We thus have the elevated range of the chalk on the one side, and on the other the high and nearly precipitous cliffs of Portland, with an intervening area at a lower level, presenting almost every- where a bare denuded surface of Jurassic strata.
Portland Mammaliferous Drift (f on Map and Sections, PI. I.).
The Isle of Portland also exhibits a singularly bare and denuded surface, with the exception of one small spot on the top of the island, which was discovered in the progress of quarrying (it has now been nearly removed) in the eastern part of the Admiralty quarries. I was obligingly conducted to the section by Capt. Clifton, Governor of the convict prison, in whose collection I had seen some specimens from it which were new to me, and to whom I am indebted for many particulars of the deposit, which he had noted before its removal. It was from this spot, we may presume, that Mr. Neale obtained his specimens. The ground is here about 400 feet above the sea-level, and a few hundred yards south of the Verne, where the summit-level is reached at a height of 500 feet. The Portland stone is extensively quarried; and over it is a capping of from 10 to 15 feet of unfossiliferous Lower Purbeck. The mammalian drift occupied an irregular trough in the Purbeck and upper part of the Portland beds.
Capt. Clifton informs me that, when first discovered, the deposit occupied a depression in these rocks, with a surface level with theirs. It was from 10 to 20 feet thick, with a width of from 50 to 60 yards, and extended N.E. and S.W. for a distance of from 200 to 300 yards. I found the remaining part of it in patches between, and spread over, the large waterworn blocks and surface of the upper Portland rock. The deposit consisted of a red clay or loam, passing into a coarse loess, in places full of angular local débris of the Portland and Purbeck beds, together with a considerable number of small blocks (some a quarter of a ton in weight) of the hard sandstone or sarsen-stone[1] of the Lower Tertiaries. At a few places this was underlain by a singular layer of pebbles, waterworn and perfectly rounded, and in a matrix of sand and red loam mixed with a large proportion of peroxide of manganese, whilst they were occasionally cemented together in a thin layer of calcareous spar. The pebbles so encased presented a perfectly clean and bright surface, as though they had been artificially polished. In this deposit I found:—
1. Small round flint pebbles ~) Origin. 2. Rolled and subangular fragments of ironstone grit I j, ,,. 3. Small subangular fragments of very hard sandstone j &' 4. Imperfectly rounded blocks of ditto J 5. Small angular fragments of flint Chalk. 6. Well-rolled rounded pebbles of chert ] TT n , 7. Subangular fragments of chert ) U ^ er G ™™™<*- 8. Subangular fragments of black flint Portland. 9. Quartz pebbles Old Gravel?
- ↑ Much worn and stained reddish brown, and sometimes blackened by oxide of manganese.
