before the Cambrian sediments were deposited upon them is clear from the fact that the pebbles of the Cambrian conglomerates which rest immediately on any portion of the series are almost invariably made up of masses of the rocks below, cemented by gritty materials on an unaltered matrix, and from which the pebbles may be easily removed. The great conglomerates at the base of the Cambrians, everywhere in Wales, indicate that there were beach- and shallowwater conditions over those areas at the time, and that the sea was then encroaching on an uneven land, becoming gradually depressed to receive the subsequent Cambrian sediment.

3. On some Precambrian (Dimetian and Pebidian) Rocks in Caernarvonshire." By Henry Hicks, Esq., F.G.S.

In this paper the author gave an account of the special examination of the great ribs of so-called intrusive felspathic and quartz porphyries which are found associated with the Cambrian rocks in Caernarvonshire, made by him in company with Prof. Hughes, Mr. Hudleston, and Mr. Homfray last summer. He described sections at and near Moel Tryfaen and across the mass from Pen-y-groes to Talysarn, in which he showed that instead of being of an intrusive nature, as hitherto supposed, the whole, with the exception of a few dykes at those parts, is made up of bedded volcanic rocks, lavas, breccias, etc., similar to those found in the Pebidian series at St. David's, and that the Cambrian rocks, instead of being intruded by this mass, rests everywhere upon it unconformably, and the pebbles in the conglomerate of the Cambrian at the base are, as at St. David's, identical with, and must have been derived from the rocks below. Similar results were obtained in the examination to the north and south of Llyn Padarn, and the conclusion, therefore, at which the author has arrived with regard to the great mass which extends from Llanellyfine in the south of St. Ann's chapel in the north is that it is entirely Precambrian, and that it belongs to the series described by him under the name Pebidian at St. David's.

The other mass, extending from Caernarvon to Bangor, he considered also entirely Precambrian; and from the mineral characters exhibited by a portion of this mass directly behind Caernarvon, he thought it would prove to be, at least at this part, of Dimetian age. The altered beds near Bangor and their associated quartz felsites he considered entirely of Pebidian age, as there is no evidence that the Dimetian rocks are exposed there.

4. "On the Precambrian Rocks of Bangor." By Prof. T. McKenny Hughes, M.A., F.G.S.

The author described a series of slates, agglomerates, and porphyritic rocks which, near Bangor, are seen to pass under the Cambrian and seem to rest conformably upon the quartz felsites and granitoid rocks of Caernarvon. He thought that the Bangor beds were the equivalents of the felsitic and porphyritic series of Llyn Padarn, and, in order to bring his interpretation into harmony with the observations of Prof. Ramsay, he explained away the apparent melting of the ends of the Cambrian beds in that section by twists, faults, and dykes. He referred the apparent unconformity recorded

by Mr. Maw entirely to rock structure, produced by cleavage on beds of different texture.

He considered that in the main the Bangor beds were the equivalents of the Pebidian of Dr. Hicks, while the Caernarvon beds nearly represented his Dimetian. But he thought there was as yet no proof of an unconformity between these formations. He would explain the apparent unconformity at St. David's by a continuation of bends and faults and joints mistaken for bedding, and would refer the brecciated rock of Low Moor, near St. David's to the Pebidian, thus taking it on the wrong side of the supposed unconformity. He thought that the green beds in the Dimetian were, in all the cases where he had been able to examine them, originally dykes.

He saw, therefore, no reason, from an examination of other areas, to suspect any different explanation from that suggested by the examination of the Bangor and Caernarvon district, viz. that we have in the Bangor and Caernarvon beds one great volcanic series, on which the Cambrian conglomerates and grits rest with a probable unconformability.

An appendix by Prof. Bonney, on the microscopical examination of the rocks referred to, accompanied this paper.

CORRESPONDENCE.

"CONCRETIONARY BANDS" OR "CONGLOMERATES" OF LAMBAY ISLAND.

SIR,

In the paper "On the Borrowda.e Series and Coniston Flags," in the Quarterly Journal of the Geological Society for Aug. 1877, p. 479, the authors speak of "Concretionary bands" "called by" the late "Mr. Du Noyer, coarse conglomerates," and according to the late Prof. Jukes, containing "pebbles" with Silurian corals attached. In next page these "conglomerates" or "concretionary bands" are said to form a portion of an ash-breccia series, but no reason is given for what would seem to be an entirely unnecessary correction of the descriptions quoted from Mr. Du Noyer and Prof. Jukes; nor is it stated why the rocks are referred to as concretionary bands."

These descriptions, quoted at p. 479, will of themselves show the difficulty of accepting the concretionary nature of the Lambay rock referred to; the matrix being of "black mud," ir closing pebbles of "cleaved slate," "grey grit," "grey limestone," "greenish-grey greenstone," "ash," and "limestone conglomerate inclosing rolled pebbles of greenstone": some of these fragments supporting attached Silurian corals.

The unqualified application of the word "concretionary," as an amendment to Mr. Du Noyer's "coarse conglomerate," to such rocks, seems a singular use of the term, though it can scarcely be meant to convey the idea that the writers quoted did not know the difference between concretionary rocks and conglomerates.

MURREE, Oct. 1877.

A. B. WYNNE.

THE

GEOLOGICAL MAGAZINE.

NEW SERIES. DECADE II. VOL.* V.

No. II. FEBRUARY, 1878.

ORIGINAL ARTICLES.

I. THE OLD MAN OF HOY.

By Prof. ARCHIBALD GEIKIE, LL.D., F.R.S., Director of the Geological Survey of Scotland. (PLATE II.)

THE tidal wave of travellers which, thanks to railroads and

Let

It

even as far as John o' Groat's, has hardly as yet risen much beyond that utmost shore. The tourist stops short at the Pentland Firth ; indeed, when he reaches its bare treeless coast, and finds that there is really no traditional house at John o' Groat's (though a good inn, with careful host and kindly hostess, should tempt him to rest there a while), he is in a hurry to get back by daylight to the busy hum of men in the hyperborean city of Wick or Thurso, and as eager to fit southwards again next morning. He makes a fatal mistake, however; for he misses the very points which it would have been worth his while to make the whole of his long journey to see. him, for instance, take up his quarters for a day or two by the side of the Pentland Firth, and sitting or lying on one of its grim cliffs, let him spend his hours watching the race of its tideway. Nowhere else round the British Islands can he look down on such a sea. seems to rush and roar past him like a vast river, but with a flow some three times swifter than our most rapid rivers. Such a broad breast of rolling eddying foaming water! Even when there is no wind, the tide ebbs and flows in this way, pouring now eastwards now westwards, as the tidal wave rises and falls. But if he should be lucky enough to come in for a gale of wind (and they are not unknown there in summer, as he will probably learn), let him by no means fail to take up his station on Duncansbay Head, or at the Point of Mey. The shelter of a flagstone "dyke" and a waterproof will save him from any ulterior consequences of the exposure, or should he be under some misgivings on this point, when he gets back to the shelter of the inn at John o' Groat's, mine host has sundry specifics of well-tried potency, at the very sight and taste of which rheums, catarrhs and the rest of that tribe of ailments at once decamp. Ensconced in his " neuk," he can quietly try to fix in his mind a picture of what he sees. He will choose if he can a time when the tide is coming up against the wind. The water no longer looks like the eddying current of a mighty river. It rather

DECADE II.-VOL. V.-NO. II.

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