Perforations in Carboniferous LimestoneAuthor(s): R. F. ScharffSource: The Irish Naturalist, Vol. 1, No. 6 (Sep., 1892), pp. 118-119Published by: Irish Naturalists' Journal Ltd.Stable URL: http://www.jstor.org/stable/25520224 .

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PERFORATIONS IN CARBONIFEROUS LIMESTONE. BY R. F. SCHARFF, PH.D., B.SC.

I HAvE been asked by Mr. Carpenter to make a few obser vations on the very renmarkable perforations which Mr. Owen Smith discovered in the Carboniferous Limestone near Nobber, Co. Meath. They were first described by him in the June number of the IRISH NATURALIST, and the following

month notes appeared by Messrs. Fallon, McBride, and Walpole, who had all seen similar perforated rocks in differenit parts of the country. The Rev. Mr. Close has also observed them by the side of Lough Mask, and he renmarks that we have to choose between the only two alternatives:-that these perforations have been produced by weatherinig or by the action of a boring mollusc. Mr. Smith is in favour of the latter view, but most of the writers of the above-mentioned notes are rather in favour of their having been caused by the action of water on the limestone.

But when we consider the position of the holes, their varying depth while the diameter remains constant, their upward tendency, their being confined to a portion of the stone only, and, as Mr. Smith justly remarks, their great "family likeness," the aqueous theory presents formidable difficulties. Indeed it seems to me impossible to conceive how either chemical or mechanical action of water could produce these perfectly smooth perforations of an inch in diameter on the face of a limestone rock, and having seen the specimen which was forwarded to the :Editors of the IRISH NATURALIST, I have not a doubt that the explanation given by Mr. Smith is the correct one.

A marine boring mollusc (Phoias crisataa), still inhabiting the Irish Sea, fits exactly into some of the holes, and the first impulse to attribute the perforations to the action of this or an allied species appears quite justified. The geological deductions, moreover, which we can draw from such a con clusion are so seductive that it is difficult to abandon a theory of such important bearings. Pholas-borings in situ at Nobber, in such a fresh condition, mean that within very recent times that locality must have been covered by the sea, perhaps to a depth of 200 feet, for the species of Phoias are known to live at a depth below the surface of the sea not exceeding 30 fathoms. We know that marine shells are found at an altitude of about IooO feet in the Dublin Moun tains, and the presence of a number of other deposits in various parts of Ireland renders it extremely probable that within a recent geological period the sea covered a large portion of eastern Ireland at any rate. In spite of a number of facts wh'ich may be adduced as evidence in support of the

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Perforations in Carboniferous Limestone. II9

theory that the Co. Meath, and with it the site of the village of Nobber, were submerged beneath the sea at a compara tively recent period, I venture to think that, nevertheless, the perforations in question are not due to the action of Pho/as, or any other nmarine mollusc, and in this view I am supported by Prof. Sollas.

It may not be out of place to refer here to a discussion which was continued for several years between i869 and I872, in the Geological Magazine. Lithodomous perforations, exactly similar to those described above from various parts of Ireland,

were discovered in the Carboniferous Limestone of the Great Orme's Head in Wales by Mr. R. D. Darbishire. He con sidered them to have been formed by a species of Pholas, probably P. crispata. Prof. Bonney, in reply to several articles on the same subject, figures a number of sections of the supposed pholas burrows which show clearly that they coilld not have been produced by that or any other marine mollusc.

Moreover, in a great many of the holes he found living specimens of Hlelix aspersa, and he comes to the irresistible conclusion that the perforations "are not the weathered burrows of departed Pho/ades, but have been, and are being hollowed out by He/ices, the principal, if not the only agent being Helix aspersa."

But not alone in England have these perforations in the Limestone been observed and recorded. In a paper entitled "Observations sur les Helices Saxicaves du Boulonnais" M. Bouchard-Chantereaux describes similar excavations at great length. He watched them for many years with great care, and measured their progress from time to time, which left in his mind no doubt of the fact that they were made by snails. He found that if he removed the snail, whilst in the act of perforating the limestone, the exudation from the mouth reddened litmus paper, showing the secretion was then of an acid nature. It is probable, therefore, that the perforating action is mainly a chemical one, though the mechanical action of the snail's tongue, which is like a little rasp, must assist it in the work to some extent.

Helix aspersa is a very common snail in most parts of Ireland, and I believe, considering the evidence we have before us, that the burrows in the Carboniferous Limestone discovered by Mr. Smith in the Co. Meath are not only due to the action of that mollusc, but are being produced by it at the present day.

The perforation in the stone from the harbour-bed at West port, sent by Mr. McBride, is undoubtedly due to the action of a species of Pho/as.

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