410 J. C. Mansel-PIeyclell — Geology of BorseL 



headland at Gad Cliff, and, pursuing its course eastward, it retires 

 from the coast-line, occupying for the most part the summit of the 

 range behind the village of Kimmeridge. Near Swyre Head a fault 

 throws it northward, and it forms the higli land south of the village 

 of Kingston to Downhays Barn, where it takes a southerly direction, 

 and touch.es the coast-line again at Emmets Hill, forming the upper 

 part of St. Alban's Head ; through Winspit and Seacombe to Durle- 

 ston Head, a mile south of Swanage, where the quarries yield a white 

 limestone containing Ammonites, Trigonice., and Ostrece; also tabular 

 veins of chert in large irregular layers and nodules. The oyster- 

 bed is eight feet in thickness, and overlies a series of the freestone 

 beds, which consist of Ostrea solitaria cemented together by an 

 infiltration of calcareous matter, and was extensively worked 

 formerly in the old quarries of Tillywhim and Winspit. The beds 

 of the Portland series have a northern dip here, while in Portland 

 they dip to the south. The upper portion, which averages from 

 three to four feet in thickness, is extensively quarried ; it contains 

 only casts of fossils — those of a spiral univalve (Nerincsa), called by 

 the quarrymen the Portland screw, is common. The Eoach-bed, as it 

 is locally termed, is very hard and especially adapted for building 

 purposes. A part of old St. Paul's was restored by Inigo Jones 

 from these beds. In the Eidgeway district, especially at Upwey, 

 the Portland Stone assumes a chalky character with chert and 

 Trigonice — it is associated with the Portland Sands from Poxwell to 

 Portisham. 



PuRBECK Bbds, — A shelly Limestone containing OstrecB occurs 

 among the lower portion of this formation, locally called Purbeck- 

 Portland. A very remarkable bed resting upon a thin band, four 

 inches thick, and crowded with scales of Histionotus hreviceps, ex- 

 tends over nearly the whole of Portland, the east of Lulworth 

 Cove, Gad Cliff, and Eidgeway, where it is still thinner. Silicified 

 trunks of coniferous trees, which sometimes stand erect, but are 

 generally broken off short, and seldom exceeding eight feet in length, 

 and remains of other plants allied to Zamia and Cycas, occur in this 

 bed. About a mile to the east of Lulworth Cove there are several 

 dome-like limestone-knobs, hollow in the centre, which are prob- 

 ably the enlarged bases of Cycadean plants. At Gad Cliff, which 

 flanks the western side of Kimmeridge Bay, a large Limestone block, 

 ten feet long and six broad, encases one of these Oolitic trees. 



The Purbeck-beds attain their fullest development in Purbeck, 

 and extend from the town of Swanage to Worbarrow, their northern 

 boundary being the valley which unites these two places ; their 

 southern boundary the coast to the east of St. Alban's Head, where 

 they turn northward to Afilington Barn, and, skirting the north side 

 of the Encombe and Kimmeridge range to Gad Cliff, are lost beneath 

 the sea at Worbarrow Tout. At Blackmanston they are squeezed up 

 between the Kimmeridge Clay on the south and the Hastings Sands on 

 the north : continuing westward without further expansion, they re- 

 appear at Mewps Bay, forming the coast-line to Durdle Door. 



The great fault which commences in the neighbourhood of Poxwell, 



