IN THE ISLE OF PORTLAND AND AROUND WEYMOUTH. 43 



and Tertiary area, obtained at the period of the Portland Drift (see 

 Section 3, PL I.). 



The disturbance which raised the Weymouth district and elevated 

 the north of Portland, is the central anticlinal of the Jurassic series 

 (A in Map, PL I.), which runs east and west and brings up the 

 Forest Marble between Broadway and Buckland Eipers ; for I find 

 that on prolonging the angle of rise of the Portland beds northward, 

 and that of the Purbeck beds (where they crop out on the south of 

 the great Eidgeway fault) southward, the two planes meet exactly 

 over the ridge of that line. The great Eidgeway fault is, I believe, 

 of older date. 



If I am right in this intepretation, then it is probable that the 

 anticlinal bringing up the strata below the Chalk at Purbeck, and 

 likewise the great upheaval at the back of the Isle of Wight, both of 

 which are on the same line of disturbance as the Broadway anti- 

 clinal, are also of the same age ; and we thus obtain a marked in- 

 stance of elevation and denudation during the Quaternary period. 



The great difference of level between the mammaliferous drift and 

 the raised beach might lead to the supposition that they were of diffe- 

 rent ages. Still, as the former is not a case of capping horizontally 

 an isolated hill, but of forming part of a sloping surface continuous 

 from the northern to the southern end of Portland — and as that sur- 

 face, for the reasons before mentioned, had its inclination given it* 

 subsequently to the deposition of the drift, and when consequently 

 its level above the sea may not have exceeded from 100 to 150 feet, or 

 about 50 or 100 feet above that of the old beach, while the old beach 

 itself (like the drift-bed) contains materials derived from the same 

 Tertiary and Greensand area, — it is probable that the drift-bed and 

 the raised beach were contemporaneous. In fact, while in the upper 

 part of the Admiralty Quarries the drift-bed attains a height of 

 400 feet, at the angle of the road near the new church it is only 

 346 feet high, still dipping in the direction of the Bill. On the 

 other hand, the Old Beach, which at the Bill is only 24 feet above 

 the present beach, rises gradually northward until near the Sand- 

 holes, where it attains a height of 36 feet. It is probable, there- 

 fore, that the old stream emptied itself a short distance off the 

 present Southwell shore into a small bay, of which the land passing 

 from the Sand-holes in the direction of the lower lighthouse formed 

 the western horn. 



The Portland raised beach is by far the most interesting one in 

 the south of England, whether for its extent, its thickness, its large 

 exposure, or its general conditions. In the first place, the materials 

 of which it is formed are partly of local, but still more are of 

 distant origin. Although it contains pebbles of Portland Stone and 

 chert, it is essentially a chalk-flint shingle with a considerable pro- 

 portion of pebbles of chert from the Greensand. It also contains 

 subangular brown -coated flints, derived apparently from an old gravel, 

 with a few which may be referred to Tertiary strata, together with 

 some angular fragments of chalk-flint showing no wear. With these 

 are a number of pebbles of red sandstone and of light-coloured and 



