344 



JURASSIC. 



Upper Purbeck b. 

 Beds. 

 ■^o to 60 feet. 



d. 



Middle Purbeck 



Beds. 

 50 to 150 feet. 



The Purbeck Beds of Dorsetshire were divided as follows by 

 Prof. E. Forbes and Mr. H. W. Bristow (see Fig. 53) ^ '— 



Paludina clays with Purbeck Marble containing Paludina 



carinifera. 

 Cypris clays and shales. 

 Unio beds, greenish shaly limestone with Unio, Cyclas ; 



Crocodile-bed at Durlston Bay,with Turtle, Crocodile, etc. 

 Broken-shell limestones, with comminuted shells, Cyreiia ; 



Fish-remains, etc. 

 Beef beds, shales, etc., with fibrous carbonate of lime (Beef), ^ 



selenite, etc. 

 Corbula beds, with Corbiila, Melaiiia, Aster acanthus. 

 Scallop beds, with Pecten. 

 Intermarine beds, with Corbida, Ostt-ea, Cyclas, Hemicidaris 



Purbeckensis ; (Swanage Stone). 

 Cinder bed made up of Ostrea distorta ; when weathered, it 



presents a rough cindery appearance. 

 Dirt-bed (local). 

 Cherty freshwater bed, limestone with nodules of chert, 



LimtiiTa, Physa Bristovii, Planorbis, Valvata, etc. 

 Carbonaceous bed at Durlston Bay, with Insectivorous 



Mammals. 

 Marly freshwater beds. 

 . Soft cockle beds ; chiefly marl, with gypsum and pseudo- 



morphous crystals of rock-salt in places. 

 Hard cockle beds, sandy and calcareous beds. 

 Cypris freestone, slaty limestones, etc. 

 Broken bands, beds made up of broken fragments of 



calcareous slate and bituminous limestone, squeezed 



together, with fragments of chert, etc. 

 Soft cap, sandstone and bituminous limestone. 

 Dirt-bed, with pebbles or fragments of limestone. 

 Hard cap, bituminous limestone, etc., botryoidal in places. 

 Dirt parting in places. 

 Portland Stone. 



The Dirt-beds, which consist of sandy carbonaceous clay, with rounded 

 fragments of limestone from the underlying beds, occur at various horizons, but 

 usually towards the base of the Purbeck strata. At Portland the principal bed 

 lies above the Caps (see p. 339), and is full of the fossil silicified stools and 

 prostrate trunks of coniferous and cycadean trees. There it is known as the 

 Black Dirt, and it was first described by Thomas Webster.'* It contains rounded 

 fragments of limestone, which Mr. Horace Brown has suggested are due to the 

 superficial weathering in Purbeck times of the underlying beds. Many limestones 

 weather in a rubbly form at the surface, and similarly the Purbeck Caps may have 

 weathered during the formation of the old soil or Dirt-bed. The stools of the 

 trees, some of which are known as "Bird's Nests," stand out in more or less circular 

 ridges, as sketched by Prof. Henslow in 1832, and they evidently mark old 

 terrestrial soils, and the remains of contemporaneous land-surfaces. 



Mr. J. C. Mansel-Pleydell has remarked that the most easterly 

 evidence of the Purbeck forest is at Gad Cliff; good examples 

 occur also about a quarter of a mile east of Lulworth Cove. The 



Lower Purbeck J /, 



Beds. ^ 



95 to 160 feet. 



1 Vertical Sections (Geol. Survey), No. 22 ; and Horizontal Sect. No. 56. 

 - Termed ' Horseflesh ' in the Isle of Portland. 



3 T. G. S. ii. 42. See also Buckland and De la Beche, T. G. S. (2), iv. 11 

 and Fitton, Proc. G. S. ii. 185. 



