224: Dr. FiTTON on the Strata below the Chalk. 



belong to the species nidiformis of Adolphe Brongniart (Cycadeoidea megalophylla of Dr. Buck< 

 land) : but it deserves inquiry whether the specimens in the lower bed agree in all respects with 

 either of those found in the upper one. 



In the Isle of Portland, no trees have hitherto been found in this lower dirt ; which, from its 

 smaller thickness, might seem to have been less adapted to their production than the upper one. 

 But if I can rely upon a single observation, part of a prostrated trunk exists in the Dirt below the 

 Cap, in a section exposed on the west of the road leading from Upton (on the main land of Dor- 

 setshire) to Poxwell. I came to this conclusion from observing that the inclined stratum of lime- 

 stone over the bed including this specimen was distinctly botryoidal, and of great thickness, — two 

 characters which accord with those of the Cap; while the stone below it contained Portland 

 fossils. But I saw no Cycadeae in the bed at this place, 



11. " Skull-Cap." The thickness of this bed in Portland varies from one to three feet, and it 

 frequently swells out suddenly from 15 or 18 inches to double that thickness, and returns as 

 rapidly to its previous dimensions. It is less uniform than the Cap, and sometimes has an 

 obscurely conglomerated appearance, but, like that bed, consists of freshwater limestone, and is 

 in some places botryoidal. It includes also, as Mr. Webster has stated, small cavities coated with 

 minute rhomboidal crystals of carbonate of lime, like those found in the slaty beds above. 



12. At the lower part, the " Skull-Cap " is closely attached to, or passes into, a seam of argil- 

 laceous matter, sometimes not more than half an inch, and seldom more than 2^ inches thick, by 

 which it is separated from the Portland stone. This seam appears never to be wanting in these 

 quarries, and is much more uniform than the Dirt immediately above the Skull-Cap, which is very 

 unequal in thickness : in some instances it is incorporated with, or adheres very closely to, the 

 top of the Portland stone *. 



13. and 14. The top of the Portland series consists of very fine-grained oolite, resembling the 

 roe of fishes, and of a very light brownish hue. With the oolitic particles, other rounded frag- 

 ments, less regularly shaped, are mixed and united by a calcareous cement, so that the whole 

 compound is very like the recent conglomerates which abound on the shores of New Holland, of 

 many of the Indian Islands, and of Bermuda ; especially resembling the newly-formed masses of 

 the last-mentioned islands, described by Lieut. Nelson in a paper lately read before this Society f- 

 This oolitic portion is seldom more than four inches thick, and passes at the lower part into dark 

 grey flint, in irregular concretions, which form an interrupted range near the top of the bed, and 

 include the same petrifactions (but silicified), with those which abound in the limestone imme- 

 diately around and below them. The stone below the flints, to a distance of from two to four 

 feet from the top, is called " Roche " by the quarry-men, and is essentially continuous with the 

 lower part of the bed ; but it contains so many casts of shells (of which, indeed, it is almost 

 entirely composed), as to be useless for the purposes of building, and is therefore always sepa- 

 rated in the quarry. The remainder of this stratum, about 8 feet thick, is known in the island as 

 the " White-bed ", and is that which is now, almost exclusively, quarried for the market under the 

 name of Portland stone. This and the lower strata of the formation, which are not worked at 

 present, have been already described by Mr. Webster. 



Among the fossils of the White-bed in Portland, are a Corbis ?, Cythercea parva, Lithodomus, 

 Pecten lamellosus, Perna quadrata, Plicatula, Terehra Portlandica, Trigonia gibbosa, T. incurva. 



* The existence of Cycadeae within three feet of the Portland beds being certain, it deserves 

 inquiry whether they may not occur also in this lower and thinner Dirt-bed immediately above the 

 stone. 



f Proceedings, vol. ii. p. 81. 



