IN THE ISLE OP PORTLAND AND AROUND WEYMOUTH. 49 



the shore-line and to a certain distance beyond. With regard to the 

 action of ice, I feel about it the difficulty before expressed on the 

 score of the presence of organic remains ; and also, although a local 

 mass of ice might have radiated from the central higher ground 

 southward toward the Bill, and in a north-westerly direction at the 

 other end of the island, the size of Portland and its height are too 

 limited for local glaciers, while if it formed part of a continental 

 ice-sheet it would have a more definite and uniform direction, and 

 we should expect to find on the adjacent mainland traces of that 

 ice-sheet, which we do not. 



The limited size of the island forms also a similar but not so 

 strong objection to the operation of accumulated snow. 



I find in the third alternative an explanation which agrees with 

 the observed conditions. Sir R. I. Murchison, with respect to the 

 Brighton Cliff, and myself with respect to the Sangatte Cliff, had, 

 so far back as 1851, arrived independently at somewhat similar con- 

 clusions regarding the condition under which they were formed, save 

 only that his generalizations were more extreme, attributing to one 

 operation what I should attribute to several. He supposed that the 

 angular debris resulted from an extended catastrophe* caused by 

 great waves produced by great oscillations and violent fractures of 

 the crust of the earth, "by which the earth's surface has been so 

 powerfully affected in former times," passing over the land, and by 

 " the large area under consideration being suddenly broken up and 

 submerged." I then limited my conclusions to the factf that the 

 accumulation of the Sangatte drift was tumultuous and of short 

 duration, and that it was formed under w T ater. 



At present none of the Middle Purbecks have been discovered in 

 situ in Portland ;£ ; nevertheless we have shown that fragments of 

 them abound in the angular debris at the Bill. Either these have 

 been derived from the mainland or from beds which formerly existed 

 on the island. Had it not been for the circumstance that the angular 

 debris is found on the level of the Kimmeridge Clay at the base of 

 the escarpment at the north of the island, as well as on the level of 

 the Lower Purbeck on the south, I might have hesitated in ascribing 

 to it a local origin, and considered that it more probably came from 

 the beds in situ on the mainland before the elevation of the anticlinal 

 line and subsequent denudation. There are, however, two objections 

 to that view — viz. an absence of specimens derived from any other 

 strata within that area, and the earlier date of the denudation. I 

 therefore imagine that the fragments are of local or Portland origin, 

 that prior to the accumulation of the angular debris the Purbeck 

 series in Portland was more complete, and that, instead of being 

 confined, as now, to part of the lower almost unfossiliferous beds, it 



* Quart. Journ. Geol. Soc. vol. yii. p. 389. f Ibid. p. 274. 



J On the slope, north of the lower lighthouse, I found in a small old quarry 

 a thin seam of stone full of Cyprides, but of the smooth Lower-Purbeck variety. 

 I failed to find on the rather higher ground above any section to show what 

 the strata were. At Blacknore the thickness of the Lower Purbeck is very 

 considerable, but fossils are absent or very scarce. 



Q. J. G. S. No. 12L e 



