212 Dr. PiTTON on the Strata below the Chalk. 



Feet. 

 4. A rugged platform at the foot of the clifF above described, is covered with 

 debris, and partially clothed with shrubs and vegetation. The beds appear to 

 be :— 



a. Soft, dark bluish grey, effervescent sand-rock, or indurated marl, including a 

 band of firmer consistence, which contains casts of Panopcea depressa and other 



^^'^^l^*-'^- t 40 to 50 



b. Brownish marly sand-rock, softer than a. 



c. A firmer stratum, including, about 20 feet from the bottom, large subglobular, 

 but nearly continuous concretions of greater firmness, which also contain casts of 

 Panopaea dej}ressa and other fossils. 



[These concretional masses, not improbably represent the great nodules of 

 Shotover Hill in Oxfordshire.] 



Total estimated thickness of the sand 120 to 140 



Springs break out at the bottom of this group, where the Kimmeridge clay might be ex- 

 pected. The total height from the top of the cliff may be between 250 and 280 feet. 



(107.) Kimmeridge clay. — The clay which has obtained its name from the 

 farm and bay of Kimmeridge^ rises from beneath the beds last mentioned^ — on 

 the east, near the point or southern prominence of St. Alban's Head, and 

 on the west, immediately under the lofty precipices of Gad ClifF. It occupies, 

 apparently, the whole of the low undulating tract which intervenes between 

 these extreme points ; and is capped, about the middle, by the projecting mass 

 of Portland stone and sand which forms the summit of Swyre Head. The 

 bituminous strata which characterize this formation are seen with greater 

 distinctness on the shore of Kimmeridge Bay, as the coarse sand, grit, and 

 limestone found between them and the Oxford oolite on the north-east of 

 Weymouth, do not come up to the surface in the peninsula of Purbeck. 

 The little promontory of Hen Cliff may be considered as the centre of an 

 almost semi-circular space, which is continued in a narrow slip or platform, 

 along the shore to Pier Bottom : Plate X. b. fig. 6. 



The strata on the shore at Kimmeridge Bay form a low dome-shaped prominence, the cul- 

 minating point of which is at some distance from the ridge of the Isle of Purbeck. At Broad- 

 bench, where they are very well displayed, they rise with a very slight inclination from the south; 

 and rise also from beneath Gad Cliff on the west; but at Hen ClifF the dip is towards the south- 

 east, conformably to the general disposition of the strata beneath St. Alban's, and thence to Durl- 

 stone Head ; while it is obvious that under the main ridge of the peninsula they must dip to the 

 north, along with Portland and Purbeck strata. The rise of the beds from the sea, where the 

 shore is cut into by the circular erosion at Kimmeridge, has produced an apparent curve in the 

 cliffs around the bay ; so that the strata seem to rise from the sea near Broadbench, and to de- 

 cline again as the line of the coast returns toward the south in approaching Hen Cliff. The 

 natural sections hereabouts are very distinct ; and a considerable surface also of some of the 

 strata is exposed upon the shore, consisting apparently of the lowest beds in the Isle of Purbeck. 

 The coast at Kimmeridge Bay is probably not less than 400 feet below the bottom of the Port- 

 land stone ; and if 100 feet of that interval be assigned to the Portland sand, it will follow that 



