J. C. Mansel-FleydeU — Geology of Dorset. 411 



and extends througli Friar Waddon to tlie north of Abbotsbury, has 

 its greatest extension at Bincombe and Eidgeway, where a good 

 section is exposed in the railway-cutting; the Purbeck-beds flank the 

 south side of the fault with a few exceptions from Poxwell to 

 Portisham, The late Prof. Edward Forbes divided them into upper, 

 middle, and lower, each marked by a peculiar assemblage of organic 

 remains : the lower is largely developed at Eidgeway, the middle 

 and upper in Durleston Bay. There are evidences of various changes 

 from sea to brackish-water, and inversely, between each of which 

 long lapses of time must have intervened. The recurrence of the 

 delicate remains and fragments of insects may be accounted for 

 under the supposition that these light spoils from the land were 

 carried down by the rivers into the estuary, and the abundant deposits 

 in some of the beds were probably occasioned by the collapse of 

 insect life, through sudden changes of temperature. Some of the beds 

 at Eidgeway are characterized by a peculiar Pupa and a small Hemi- 

 pterous insect, JVotonecta, which belonging to an aquatic species may 

 probably be the cause of its being found entire, and not fragmentary, 

 as are the dragon-flies and other entomological remains. At Eidge- 

 way the corresponding beds of the Lower Purbeck differ by a greater 

 development of limestone, and the dark shaly clays are entirely 

 absent, which chiefly constitute the Durleston Bay section. At 

 Durleston Bay the lowest beds commence with the lower insect-beds 

 of Prof. Edward Forbes, and are of marine origin. The quarries which 

 give the Middle Purbecks their valuable economic character lie 

 beneath the Pecten beds, and are chiefly of fresh and brackish water 

 formations. It is from this zone the Fish and Turtle remains are 

 chiefly obtained. 



The Cinder bed is the first evidence of the invasion of the sea 

 during the Middle Purbeck age. It is mostly formed of the remains 

 of Ostrea distorta, and contains the Hemicidaris FurbecTcensis, the 

 only Echinoderm of the Purbeck series hitherto known. This bed 

 forms a marked phj^sical feature throughout the island, and by the 

 experienced eye can be detected with certainty. The Cinder bed is suc- 

 ceeded upwards by (H.) Turtle beds, (G.) Pecten beds, (E.) Corbula 

 beds, and (F.) Beef beds, of Eev. J. Austen's list, which are capped by 

 (D.) the uppermost of the Middle Purbeck. The upper division at 

 Swanage is about thirty feet thick, including a band of Paludina 

 elongata, which takes a high polish. It was held in high repute by 

 the architects of the middle ages, and there are but few of our 

 cathedrals and old churches which do not contain pillars and window 

 shafts of this material. Mr. Bristow considers there is sufficient 

 evidence that the greatest part of this material for ancient eccle- 

 siastical ornamentation was procured from the neighbourhood of 

 Swanage. 



The remainder of the Upper Purbecks consists of a series of bands 

 of clays and stone, the clays being charged with Cijpris shales, and the 

 stone with UnionidcB. At Eidgeway these beds are coarser and more 

 sandy, and contain lignite ; at Mewps Bay they are thinner, but contain 

 a greater variety of species of Gypris than at Durleston. A remark- 



