Dr. PiTTON on the Strata below the Chalk. 



219 



and still retains its width at the bottom. The thin slaty beds of the Purbeck are in this instance 

 disturbed and bent, downwards and towards the fissures, to some feet on both sides ; but the soil 

 above is perfectly level; and in some other cases, where two or more cracks are near each other, 

 the intermediate masses are still more contorted and displaced. It will be shown hereafter that 

 appearances of the same kind are observable in the Portland-stone quarries in Wiltshire, Oxford- 

 shire, and Bucks. 



Sectionof one of the Portland Quarries. 1834;. 



1. Vegetable Soil 



2. " Slate " 



s 



3. Clay, or " Dirt ' 



4. "Bacon-tier" 



5. Clay 



6. "Ash 



7. " Soft Burr " 



8. " Black Dirt 



9. " Cap ' 



[fiSS^I^^^^S 



10. "Dirt", with Cycadees 



11. "Skull-Cap" 

 . 12. "Clay" 



13. "Roche" 



"3 



S 



' (^^';g^ '_;<2s^^'^^'"^i5^'-'^^"'^pV^;^;^._<^p 



14. " White-Bed" 



Bottom of the Quarry. 



2 



-60 



2. The " Slate" of the preceding section is nothing more than the coarsely fissile limestone 

 which pervades the whole of the Purbeck formation, but at the lower part less frequently alter- 

 nates with clay. Its thickness varies in different parts of the Isle of Portland, from about 

 fifteen to less than six feet. In some places it includes small veins of rhombic carbonate of 

 lime in minute crystals ; but has commonly the general aspect and flat conchoidal fracture of 

 freshwater limestone. On a close examination these beds are found to contain the remains of one 

 or more species of Cypris*; and I saw a few traces of small Modiolae, like those which abound 

 in the corresponding strata in the Vale of Wardour and Buckinghamshire. The Cypris, in fact, 

 is found in all the beds above the oolite (13. of the Sketch above); so that no doubt remains 

 respecting the boundary between the Purbeck and the Portland formation below. Nothing can 



* As, with few exceptions, casts only of the interior of the crusts are obtained here, it is difficult 

 to determine the species of these fossils. Mr. Sowerby is disposed to refer most of them to a 

 new species, C. tuberaclata, PI. XXI. fig. 4. ; and a few perhaps to the C. Valdensis, PI. XXL 

 fig.l. ; both of which abound in the corresponding part of the Purbeck formation in the interior. 



