Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/366

 served by him in West Somersetshire in 1866, and stated that in 1849 he had himself called attention * to the marks of glacial action shown in the terminal curvatures of slaty rocks in Forfarshire, and suggested that they were to be explained by the progression of a thick covering of ice along the edges of the slaty beds.

Discussion.

Mr. Gwyn Jeffreys read extracts from the Rev. Mr. Hodgson's 'History of Northumberland ' (published in 1827), in which these borings in limestone were referred to the action of snails. Mr. Jeffreys considered the foot to be the sole instrument employed by the boring Mollusca in excavating their burrows. He exhibited specimens of Lias from Lyme Regis perforated by Pholas, and of hard limestone from Malta perforated by Lithodomus, and remarked, in connexion with the notion that asperities on the shell might be boring agents, that the shell of Lithodomus is perfectly smooth.

Prof. Ramsay mentioned that he had seen Helices taken out of these holes at Tenby by Dr. Buckland, who believed that the snails effected the perforations by the agency of an acid.

Mr. Charlesworth thought that if so much uncertainty could prevail upon such a subject, it threw great doubt upon some of the grandest generalizations of geology. He referred to the evidence connected with the glaciation of the Great Orme's Head, in which the origin of the perforations under discussion was of much importance, Mr. Darbishire maintaining that they were the work of Pholades, while Mr. Bonney asserted that they were produced by snails. In the same way the origin of the celebrated borings in the Temple of Jupiter Serapis might be disputed, and the generalization founded upon it rendered doubtful. Mr. Charlesworth noticed the necessarily small proportion of borers to the whole snail-population of Britain, and remarked especially upon the absence of perforations in the chalk districts. He considered that repeated observations were necessary before this snail-engineering could be admitted, and suggested a systematic course of experiments.

Mr. Boyd Dawkins suggested that the carbonic acid exhaled by snails in respiration might act upon limestones, and remarked that chalk weathers too rapidly to preserve the excavations.

2. On the probable Cause, Date, and Duration of the Glacial Epoch of Geology. By Lieut.-Col. Drayson, R.A., F.R.A.S.

(Communicated by Alfred Tylor, Esq., F.G.S.)

[Abstract.]

After referring to the evidence of the occurrence of a Glacial Epoch, and to the various hypotheses which have been proposed to


 * Jameson's Edinburgh Journal, vol. xlvi. p. 377.