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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 . By the DUKE OF , 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 , 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

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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 pre- cipices 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 ter- races 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 at- tention ; 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 , 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 to visit the ruins of , 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 , &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 re- markable order of the rocks overhead did not attract the particular at- tention 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 vi- sited 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 ba- saltic 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 contain- ing 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 dis- posed 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. Mr. Mills's estimate of the thickness of the basalt may be tolerably correct, viz. 30 feet. The jointed character of this basalt renders it peculiarly liable to the disintegrating effects of weather. The bottom of the ravine is covered with its fallen blocks. 2. The first leaf-bed, a thin seam, about a foot thick, of shaly matter, bearing impressions of leaves and stems of plants. 3. A bed of volcanic ashes or tuff; being an ashy paste full of white angular fragments or lapilli, disposed in a manner characteristic of erupted volcanic matter, and closely resembling, as I am informed by Sir C. Lyell, similar products found in Mont D'Or in Auvergne. It appears to me to resemble very closely also some of the tufts in those remarkable ravines across which the road passes from Castel a Mare to Sorento, in the vicinity of Naples and Vesuvius ; and, further, it was recognised by Mr. Smith, of Jordan Hill, as very similar to some of the volcanic products of the island of Madeira. This bed varies much in the fineness or coarseness of its component fragments; at some points consisting of close fine-grained matter, with white specks more or less abundantly interspersed; at others presenting an ex- ceedingly coarse texture, the paste containing large fragments of a pumiceous matter, with the white lapilli of corresponding size. But there is one peculiarity of a remarkable character ; the whole of the beds, although not far removed from the horizontal, dip slightly towards the S.E. or landward end of the ravine ; and upon the dip, this bed of tuft passes into a conglomerate of flints, cohering by a cement so tenacious, that the flints themselves frequently break rather than quit their matrix. These flints present, when wet and freshly broken, the most brilliant tints of red and orange, and are evidently more or less in a burnt condition. Some of them, however, are less altered than the rest in texture and colour. One specimen I obtained from external appearance alone was easily identified as an unequivo- cal chalk flint ; and after I had shown it at the late meeting of the British Association, a fossilized organism was discovered in it, which placed this conclusion beyond a doubt. The white lapilli, throughout the whole course of the bed, are generally siliceous ; although some of the minuter particles have the appearance of unaltered chalk. 4. The fourth band in the descending order is the second leaf-bed ; that which is by far the richest in vegetable remains. It is about Downloaded from http://jgslegacy.lyellcollection.org/ at University of Melbourne on June 22, 2016

1851.] DUKE OF ARGYLL ON ARDTUN LEAF-BEDS. 95

2 feet thick; and the lower portion of it is not so much mineral matter with organic impressions as a compressed mass of leaves, not a few of which, when the layers are partly divided, seem still to retain the damp obscure colours of vegetable decay. The composition of the bed becomes harder, as it passes upward, where there is more and more mineral matter, with fewer impressions of plants. These are still, however, frequent, preserving in large and small leaves the most delicate tracery of the skeleton. Some few impressions of twigs are found even above the limits of the bed, and here and there mark the lower portions of the superincumbent tuff. 5. The fifth bed is a second band of tuff, similar in composition to the one above described, but somewhat thicker. Owing, however, to its dipping under the sloping base of the ravine, less of its course is visible than of the one above. 6. The sixth bed is a seam of what may be best described as baked clay or very fine mud. It is very brittle, but without any particular form of fracture. From its general appearance and relative positiou among the beds, it at once suggests itself to be a third leaf-bed. Accordingly, after some search, a few impressions were obtained of leaves apparently similar in character to those found in the superior deposits. But the nature of the material prevents more than mere fragments being obtained. 7. Below this third and last leaf-bed the cliff is composed of a dark amorphous basalt. Like most of the traps and basalts of the district, it is of an amygdaloidal structure, the cavities being filled with various mineral crystals. On the surfaces exposed to the action of the air and sea-spray these have decayed out, and the empty holes give here and there a honey-combed appearance to the rock. 8. Lastly, the cliff ends in beautifully columnar basalt, dipping into the sea. I have not ascertained the soundings nor the nature of the bottom, and consequently am unable to say what may be the height of the columns. -They may be seen, however, to a consider- able depth in the clear waters of that sea. They are sometimes per- pendicular, sometimes bent in various directions ; a common disposi- tion here being, as at Staffa, a gentle outward curve, as if bending under the weight of the superincumbent cliff*. The point, at which the ravine exhibits a section of these beds, is that at which the headland reaches its highest elevation. To the W. and E. it declines in height ; but the beds containing the vege- table impressions can be traced for a considerable distance along the sea-face ; and excellent specimens have been obtained from the second leaf-bed about 100 yards farther to the E. It has been mentioned that the beds dip gently towards the S. ; that is, in a direction nearly parallel to the line of Loch Laigh. The surface of the headland of Ardtun follows the same slope ; so that at the head of Loch Laigh-- that is, in the course of about a mile along the line of dip--the land is but slightly elevated above the level of the sea, and it is remark- able, that at both sides of this he adlandmupon the shore of Loch Scridden and that of Loch Laigh--a seam of coal has been found * Since writing the above, I have received from Mr. M'Quarrie a measurement Downloaded from http://jgslegacy.lyellcollection.org/ at University of Melbourne on June 22, 2016

96 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 8, immediately under a sheet of basalt (see Map, p. 90,andFig. 2). On the Loch Scridden side this basalt is regularlycolumnar, some 20 feet high ; on the Loch Laigh side it is much thinner, and somewhat resembles a rotten trap. It is impossible not to conjecture from the line of dip of the Ardtun strata, that this coal-seam is a prolongation of one or other of its leaf-beds ; and, if so, it would be an interesting instance of the passage of vegetable matter from one condition to another ;- from a state, in which it is so slightly altered that every fibre of its original structure remains, to one, in which it is converted into the highly altered mineral, coal. To return to the strata exhibited at Ardtun. The geological epoch, to which all the beds above that of the amorphous basalt belong, is determined by the character of the organic remains. The leaves are of considerable variety, but all belonging to well-known existing families of the Dicotyledonous order. They are therefore remains of the tertiary period ;--a conclusion farther confirmed by the position of the chalk flints in the tuff conglomerates with which they are associated. Further, these beds seem to me to furnish indisputable evidence of suba~rial volcanic action, alternating with periods of repose. The second leaf-bed is the one which throws the clearest light on the cir- cumstances of its formation. It is to be observed, in the first place, that the leaves are not torn or shattered ; those of the large palmated planes, as well as those of the small buckthorn, &c., being fully ex- tended, and showing unruffled surfaces. Leaves, violently cast from the trees on which they grew, would not have presented such appear- ances. They do not even consist with the brittleness of dead leaves, when dry. Two other remarkable circumstances remain to be noticed; first, that no trunks of trees, no branches, nothing beyond of the various beds at Ardtun. At a point east of the ravine where the lower basalts are more easily measured, the whole series stands thus :- Feet. Uppermost basalt ...... 40 First leaf-bed ...... 2 First ash-bed ...... 20 Second leaf-bed ...... 2~ Second ash-bed ...... 7 Third leaf-bed ...... 1~ Amorphous basalt ...... 48 Columnar basalt to the level of low tide.. 10 Total ...... 13I At the ravine the measurement of the upper series of beds varies considerablyfrom those figures :- Feet. Uppermost basalt ...... 16 First leaf-bed ...... 2 First ash-bed ...... 8 Second leaf-bed ...... 2~ Second ash-bed ...... 6 Third leaf-bed...... 189 From this it will be seen that whilst the leaf-beds preserve a remarkable uniformity of thickness, the associated ash-beds and basalts vary. much. In the uppermost ash-bed there is a difference of 12 feet, and this within a short distance. Downloaded from http://jgslegacy.lyellcollection.org/ at University of Melbourne on June 22, 2016

1851.] DUKE OF ARGYLL ON ARDTUN LEAF-BEDS. 97 the size of the merest twig, has been yet found assodated with the leaves; secondly, that plants of a reedy texture--some of them at once recognisable as Equi, eta--are associated in great abundance with the leaves, especially with that lower portion of the bed which almost exclusively consists of the vegetable remains. From all this the con- clusion is obvious, that these leaves must have been shed, autumn after autumn, into the smooth still waters of some shallow lake, on whose muddy bottom their were accumulated, one above the other, fully expanded and at pertect rest. It cannot have been a water .a~- tated by tides or currents ; for these would have swept such remains away, or left evidence in their disposition of disturbing agency. It cannot have been water of any depth ; for it is well known that reeds, and especially the Equisetum and other kindred families, do not affect such situations. But there is another ground for this latter conelu- mon: the bed of ashes or tuff covering the leaves shows clearly, from the arrangement of its materials, that they cannot have undergone the sifting process inseparable from subsidence through water. The light pumiceous particles and the heavy flinty white lapilli, are dis- seminated indiscriminately without any reference to the order of gravity, although the former are composed of a substance which will frequently float in water, whilst the latter are particularly dense and heavy. All these circumstances taken together, as also the absence of any freshwater shells, or other organisms, indicative of a permanent lacus- trine condition, seem to me to afford the strongest evidence, that the situation in which these leaves were overflowed by volcanic mud and ashes, was one, which may rather be described as a marshy terrestrial surface, than the bottom of a lake, properly so called. If this con- elusion be correct, it follows that the materials which overlie the leaves were emitted by a volcano in subatrial action. The condition of this matter at the time of its eruption seems pretty clearly indi- cated by its condition now. First, the damp and bedded leaves have had poured upon them a stream of liquid mud, insinuating itself be- tween their planes, lifting and holding those most easily detached from the surface, and leaving in its original state of rest "the lower portion of the bed. To this matter,--which although now appearing in its upper portion as a hard blue stone, bears in the perfect state of its vegetable impressions indisputable evidence of its once liquid eondi- tion,--has succeeded an overflow or a shower of matter of very dif- ferent composition. In respect to the latter, it is more difficult to conclude with certainty what was the original condition. It seems to have followed the mud after a very short interval of time, although long enough to have allowed a partial consolidation. That stray twigs and leaf-stalks were still sticking out of the surface of the mud, is sufficiently proved by their traces, generally much carbonized, in the lower part of the tuff. The line of junction between the bottom of the tuff and the top of the leaf-bed is, in a general view, sharp and definite enough ; whilst a closer inspection shows just enough subsi- dence of the particles of ashes into the substance of the stone below, to indicate the degree of consolidation to which the latter had attained. Downloaded from http://jgslegacy.lyellcollection.org/ at University of Melbourne on June 22, 2016

98 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 8,

These appearances, however, are hardly sufficient to determine whether the tuff was erupted in the form of a muddy flood, or in that of a volcanic shower. A fall of any height through air would, to a certain extent, have the same effect as subsidence through water, in producing an arrangement of particles, determined by their specific gravity. Subsequent heavy, falls of rain, however, such as have fre- quently been known to accompany similar eruptions in the present epoch, by washing down the finer particles, might considerably modify the original conditions. It must be remembered, however, that in the disposition of matter erupted by a volcano, much must depend on the degree of proximity to the seat of action. The nearer that point is, the more completely will the laws of gravity be liable to be counteracted by more violent temporary forces. Now, I think there is some evidence, that the seat of action was at no great distance from the spot described. The tuff-bed is traceable for some distance along the coast ; and at one point, the farthest at which I have ex- amined it, the composition is fine-grained and homogeneous, indicating that at the spot referred to, the eruption had undergone a change similar to that which distinguished the fine white ashes which covered the country at Misenum, from the coarser and mixed materials which overwhelmed Pompeii. I have already described also the remarkable change which takes place in the composition of the bed within a few feet of space at the Ardtun ravine. So rapid a transition in charac- ter, from a conglomerate of coarse and heavy materials to one of much finer composition, seems to indicate a corresponding rapid change in the intensity of the forces to which the bed owes its origin. The condition of the flints tends to prove the agency of heat ; whilst it equally proves that the degree of intensity to which they were ex- posed was very variable. Some of them have the appearance of having been much burnt, although I observed none in a state of vitri- fication; whilst others are so little altered as to preserve in good form minute Organic remains. From these circumstances, although on this point sufficient data are still wanting, I should be inclined to conclude that the flints had not been thrown out in a fiery shower, but rather having been subjected to considerable heat, modified by the earthy matter with which they were associated, were poured forth with it in a mud-stream. But, whatever may have been the particu- lar process by which the tuff-beds, and this one especially, may have been formed, it is certain that it must have been repeated after a con- siderable interval of time, and that the volcanic eruption was not of such violence as to change materially the conditions of the surface. The hollow in which the marsh had originally been formed, and in which the first or lowest leaf-bed had accumulated, continued to be a hollow after the mud and ashes had overflowed it. Water again ac- cumulated, and autumnal leaves were again cast upon its surface in greater numbers and variety than before. An eruption similar to the first for a second time covered its deposits ; still its condition re- mained sufficiently unchanged to admit a repetition of the same pro- cess, and once more it continued to receive the annual sheddings of a forest vegetation. But the third eruption must have been one of a Downloaded from http://jgslegacy.lyellcollection.org/ at University of Melbourne on June 22, 2016

!851.] DUKE OF ARGYLL ON ARDTUN LEAF-REDS. 99 very different kind; sheets of lava of great solidity and thickness were now poured forth upon the ground, and if surfaces completely vitrified, such as well-marked obsidian, be any indication of suba~rial exposure, such must have been the condition of this lava. The confi- guration of the country no longer remained the same, and so complete was the change effected by this and subsequent convulsions, that the spot which had so long been the receptacle of calm stagnant waters under the lee of some great forest, became as we now see it, cut into the sea-cliff of a naked headland, so peculiarly exposed to the surf of a stormy ocean, as well to deserve the description of its Gaelic name, "the Point of Waves." It is true, that no evidence remains in the form of visible craters to mark the site of volcanos to which the traps of the Hebrides may owe their origin, and to prove.their sub~rial character. Such indeed may not have been the character of all of them during the immense periods of time in which their activities were exerted. But in the particular case of the beds here described, with other evidences so strongly marked, I cannot feel that the absence of this particular proof stands .much in the way. Of nothing, perhaps, does the whole geology of the Hebrides present more conclusive demonstration than of the enormous changes in the relative position of sea and land, which have been effeeted since the latest period of volcanic activity of which any evidence remains to us. The position of the leaf-beds in the cliffs of Ardtun is one example. A great portion of the later tertiary period, as well as the whole of that period of submergence to which the Drift is referable, lie between existing times and that to which the sealing up of these beds may be referred. The , like all the rest of Scotland, presents banks of sea-worn gravel far above the level of the highest Ardtun basalt, and its rocks and boulders are deeply marked by those remarkable abrasions, which, whatever be the particular material which caused them, are apparently due to the action of something impelled by powerful currents. There is some evidence, that the sheet of basalt which caps the Ardtun beds was by no means the last or highest which once occupied the same area. From out of the mosses, now covering its surface, tablets of similar material are seen elevated here and there, with broken joints strewn about at the foot of their little escarpments. They have all the appearance of having formed part of a sheet which over- lay the other, and of which these isolated portions are the only rem- nants (see Fig, 2, p. 92). Unless therefore the original craters of erup- tion had been above the highest level accessible to such changes, it is not surprising that no vestige should remain of cones of seorise, or other accumulations of loose materials. Nor, when they had been once given up to the sea, could we reasonably expect much evidence of this former existence. The ocean cannot often be successfully called to account for such acquisitions ; it is, however, a curious fact, that on the shore of the island of Tyree, opposite the basalts of Mull, at no great distance above the reach of the present tides, balls of pumice have been found in considerable abundance. These are of course sea- borne, and, although there is no proof of their having been Hebridean Downloaded from http://jgslegacy.lyellcollection.org/ at University of Melbourne on June 22, 2016

100 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 8, products, it may be at least safely asserted that this is as probable an on~n as any other. We may safely conclude that the spot where the leaves are now found, could not have been at any very great distance from at least the border of the forest which yielded them. But it is probable that the country on which it stood has foundered among the subsequent convulsions, which seem to have broken and disjoined so many tracts, once continuous. We may possibly even have a doubtful guess of the direction in which that country lay. The headland of Ardtun does not seem to bear any close relation to the rest of the traps, in the same (southern) district of Mull. At a very short distance to the east upon the southern shore, the terraces of trap are associated, so far as yet examined, with no other formations than those of the oolite and lias. From Macculloch's d&cription, they both underlie and overlie various members of those formations, in a manner very similar to that in which the traps of Skye and many of the smaller islands are found associated with the same series of rocks. The recent observations of Prof. E. Forbes show, that in the ease of Skye, this association takes place in a manner which indicates with singular pre- cision the ageofsome at leastofthe basaltic sheets (s.ee.page 108). Ido not believe that the higher basalts of the great precipices of the south coast of Mull ever have been, or perhaps ever can be, examined with very great minuteness. But no evidence certainly exists that any of them are of later date than the secondary period; whilst the great differ- ence of elevation renders it improbable that any of them can belong to the same epoch with the Ardtun beds. I think the only indica- tions of relation in the latter to any of the surrounding formations, point in the northerly direction. No one who has followed the de- scription of the Ardtun Head, and is acquainted with Staffa, will fail to recognise a remarkably corresponding feature. The lowest two members of the Ardtun series--the massive amorphous basalt, pass- ing into and resting upon the columnar,--offer a precise representa- tion on a smaller scale of that wonderful front which lies opposite at some five or six miles' distance. It is to be observed too, that the greater elevation to which these two formations rise in Staffa, corre- sponds with the line of dip (rising to the north) of the same beds of Ardtun. The whole group of the Treshnish Islands, "which'guard famed Staffa round," would seem from their low tabular appearance to belong to the same prolonged sheets of trap, and may represent the skeleton of that country now destroyed, from whose forests the Ardtun leaves were shed. I think it not improbable that by future researches among the conglomerates and other stratified matters as- sociated with the traps in Mull and the neighbouring islands, portions of the more substantial parts of those forests will yet be found. It appears from Dr. Maceulloeh's account of the traps of the middle district of the island of Mull, that he did actually find the carbonized stem of a tree*, whose structure proved it to be coniferous. His notice of the "vein" in which it occurred is an accurate description of the tuff which covers the leaves at Ardtun ; but he expressly says * Loe. cir. vol. i. p. 568, and voh iii. p|. 21. Downloaded from http://jgslegacy.lyellcollection.org/ at University of Melbourne on June 22, 2016

1851.] DUKE OF ARGYLL ON ARIYI'UN LEAF-BEDS. 101 that it occupied a perpendicular instead of a horizontal position in the cliff, and the headland of Bourg seems to be indicated, although not very dearly, as the locality. Mr. M'Quarrie, of Bunessen, to whose intelligent interest I owe many of the best specimens obtained from the leaf-beds, reports to me that he has coasted round the headland of Bourg, and could see no such vein of tuff as that de- scribed by Dr. Macculloch. There is one very remarkable dreumstance which may serve, if not to confirm, at least to strengthen the conjecture which would connect the lower basalts of Ardtun with those of Staffa. So far as I am aware, there is only one sheet of trap in the British Islands which can be identified in point of geological age with the ulrpermost basalt of Ardtun. That one sheet of trap is on the coast of Antrim, and it bears to the columnar basalts of the Giant's Causeway the same re- lation which I have supposed between the corresponding Ardtun bed and the basalts of Staffa. I am indebted to the kindness of James Nasmyth, Esq., of Manchester, for a minute description and relative sketches of the order of the strata in that part of the coast of Antrim, and for excellent spedmens of the bed of charred wood, which, as it will be seen, there occupies a position similar to that of the leaf-beds of Ardtun. 1. The first bed (counting, as before, downwards from the sur- face) is 50 feet of basalt; the upper part being of small, the lower of larger and rude columnar form. 2. A bed of charcoal and lignite. Some specimens show the fibres of the Wood as perfectly as if taken fresh from a charcoal kiln. The wood is dicotyledonous. 3. Immediately under the bed of lignite succeeds a great mass of amorphous basalt, precisely as in the case of the Ardtun leaf- beds. 4. Again as at Ardtun, the mass of amorphous basalt rests upon a bed remarkable for the very perfect regularity of its co- lumnar form. 5. A band of matter highly coloured with red oxide of iron suc- ceeds, maintaining its position with great regularity along a great part of the coast. 6. Another bed of amorphous basalt. 7. Another of rude columnar basalt, of a starch-like wavy form. 8. A thin band of red oxide of iron. 9. A very black amorphous basalt. 10. Chalk, on which, dipping into the sea, the whole series of the 9basaltic beds rest. From the top of the diff" to the chalk, these beds are no less tha~ 460 feet in height. The amorphous and columnar basalts on which the Mull leaf-beds rest may possibly not belong to the same epoch with the closely similar Antrim beds ; because we have no positive proof that, like the latter, they either rest upon, or have burst through chalk. The flints of that formation which are found above them may be the de- bris of chalk, originally deposited in the same position and subse- Downloaded from http://jgslegacy.lyellcollection.org/ at University of Melbourne on June 22, 2016

102 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 8, quently washed away; or they may have been rolled by currents from a distance and thrown by volcanic agency on the top of basalts belonging to a former epoch. But I think the most probable con- jecture is that, if these basalts do not even now rest upon chalk, they have burst through it, and belong to the same period as the corre- sponding beds of the Antrim coast. At all events it is clear that the tertiary volcanos which at Ardtun give such clear evidence of inter- mittent action, have been in a state of still more tremendous activity on the neighbouring coast; and were powerful enough to produce repeated sheets of basalt of a thickness greater than, and of a form very similar to, those on which the leaf-beds rest. It will be very singular if the comparatively thin sheet of basalt which overlies those beds is the only one in the Hebrides which can be proved to belong to the same period. It would in this case occupy a position of very remarkable isolation; the nearest basalts of the same period being at a distance of not les~ than 75 miles, with inter- vening islands, which do not exhibit any development of the trap formation. It is however but a new proof in support of a geological conclusion of much interest, viz. that the basalts of the Hebrides, as we now see them, are the accumulated results of plutonic and vol- canic action going on from time to time during an indefinite series of ages,--and frequently not only at immensely distant points of time, but also within very limited areas. Dr. Macculloeh intimates, that he 'could observe very little corre- spondence between the beds of trap, even in islands very near each other, although a large number of them over all the islands from Skye to Mull, seem to be referable to periods included in the oolitic and the liassic epochs. Not unfrequently it has been observed by the same writer, that many successive beds of trap rest upon, and include conglomerates consisting chiefly of water-worn remains of the same material ; thus indicating that some of the intervals had been long enough to witness vast physical changes--the submergence, de- struction, and reaggregation under water of still older rocks of the same formation. I have very lately had occasion to observe on the coast of Kintyre columnar basalt, which seems to have risen through the old red sand- stone whilst the latter was yet in the state of sand, each column being separated from the next on all its facets by cakes of sandstone, now highly crystallized and very brittle ; but which, when in a state to follow such labyrinthian lines, must have been soft and plastic. Before concluding this paper, I may mention that my attention has been drawn by Prof. Nicol to a post-tertiary leaf-bed in Kintyre, which has been discovered in cutting an outfall drain through the fiat area called the Laggan. In respect to the manner of deposit, this bed presents a remarkable identity of character with the leaf- beds of Ardtun. It consists of a mass of leaves, fully expanded and mixed with very little mineral matter, associated with reedy plants ; the whole still preserving the colours of damp leaves, and, although very rotten, much of their original texture. They have evidently been collected in the same mannermin a shallow marsh, and are Downloaded from http://jgslegacy.lyellcollection.org/ at University of Melbourne on June 22, 2016

1851.] DUKE OF ARGYLL ON ARDTUN LEAF-BEDS. 103 covered by a bed of clay full of vegetable fibre, which forms the sin:face of the ground. The leaves however, which when still wet can be lifted from each other quite perfect, belong to a very different age from those at Ardtun ; being apparently all of such plants as now grow in marshy situations in the West Highlands,--bog myrtle, willows, alders, &c. The Ardtun leaves belong to species and even families which have long ceased to be indigenous in that country, and indicate the occurrence of changes since the period of their growth, not less great in climate than in th~ geographical forms of land and sea. NOTE on the FossIL LEAVES represented in PLATES II. III. AND IV. By Prof. E. FORBES, F.R.S., V.P.G.S. &c. From among the numerous vegetable remains contained in the Ard- tun beds, a few of the most perfect impressions of plants, mostly of leaves only, have been selected for illllstration. In accordance with custom, specific names are assigned to these provisionally, for the sake of distinguishing between the kinds found, and of comparing them with fossils of a similar kind found elsewhere. Without much more data than such impressions, however perfect, afford, anything like a specific diagnosis, satisfactory to a botanist, could not be con- structed. The general assemblage of leaves when judged by the present state of our knowledge of tl~e vegetation of ancient epochs is decidedly tertiary, and most probably of that stage of tertiary termed Miocene. Their climatal aspect is more mid-European than that of our eocene flora. There is a striking resemblance between many of them and fossils from Styria and Croatia ; but so far as I have had op- portunities of comparing either with specimens or good figures, the Mull fossils are in all probability distinct from any recorded species. I cannot identify any of them with British eocene forms. PLATE II. fig. la, lb. Taxite,? Campbellii. Fragments of a co- niferous tree, possibly a Taxus; allied to the Taxites Ro, thorni of Unger, from the miocene lignite of Carinthia. PLATS II. fig. 2 a, 2b. Part of a frond, probably that of a fern, but presenting some anomalous features which future specimens will probably explain. For the present it may be called Filicite,? hebri- dicu~. PLATE III. fig. 1. An inequilateral leaf, the affinities of which are doubtful. Fig. 2. Rhamnites ? multinervatus. Fig. 3. Rhamnites 7. major. Fig. 4. Rhamnite8 ? ? laneeolatus. Fig. 5. Platanites hebridicus, var.? Fig. 6a, 6b. Equisetum Camp6ellii. PLATE IV. fig. 1. Platanites hebridicu,. This leaf is one of the most abundant and characteristic of all those found at Ardtun. It has a close affinity with the Platanu8 hercules (Unger, Chlor. protog. p. 138. t. 46) from the marly slates (said to be eocene) of Croatia. Fig. 2. Affinities doubtful. Fig. 3. ~41nites ? MaeQuarrii.