The Quarterly Geological Journal Society of London

Downloaded from http://jgslegacy.lyellcollection.org/ at University of Melbourne on June 22, 2016 THE QUARTERLY JOURNAL OF THE GEOLOGICAL SOCIETY OF LONDON. PROCEEDINGS OF THE GEOLOGICAL SOCIETY. JANUARY 8, 1851. Prof. tteinrich G. Bronn, Prof. Wilhelm Haidinger, Colonel Gregorius Helmersen, and James Dana, Esq., were elected Foreign Members. The following communications were read :-- l. On TERTIARY LEAr-BEDs in the ISLE OF MULL. By the DUKE OF ARGYLL, F.G.S. With a NOTE On the VEGETABLE REMAINS from ARDTUN HEAD. By Prof. E. FORBES, V.P.G.S. THE Island of Mull is deeply indented in a direction nearly east and west by two long arms of the sea, Loch na Kael and Loch Scridden, forming the three natural divisions described by Macculloch as the Northern, the Middle, and the Southern Trap Districts. The North- ern division is of comparatively low elevation, and composed chiefly of terraces of trap. I am not aware that it has ever been carefully examined in detail ; but it has been understood not to present any features of remarkable geological interest. The middle division is a lofty and rugged tract, containing the fine summits of Ben Tulla and Ben More, the latter being one of the highest mountains on the VOL. VII.~PART I. H Downloaded from http://jgslegacy.lyellcollection.org/ at University of Melbourne on June 22, 2016 I Downloaded from http://jgslegacy.lyellcollection.org/ at University of Melbourne on June 22, 2016 Jan. 8, 1851.] D~KV.OF ARGYLL ON ARDTUN LEAF-BEDS. 91 western coast, and visible from a great distance among the Hebrides. These mountains are raised far above the highest level to which the traps of the island attain. A portion of it is coloured by MaccuUoch as syenite, and specimens in my possession from the Peak of Ben More belong to this class of rock. But the whole of the long and high promontory stretching westward from the flank of Ben More, and lying between Loch na Kael and Loch Scridden, exhibits great terraces of trap, piled one above the other, and terminates in that striking headland of Bourg or Gribon, whose lofty horizontal lines rise from the ocean with almost perfect regularity in a pyramidal form, until the final cap attains an elevation of about 2000 feet. The southern division has been long known for the magnificent coast scenery it displays ; presenting a continuous line of mural precipices of great elevation, frequently based on and capped by basalts of every variety of form, and including extensive strata of the oolite and lias. Sections of this coast have been given by Macculloch* and by Sir R. I. Murchisont. This division is prolonged considerably farther towards the west than the other two, ending in the long promontory called the Ross. The same geological character, however, is not preserved throughout. At a point nearly opposite to the headland of Bourg, the trap terraces of this division likewise descend, but less abruptly, to a lower level. An interval of mica slate succeeds ; and the remainder of the Ross consists of low round hills, entirely composed of a fine hard red granite. Along the line of junction between the trap and mica slate, the Ross is indented in a direction nearly north and south by a deep bay or arm of the sea, called Loch Laigh. The head of this bay is mica slate, the western side is granite, whilst the eastern is a prolongation of the last and lowest of the trap terracesmthe last, I here mean, in the westerly direction, but, as it now appears, the last also probably in respect to age. In most other situations the headland of Ardtun, the termination of this terrace, would have attracted prominent attention ; but its basaltic columns, although very perfect and beauti- ful, are small when compared with those wonderful pillars, which in the same landscape are seen bending round the cave of Staffa, whilst in height it seems an insignificant cliff upon a line of coast marked by the towering precipices of the Inimore of Carsaig and the lofty terraces of Bourg. The first public mention I can find of the headland of Ardtun is from the pen of Dr. Samuel Johnson. Sir Allan McLean, in con- veying him from the island of Inch Kenneth to visit the ruins of Iona, selected this spot as a resting-place ; and the Doctor mentions that its columnar basalt, on whose broken shafts they sat, was pointed out to him as scarcely less deserving of notice than that of Staffa. This was in 1773, and the visit of Sir Joseph Banks in the previous * Description of the Western Islands of Scotland, &c. 1819, vol. i. pp. 559, 561 ; and vol. iii. pl. 20. fig. 11. t Trans. Geol. Soc., 2nd Series, vol. ii. Pt. 3. p. 359. pl. 35. H2 Downloaded from http://jgslegacy.lyellcollection.org/ at University of Melbourne on June 22, 2016 92 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jail. 8, year had not yet made fully known to the world the wonders of the cave of Fingal. It is hardly surprising that the remarkable order of the rocks overhead did not attract the particular attention of Sir Allan and his party. But it is more curious that the very spot where that order is best displayed, and where the unusual character of the strata did actually attract .4 minute attention, was visited and examined so *4 long ago as 1790 without ~'~ anydiscovery being elicit- ed of the organic remains which have since been brought to light. In the Philosophical Transactions for the year 1790 (p. 73 et secl. ) there ~. is a paper entitled "Some account of the Strata and '~ Volcanic appearances in qo = the North of Irdand and Western Islands of Scot- land," in two letters from I Abraham Mills, Esq. to e~ John Lloyd, Esq.,F.R.S. From this paper I have extracted the following account of the headland of Ardtun (2. 78) :-- " Hence we steered for Ardtun Head ..... when we approached the Head, we stopped the rowers and sat some time con- templating the wonderful arrangement of the basaltic columns, and as we rowed along shore to the eastward, had a fine view of the various situations into which the columns are thrown. The coast being everywhere steep, it was some time before we could get a convenient place to Downloaded from http://jgslegacy.lyellcollection.org/ at University of Melbourne on June 22, 2016 1851.] DUKE OF ARGYLL ON ARDTUN LEAF-BEDS. 93 land, but having at last got on shore, we marched to the extreme point or head. About a quarter of a mile from this spot is a deep glen, running N.N.E. to the sea. It is about thirty yards in length and twenty in breadth. The strata are disposed in the following extraordinary manner. The uppermost is ten yards of lava with horizontal divisions and vertical joints taking the form of rude pillars. Under this is a horizontal bed of a perfectly vitrified substance, which appears to have been a shaJe, and is from one to two inches in thickness. Beneath this there is about three yards of a siliceous gravelly concrete, below which are horizontal beds of indurated marl of various thicknesses, from six to twelve inches. The whole of these beds taken together are about four yards ....... Lastly, there are ten yards of rude lava, containing specks of quartz and mica unaltered, pieces apparently of granite, and some nodules of calcined chert. The whole is incumbent on regular basalt pillars of various dimensions from eighteen to six inches in diameter." With the exception of the total omission of the three beds containing the vegetable remains, two of which, although comparatively thin, are sufficiently conspicuous, this description, as will be seen, is tolerably accurate. Since the visit of this gentleman, I am not aware that this ravine, or "glen," the only point at which the strata are sufficiently accessible to be examined in detail, has been seen or known; the neighbourhood only having been visited at different times by Professor Jameson, the Marquis of Northampton, then Earl of Compton*, and by Murehison and Sedgwick; and although there is some ambiguity in the precise localities alluded to in some of Macculloch's remarks on this district, I think it probable that he had at least coasted round this headland in a boat and observed the lines of stratified matter between the upper and lower basalts. Fig. 3.--Pietorlal Section of ztrdtun Head. Although aware that fossil leaves had been accidentally found a few years ago in the promontory of Ardtun, I had not until this year * See Trans. Geol. Soc. vol. v. Part 2. p. 369. Downloaded from http://jgslegacy.lyellcollection.org/ at University of Melbourne on June 22, 2016 94 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 8, an opportunity of examining the spot from which they came. Having now had the advantage of doing so in company with James Smith, Esq., of Jordan Hill, I shall proceed to describe the order and nature of the beds presented in the natural transverse section formed by the ravine. The cliff was roughly estimated,' by help of an aneroid barometer, at the height of 130 feet. In the descending order the beds are disposed as follows :-- 1. Basalt, accurately described by Mr. Mills as having "horizontal divisions and vertical joints," taking the form of rude pillars. This bed of basalt is thicker on the western than on the eastern side of the ravine, and appears to consist of two sheets, separatedby a thin seam of highly vitrified matter or obsidian.

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