Page:Quarterly Journal of the Geological Society of London, vol. 31.djvu/95

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