42 J. PRESTWICH ON THE QUATERNARY PHENOMENA 



of the fissile beds of the lower Purbecks, presents a bare rock-surface. 

 There is no trace of anymore recent Secondary or of any Tertiary strata 

 anywhere in the island ; nevertheless the mammaliferous drift of the 

 Admiralty Quarries contains well-worn pebbles of chert from the Upper 

 Greensand, and of iron sandstone and Sarsen-stone, together with 

 small blocks of the latter from the Tertiary strata, and chalk-flints from 

 the hills between Up way and Dorchester, a distance of from eight to 

 ten miles to the north of Portland, from which it is separated by a 

 low plain of older strata, where none of the few scattered drift-beds 

 has any resemblance to the Portland drift. This latter stands alone ; 

 and although at a greater distance from the Tertiary strata in situ, it 

 contains nevertheless a much larger proportion of their debris than 

 do any of the nearer and lower-level drift-beds. 



These Tertiary and Greensand materials could only have been 

 transported to their present position by a floating iceberg or by a 

 running stream. The former is not probable, as the transported 

 rock-specimens are all waterworn and mostly well rounded, and they 

 are associated with mammalian remains and a silt which has the 

 ordinary characters of a freshwater loess, although, as is common 

 with the loess itself, no shells have been found in it. I can only 

 conclude, therefore, that this deposit has been formed by direct 

 transport by a stream running southward from the Greensand and 

 Tertiary area and passing over Portland ; and this, necessarily, could 

 only have taken place when the intervening district was bridged 

 over by strata since removed. It follows that the denudation of 

 the plain of Weymouth and the deposition of the several drift-beds 

 found thereon are of more recent date than the mammiliferous drift 

 of Portland (see Sections 1 and 2, PI. I.). 



If the dip of the Portland and Purbeck beds at Portland be 

 prolonged northward, they would reach an elevation of about 

 1000 feet at Broadway, and of about 1500 feet at the Eidgeway, 

 and thus interpose a high ridge between Portland and the newer 

 strata from which the pebbles of the Portland drift have been de- 

 rived; consequently either the crest of the anticlinal had then 

 been removed, and a high level plain of the Jurassic strata extended 

 from Portland to the Greensand and Tertiary area, or else at that 

 time the north end of Portland had not been upheaved, and a con- 

 tinuous plain of Portland and Purbeck extended to the Bill of 

 Portland from the point where those formations were brought into 

 contact with the newer strata by the great Eidgeway fault. Looking 

 to the facts that the Portland drift is at a height of 400 feet, or 

 nearly that of the supplying Greensand and Tertiary strata — that the 

 Portland plateau i3 prolonged northward to 100 feet higher, and is 

 then abruptly truncated — and that the drift-bed, although rich in 

 Tertiary sandstone, does not contain a fragment of the Coral Eag, 

 Forest Marble, or other rocks which, under the first-named condi- 

 tions, must have formed the surface of the plain over which the 

 stream would have passed, it is, I think, more probable that the 

 second condition, or that of a gradually sloping plain of Portland 

 and Purbecks extending from the Bill of Portland to the Greensand 



