IN THE ISLE OE PORTLAND AND AROUND WEYMOUTH. 31 



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 (/ 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 Yerne, where the summit-level 

 is reached at a height of 500 feet. The Portland stone is exten- 

 sively 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 debris 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* 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. 



