210 Dr. PiTTON on the Strata heloio the Chalk. 



part of the Purbeck series, was formerly much quarried for roofing, in the hill above Tillywhim, 

 and near the south coast of the peninsula : and there, as well as in the corresponding beds of 

 Portland and the north of Weymouth, I have found in it casts in calcareous spar of one or more 

 species of Cypris. 



(105.) Portland Stone. — This formation first appears upon the coast be- 

 neath the Purbeck strata at Durlstone Head, and rises slowly in the rocky 

 cliflFs on the west of Tillywhim to a point about midway between Winspit 

 and St. Alban's Head, where it is succeeded by the Portland sand. The stony 

 strata there retire as they rise from the shore, and occupy the margin of an 

 irregular space thence to Gad Cliff, where they return to the coast and sink 

 under the sea : so that between the extreme points of Durlstone Head and 

 Gad Cliff (or rather Worbarrow Knob, a rocky hummock on the west of that 

 place), a real curvature combined with the general inclination of the strata 

 towards the north, which is in some places very rapid, has caused an extensive 

 disclosure of the lower beds, excellent sections of which are visible on the 

 west of St. Alban's Head. The component strata of the Portland stone in 

 Purb eck, agree with those of the Isle of Portland, which have been described 

 by Mr. Webster. 



(106.) Portland Sand. — I propose to give this name to a group of strata 

 which holds the place of that mentioned by Mr. Conybeare as occurring 

 beneath the equivalent of the Portland stone at Shotover Hill in Oxfordshire, 

 and in Bucks; and which in the interior contains a large proportion of 

 sand, including green particles, ascertained by Dr. Turner to be of the same 

 composition with those of the green sands beneath the chalk*. I have found 

 a similar group abounding in green matter, and containing, as at Shotover, 

 very large concretions of grit, in a corresponding place in the lower Bou- 

 lonnoisf, where a great part of the formation consists of sand. But on 

 the Dorsetshire coast, and I believe, in the Vale of Wardour also, the beds 

 are generally of a dark grey colour, more coherent, less sandy, and the 

 calcareous matter which they contain, is more uniformly diffused;};. The 

 group is certainly of sufficient importance to require a separate name, and 

 that of Portland sand, while it expresses the more usual character, indi- 

 cates also its intimate connexion with the Portland stone. On the other hand, 

 it graduates into the Kimmeridge clay beneath; and its position seems to be 



* See above, (10.) note §. t Geol. Soc. Proceedings, vol. i. pp. 9 and 27. 



J On the west of Upway, however, near the coast of Dorsetshire, nearly half the substance of 

 this stratum is made up of grains of green earth; and along the whole line of the formation in 

 that quarter, the prevailing character is that of siliceous sand and green earth. — See Dr.Buckland's 

 and Mr. De la Beche's Memoir, in the preceding part of this volume : p. 20, and note. 



