy an area of from 

 five to six miles square at Teffont Ewias, Chicksgrove, Lady Down, 

 &c. in the centre of the Yale of Wardour, and rest conformably 

 upon the Portland group, sometimes, but not always, separated by a 

 thin band of dark clay. These estuarine deposits have been consi- 

 dered to belong to the Jurassic system, in consequence of their pre- 

 senting remains of animal life nearer in affinity with the Jurassic 

 period than with the Wealden above, and also from the fact that the 

 marine Portland beds pass into the freshwater Purbecks without un- 

 conformity, and generally with an ancient land surface at or near the 

 junction ; the same area too (for the Portland beds are always capped 

 by the Purbecks) which formed the bottom of the Portland sea, became 

 when raised, first the support of the ancient forest, and then, when, 

 again slightly depressed, the delta of some great river, and this 

 without any unconformity or denudation, which would imply a great 

 lapse of time. 



"The Purbeck beds in the Yale of Wardour are comparatively much 

 thinner than those exposed on the Dorsetshire coast, and here pre- 

 sent only about 60 or 70 feet, belonging entirely to the lower and 

 middle divisions. 



" Whether the Upper Purbecks were ever deposited here, it is im- 

 possible to' say. These Purbeck beds thin out rapidly on the coast, 

 passing from E. to \Y., or from Durlston Bay to Portland Bill, and 

 also from S. to IS"., passing from the same exposure at Swanage 

 through the Yale of AYardour to the thin capping at Swindon. 

 This thinning-out may have been due to the north and west sides 

 of the estuary being raised and above water when the Upper Purbecks 

 were being deposited in the south and east — a supposition which 

 is borne out by the Purbeck at Swindon, shown by Mr. Blake to 

 be ' in point of time as old as some parts of the Portland.' Or, 

 on the other hand, the upper beds may have been deposited and 

 denuded — a supposition not improbable when we remember that 

 the Cretaceous system rests quite unconf ormably on these freshwater 

 beds, e. g. the Gault in the Yale of AYardour overlapping the beds 

 below. 



" Xew quarries, opened since Pitton wrote his memorable paper, 

 have supplied some very interesting fossils, amongst which is the 

 new Trigonia (T. densinoda). 



" At the junction of the Purbeck beds with the Portland strata 

 there generally occurs a thin bed of dark clay; and some few feet 

 above may be seen an ancient earth or land surface, which is, in 

 places, as much as 2 feet thick. This bed has in it large pieces of 

 coniferous wood and a Cycad, as in the Isles of Portland and Pur- 

 beck. The " cap " as it called, is here visible, and has yielded 

 some interesting Mollusca, besides several species of fishes, and 

 a very much larger form of Archceoniscus than A. Brodiei. The 

 beds immediately above the Middle Purbecks are not well exposed, 

 they consist (as on the coast) of soft shales with Cyprids and marls, 

 affording no good stone worth the quarryman's labour. These, 

 I presume, would be equivalent to the soft cockle-beds of the 

 Durlston section. 



