Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/55

Rh

-beaches are common around our coast lines; but none seem to have attracted so much attention as the Chesil Beach on the north-east margin of Lyme Bay, Dorsetshire. This is probably due to the circumstance of the beach being situated on a much resorted-to coast, its great length, and the peculiar assortment of the fragments composing the accumulation, these being arranged as if they had been passed through a series of sieves of gradually decreasing coarseness. This last particular has even attracted the attention of the fishermen, who say they can tell, on the darkest night, at what part of this beach they have landed, by the size of the pebbles at the place.

The latest published paper on the Chesil Beach is that of Prof. Prestwich, read before the Institution of Civil Engineers, February 2nd, 1875; and from the discussion at the reading of it it is evident that opinions still differ much on the subject. This has emboldened the writer of this paper to come forward, more especially as no previous writer seems to have taken into consideration the special effects that the "flow" of the tide, which is most accelerated at the nodal or hinge-line in the English Channel, must have on the east shore of Lyme Bay—also because he has specially studied Cahore shingle beach, co. Wexford, which in many points is analogous to the Chesil Beach.

At Portland Bill on the English Channel, and at Cahore Point on the Irish Sea, the tidal currents are considerable, and must work more or less actively; but to understand them, it is necessary to give an epitome of what is known about the tides in these seas. The tidal waves and currents in the seas surrounding Great Britain and Ireland were but imperfectly known until after the investigations carried on by the late Admiral Beechey and Lieut. F. W. L. Thomas, R.N. The result of their researches in regard to the English Channel and Irish Sea is given in the manual of Tides and Tidal Currents by the Rev. S. Haughton, F.T.C. Dublin; and to it we are indebted for much of the following.

The tides rise and fall twice in a little more than twenty-four hours ; but the risings and fallings are not always of equal extent,