RECORD: Smith, James. 1838. On the Last Changes in the relative Levels of the Land and Sea in the British Islands. Edinburgh New Philosophical Journal 25: 378-394.
REVISION HISTORY: Transcribed (single key) by AEL Data 10.2008. RN1
On the Last Changes in the relative Levels of the Land and Sea in the British Islands. By JAMES SMITH, Esq. of Jordanhill, F.R.S.L. & E., F.G.S. & M.W.S.*
THE occurrence of recent marine remains at higher levels than those at which they could have been deposited by our present seas, early attracted the notice of the Wernerian Society; and its memoirs contain a valuable collection of facts illustrative of this subject. The communications of Messrs Stevenson,† Bald,‡ Home Drummond, || Blackadder, § and others, ¶ furnish numerous observations respecting indications of changes in level on the eastern coasts of Scotland, whilst those of Captain Laskey** and Mr Adamson†† record similar phenomena in the basin of the Clyde and Lochlomond.
* By permission, from vol. viii. of "Memoirs of the Wernerian Natural History Society," about to appear.
† Wern. Mem. iii. 327.
‡ Ib. i. 483; and iii. 125.
|| Ib. v. 440.
§ Ib. v. 424, 572.
¶ Ib. ii. 342, 348; v. 572, 575.
** Ib. iv. 568.
†† Ib. iv. 334.
My attention was first called to the subject by the discovery of marine shells, agreeing in general with those of the adjoining seas, embedded in blue clay, at Ardincaple, the seat of Lord John Campbell, in Dumbartonshire. At that time it was usual to ascribe all such appearances to diluvial action; and although the shells bore no marks of violent transportation, the bivalves being entire, with the epidermis uninjured, and in their natural position; yet, as the distance from the sea was small, I imagined they might have been protected from injury by having been lodged in an eddy. Two of the shells appeared to differ from any known species; one of them, a Tellina (T. approxima), is so common, as in many localities to become characteristic of this deposit. It resembles the T. tenuis, but is distinguished by a brown epidermis. The other resembled a Natica, but was destitute of the umbilicus. The only specimen procured of this shell I unfortunately broke, but not until a sketch of it had been taken.* Lord John Campbell was kind enough to order a new excavation to be made, in hopes of finding other specimens, but without success.
Soon after this, Mr Thomas Thomson gave an interesting description of a similar deposit at Dalmuir in Dumbartonshire, in the Records of General Science.† He collected twenty-nine species, which were submitted to the inspection of Mr Sowerby, who pronounced three of them to differ from any known recent British shells; one of them was said to be Natica glaucinoides, a crag fossil; another, Fusus lamellosus, which had only been observed about the Straits of Magellan; and a third, Buccinum striatum, an unknown species. This remarkable fact, coupled with my own observations, led me to imagine that the term "recent," which had usually been applied to such deposits, was perhaps not rigidly correct. In order to ascertain how far it was so, I determined to collect as many of the shells belonging to them as I could. In a fresh excavation I made at Dalmuir, I increased the number of species, from that locality alone, to upwards of seventy. The Rev. Mr Landsborough of Stevenston, in Ayrshire, was kind enough, at my request, to collect marine remains from the elevated shelly deposits in
* Plate XLV. Fig. 18.
† Vol. i. p. 131.
his parish; and Mr Witham sent me a collection from similar beds on the Yorkshire coasts. In order to render the comparison between the existing and more ancient races of testacea as exact as possible, I determined, at the suggestion of Mr Lyell, to avail myself of the facilities which the possession of a yacht afforded, to collect and form a catalogue of those now existing in the same seas. Amongst the shells dredged up, several new species have been discovered. I failed, however, in finding any of the unknown subfossil ones. As by far the greatest number of the shells, from the ancient deposits, have been found in the basin of the Clyde and north of Ireland, I have confined the catalogue of recent shells to those which are now to be found in the same seas; a comparison of the two catalogues will thus shew how far their former inhabitants coincide with the existing species.
In the prosecution of this inquiry, I discovered marine remains so frequently, that any attempt to describe or enumerate the localities would exceed the bounds of this paper. When once I was furnished with a clue, I found them in places where their presence had never before been suspected; sometimes in great numbers, whilst at others the very same beds were altogether destitute of them. This is peculiarly remarkable in a finely laminated clay, which I have traced to a great extent in the counties of Lanark, Renfrew, and Dumbarton. It is equivalent to the carse clay of the Forth and Tay, and must have been deposited at the bottom of a tranquil sea, at such a depth as not to have been disturbed by the agitations of the surface. The shells and other marine remains with which it abounds are almost invariably found in the lower part of this bed, a circumstance which can only be accounted for by supposing a sudden depression, which has converted a half-tide deposit into a deep-sea one. The testaceous animals have thus been entombed alive in the beds subsequently formed, and their remains are preserved with all the perfection of recent specimens. Associated with this clay, we frequently find extensive beds of pure gravel and sand also destitute of organic remains, although there can be no doubt of their marine origin. Mr Lyell has observed the same thing in similar beds in Sweden.*
* Phil. Trans. 1836, pp. 11 and 15.
We must be cautious, therefore, in concluding that alluvial beds in which we do not find such remains, are fresh-water ones; and, of course, equally so in deciding on their marine origin, till confirmed by the presence of their appropriate remains.
These deposits are much more extensive, both as to the amount of change of level and superficial extent, than has been generally supposed. We have conclusive evidence that the whole of the British islands have, at periods which, geologically speaking, are by no means remote, been subjected to changes both of depression and elevation. The submarine forests which have been observed on so many parts of our coasts, are proofs of the former kind of changes, whilst those of elevation are evidenced by raised beaches, sea-worn cliffs and caves, stratified beds of sand, gravel, and clay, and above all, by the marine exuviæ which they contain.
The deposits thus formed must, in Scotland at least, be intercalated between the two first groups in Mr De la Beche's classification of rocks, viz, the modern group and the erratic block group. We infer that they are posterior to the latter, from their superposition, and that they do not belong to the former, from the absence of the remains of man or of works of art. The erratic block bed, which has also been termed diluvium, has in Scotland received the provincial name of Till. It is very accurately described by Mr Bald,* under the name of the old alluvial cover, in his paper on the coal formation of Clackmannanshire. It generally consists of stiff unstratified clay and gravel, confusedly mixed with water-worm masses, and also with angular fragments of sandstone, shale, and coal, which have not suffered from attrition, although comparatively soft in their structure. Organic remains are excessively rare in it. Mr Bald, who remarked this, afterwards found the tusk of an elephant embedded in it in the excavation of the Union Canal; but, unwilling to draw an important inference from a solitary fact, he supposed it might have been placed in the situation in which it was found, from some accidental cause. Since that time, however, elephants' bones and tusks have been found near Kilmarnock, and at Kilmaurs, in Ayrshire. I am assured
* Wern. Mem. vol. i. p. 481; vol. iii. p. 126.
VOL. XXV. NO. L.—OCTOBER 1838.
by Dr Scouler of the Royal Society of Dublin, and Dr Cowper of the University of Glasgow, who visited these localities, that, in both instances, they were embedded in the till. At Kilmaurs they were associated with sea-shells; and, on one occasion, I also found shells embedded in it, much broken, and deprived of their colour. Mr Trimmer, in describing the diluvial deposits in Caernarvonshire, in the proceedings of the Geological Society,* states that he found broken shells in the diluvium of the low cliff near Beaumaris. He also found broken shells in a bed of sand, on the summit of Moel Tryfane, 1400 feet above the level of the sea. The expression seems to imply alluvial rather than diluvial agency. Mr Trimmer, however, informs me that he ascribes their presence at so high an elevation to the latter cause, the beds having all the appearance of violent action, and the subjacent rocks worn and scratched by friction of transported pebbles. Mr Phillips also is inclined to think that in Holderness the irregularity of deposition of the shelly gravel seems to point to diluvial currents rather than to change of level.†
It is not, therefore, a necessary inference that the mere discovery of sea-shells at high levels is a proof of permanent submergence. Their fragments, like those of coal, sandstone, and shale, mark that the distance from which they have been transported is a short one. It is only when found in situ in regularly stratified beds that we are entitled to draw such conclusions; but their presence in diluvial beds must be held as an exception to the general rule.
Although this is not the place to offer any speculation respecting the origin of the till, I think it evident that it must have arisen from causes altogether different from those which have produced the marine alluvia. Whatever they were, they must have been violent and transitory. Of their violence we have ample proof in the size of the fragments they have transported, as well as the erosion of the rocks over which they have passed, but that they suddenly ceased must be inferred from the confused manner in which the different parts of the till are arranged. Submarine currents might indeed have moved
* Vol. i. p. 332.
† Treatise on Geology, p. 198
the largest boulders, but they must have been deposited somewhat in the order of size and gravity; the sand, clay, and smaller fragments being swept forward till the diminished velocity of the current was unequal to bear them farther, and banks of gravel, sand, and clay, would be formed. No inference can therefore be drawn from it as to the former level of the land, as rushes of water capable of producing such effects must have disturbed the alluvial covering both above and below the surface of the sea.
All observers concur in supposing that the cause which produced the diluvial covering of the great-coal filed of Scotland, must have had its origin to the westward, modified, however, by the form of the ground. Near Glasgow, it is quite evident that its action must have been from the north-west. In levelling a mass of it, the workman laid into a heap all the boulders which were too large to be lifted by the spade: this afforded an opportunity to estimate the relative proportions of the different rocks, which I found to be as follows:—
|White sandstone and shale,||60 per cent.|
|Clay-slate and greywacke,||10 …|
The sandstone was evidently derived from the subjacent coal-formation, the trap boulders from the Kilpatrick hills, which are about ten miles to the north-west, their identity being proved by the zeolitic minerals which they contained; the slate and greywacke from hills in Dumbarton and Argyleshires, about double that distance; the granite blocks must have been transported from still greater distances. Beyond the Kilpatrick Hills the trap and white sandstone boulders disappear, and are replaced by greywacke, clay-slate, and red sandstone, whilst those of granite and mica-slate become numerous. Near Helensburgh, twenty-three miles to the north-west of Glasgow, the granite strongly resembles that of Ardnamurchan. At Roseneath, I have seen rolled fragments of a compact reddish granite so much resembling that of Inverary, as to leave little doubt of its identity. In all of these cases, the bearing of the
supposed original rocks is north-west, but in all of them the intervening space is intersected by deep arms of the sea and steep precipitous mountain ranges. It appears to me, therefore, that the till is as ancient as the period of their elevation, and was most probably caused by the violent geological action by which it was accompanied.
It is, at all events, in Scotland anterior to the marine alluvia which I am describing, and which have been observed reposing upon it in many places. It is proper, however, to observe that in some instances we find stratified alluvium below the till. I have observed this near Glasgow, and on the west coast of Ireland; and Mr Bald, in describing that of the Forth, remarked, that in one case, when it was cut through to the depth of 162 feet, the lower bed appeared to have been deposited in water in the most quiescent state, as it was divided into the finest laminæ. In neither of these cases were marine remains detected, but Mr Mantell has described an ancient beach as passing under the elephant bed in Sussex, and Sir Philip Egerton found a bed of shells under the ordinary sand diluvium of Cheshire.* These facts do not invalidate the conclusion, that the changes in the level are posterior to the deposition of the till; they only prove that it has not swept away the whole of the pre-existing alluvia. I have observed the marine beds resting on Till neat Glasgow, and in the excavations of the railway from Edinburgh to Newhaven. Mr Thomson has observed it in Dumbartonshire.† At Johnstone, near Paisley, in digging a well, a marine deposit, containing the bones of fishes and sea-fowl, the claws of crabs, sea-weed, and shells, was found to rest upon a bed of it, upward of 70 feet in thickness. Mr Robberds‡ and Mr Rose || have observed the same order of position in the county of Norfolk.
We can, therefore, have no hesitation in considering that in these localities changes of level have occurred posterior to the deposition of the diluvial covering, although it is not improbable that in some parts of the British islands it may have been lodged on the surface subsequent to the period when the sea
* Proc. Geol. Soc. vol. ii. p. 190.
† Records of General Science, i. 132.
‡ Phil. Mag. Oct. 1827, p. 281.
|| Ib. Jan. 1836, p.34.
had become stationary at it present level. I am inclined to think that this has been the case on the west coast of Ireland; in the counties of Clare and Kerry I observed no stratified beds above the diluvium, and, on the shores of the Shannon which divides them, no terraces except those forming at present. These facts, however, seem rather to prove different periods of diluvial agency than of elevation and depression.
The changes of level must have taken place anterior to the historic period, which in this country dates from the invasion of the Romans. Diodorus Siculus,* who wrote during the reign of Augustus, describes St Michael's Mount in Cornwall under the name Izrig, as an island connected with the mainland by a space covered every tide, but dry at low water,—a description which would apply accurately at the present day. In Scotland, the Roman wall, which crosses the island from sea to sea, has evidently been formed at both ends with reference to the present level. The same observation applies to British tumuli and vitrified forts, which are perhaps of still greater antiquity. It is, therefore, highly probable, that no changes of level have taken place since the British islands have been tenanted by man.
We have ample proof that traces of these changes occur in every part of our coasts. In England, the observations of Messrs Phillips,† Rose,‡ Robberds,|| Sedgwick, § &c., on the east coast; Messrs Mantell, ¶ De la Beche,** Sedgwick, and Murchison,†† on the south; and Sir Philip Egerton, Messrs Murchison, Gilbertson, &c.,‡‡ on the west, shew, that in all parts of the English coasts they are to be met with. In Scotland, in addition to the Notices in the Wernerian Memoirs, already adverted to, the Statistical Account abounds in direct or incidental notices of similar phenomena. My own observations, and those of every well qualified observer, confirm their universality in this part of the island.
* Diod. Sic. Book v. Quoted in Thomson's Outlines of Mineralogy and Geology, vol. ii. p. 45.
† Geol. of Yorkshire, vol. i. p. 23.
‡Phil. Mag. Jan. 1836, p. 30.
|| Phil. Mag. March 1827, p. 223, &c.
§ Geol. Soc. Proceedings, vol. i. p. 409.
¶ Geol. of Sussex, 285. Geol. Proc. ii. 203.
** Geol. Manual, 149.
†† Proc. Geol. Soc. Dec. 1836.
‡Dagger; Fourth Report Brit. Assoc. p. 654.
In Ireland, I have seen them on the east, north, and west coasts. I am informed by Mr Griffiths, that he has observed them in Cork and Waterford, and Captain Portlock has recently found them in stratified beds at an elevation of 400 feet. Proofs of such changes have also been observed in the Channel Islands and on the opposite shores of the Continent, all probably referable to the same geological epoch.
These marine beds have been discovered at every elevation, from that of the present level of the sea to a height of at least 400 feet above it; and in the solitary instance of Moel Tryfane shells have been found at the height of 1400 feet; but as the cause of their occurrence in that situation is doubtful, we may conclude that the highest elevation at which proofs of such recent changes have been hitherto discovered is limited to 400 feet.
At this height Mr Gilbertson found sea-shells in stratified beds of gravel and sand near Preston in Lancashire. Mr Murchison,* who visited this locality, observed "similar phenomena over a very considerable tract of country occupying the ancient estuary of the Ribble. Sands, marls, and gravels, occasionally forming terraces, are spread over this great area, sometimes in finely laminated beds, but for the most part loosely aggregated, and bearing a great resemblance to the arrangement of the same materials now in the act of formation on the adjoining shores. Many of the shells found in these beds far inland, and at heights extending to 300 feet above the sea, are perfectly identical with existing species." Mr Murchison justly infers, that such appearances must be ascribed to actual elevation rather than to the action of diluvial currents. Sea-shells were found by Mr John Craig, mineral surveyor, at Airdrie about ten miles to the east of Glasgow, at the height of about 350 feet; they where found between a mass of blue till and a bed of yellow stratified clay, which rested upon it. Mr Craig was inclined to suppose they belonged to the till, the shells having been filled with blue clay; but I have observed the same thing in shells which certainly belonged to the stratified deposit, and it is easily accounted for. The action of the sea upon such a bottom would
* Address to Geological Society, Feb. 1832.
naturally stir up the clay so as to fill dead shells. Those found in this locality do not bear marks of violent transportation, and the distance from the sea is so great that it is difficult to suppose that such fragile shells as the Mytilus edulis and Tellina approxima could have been borne along uninjured by diluvial action. I am, therefore, inclined to consider, that the shells found at Airdrie belong to the alluvial beds, and have been confirmed in this opinion by having had some specimens of the Tellina approxima, a species which has only been found in this deposit, sent to me from the same locality.
Mr Prestwich* also found, at the height of 350 feet, in beds of sand, gravel, and clay, at Gamrie, near Banff, the following recent shells: Astarte Scotica, Tellina tenuis, Buccinum undatum, Natica glaucina, Fusus turricola, Dentalium dentalis. They were extremely friable, but perfectly uninjured.
The promontory of Brayhead, in the county of Wicklow, is formed by a cliff of alluvial strata of coarse gravel and sand, containing sea shells; it is at least two hundred feet high, and the hill of which it is a part, and which is evidently composed of the same beds, is perhaps a hundred feet higher. Here, therefore, this deposit reaches to the height of three hundred feet. At Howth, on the north side of Dublin Bay, are similar cliffs, at the height of about a hundred feet, also containing shells and other marine exuviæ.
In the Isle of Sheppey,† recent shells have been found in a bed 140 feet above the present level. In Norfolk,‡ and in Yorkshire,|| they have been found at the height of a hundred feet. Near Berwick, Mr Milne§ observed a tract of table-land at the height of a hundred feet above the level of the sea. It consists of vertical strata, which have all had their edges worn down to a level plain, just as would have been the case if the rocks had been exposed to the action of marine currents incesantly sweeping over their edges. When the tide is far out, exactly the same appearance is presented by the vertical rocks which form the bottom of the shore for a considerable distance out from the existing cliffs, and were there to be an elevation
* Proceedings Geol. Soc. May 3. 1837.
† Ib. vol. i. p. 410.
‡ Phil. Mag. Jan. 1836, p. 30.
|| Phillip's Geology, p. 198.
§ Fourth Report Brit, Assoc. p.638.
of the coast, another table-land would be formed exactly resembling, but a hundred feet above, the former.
In the basin of the Forth, beds of razor-fish (solen), and bones of the seal, have been found at the height of ninety feet.* At that of seventy feet, marine remains have been found on the banks of Loch Lomond,† on the Yorkshire coast,‡ in Devonshire,|| and in the Island of Skye.§ I have found them in several localities in the basin of the Clyde, at the height of from seventy feet to the present high-water mark.
At an elevation of about forty feet there has been observed on many parts of our coasts a series of raised beaches and terraces, which, by their magnitude, indicate the prodigious length of time at which the sea level must have been stationary at this height; and if we may judge of its duration from the relative size of the ancient terraces with those now forming, it must have exceeded the recent period of which two thousand years is but a part, by an immense amount; but this is but one of the epochs in the history of this formation; between the great terrace and the sea several subordinate ones and beaches have been observed, each of them marking long continued periods of repose, whilst a sudden deepening, two or three fathoms below the low-water mark, is probably caused by another line of terraces now covered by the sea.
The great terrace, the base of which seems very generally to be between 30 and 40 feet above the sea, forms a marked feature in the scenery of the west of Scotland, in those parts where the violence of the Atlantic has not swept away the plateau of marine alluvia which, in the less exposed situations, is always interposed between it and the sea.
The northern part of the country of Ayr, which is composed of a coarse red sandstone or conglomerate, has been worn by the former action of the sea into a magnificent range of cliffs, in some places rising to the height of 300 feet; the two islands of Greater and Lesser Cumbra, lie opposite to it, and have corresponding terraces. The former of these islands is composed
* Wern. Mem. vol. v. p. 572
† Letter from Mr Buchanan of Arden. Phillip's Geol. of Yorkshire.
‡ By Mr Witham of Lartington.
|| Geol. Soc. Proc. Dec. 14, 1836.
§ M'Culloch's Western Islands, vol. i. p. 293.
of the same sandstone intersected by trap veins; both the trap and sandstone have been worn away, but in different degrees, and the dykes are left standing out from the cliffs like ruined walls, affording no doubtful evidence of the length of time during which the sea formerly washed their bases.*
Similar phenomena have been observed in Jura, Mull, and Isla, at elevated levels, as well as at that our present seas, and they furnish, as Mr M'Culloch observes, "the most perfect record which geology affords of the wasting action of the sea upon the land."† After remarking that the destroying causes of such as are found on our present shores are so obvious that it would be superfluous to point them out, he offers the following speculations on the origin of those in question. "The other case, that of outstanding inland dykes, such as those of Cumbray, and the more conspicuous examples in Isla and Mull, is more difficult of explanation; it is equally evident, however, even in these two instances, that the surrounding strata must
* The time is not yet gone by with geology, as it has with astronomy, when the conclusions drawn from its phenomena are supposed to be inconsistent with the word of God. I rejoice, however, to feel assured that, in yielding to evidences which it is impossible for me to resist, I am neither denying its truth, nor wresting it to my own purposes. That interpretation, which admits, to the fullest extent, the remoteness of the "beginning," was not invented to meet a geological difficulty, but has been held by learned and pious men of all ages. To those who, unacquainted with the science, think the conclusions drawn from its investigation too uncertain, and too contrary to each other, to be worth attending to, I would say, that such discrepancies of opinion are every day disappearing as the science advances; and on the point in question, there is no controversy which deserves the name. There is, indeed, no rule without exception. At the meeting of the British Association held last year at Liverpool, I remember an elaborate paper was published to prove that the theory of gravitation was contrary to Scripture: it, of course, called forth no remark. At Newcastle, a gentleman, well entitled from his labours in one department of the science, to be listened to with respect, more especially as he did not impugn opinions differing from his own, took what I must call the sceptical side of this enquiry, by endeavouring to prove the uncertainty of geological evidence. The paper was honoured by a reply from Professor Sedgwick, whose reasonings were responded to by an audience containing a greater amount of high geological authority, than perhaps was ever before congregated under one roof, in a manner which proved that on this point at least there was no dispute.
† M'Culloch's Western Islands, vol. ii. p. 480.
once have existed at least at the same level as the summits of the present dykes. Nor can any obvious causes now be traced by the operation of which so great a removal of land has been effected; there are no rivers in any of the instances enumerated, to which it could be attributed, nor, indeed, could any action of a river be imagined capable of producing those effects on surfaces so irregular." He supposes they may have resulted from the tedious operation of the atmosphere, but the actual change of level affords an easy solution of the difficulty, and in each of the cases cited, we have the additional evidence of such an origin from marine remains, embedded in the alluvial strata which accompany them.
Although we have traces of changes of level on every side of the British Islands, it would be premature to say whether or not they are all universal, or whether some of them may not be confined to particular districts. Then can, I apprehend, be no doubt as to the lower levels under the great terrace; the plateau at its base, except where since worn away by the action of the sea, is invariably composed of marine beds of sand, gravel, or clay; but the case is doubtful as to those at higher elevations; and if the shells at the top of the mountain of Moel Tryfane be considered as a proof of elevation, we may safely assume that it must have been a local one.* Although we do
* Since writing the above, I have read with much pleasure Mr Trimmer's paper on the diluvial drift in Wales and Ireland in the Journal of the Dublin Geological Society. I agree with him entirely as to the well-marked difference between diluvial deposits and those caused by permanent submergence; and if I differ with him as to the origin of the gravels of Howth and Bray, it does not in the slightest degree affect the argument. He appears, for he has not come to that part of the subject, to consider them the result of diluvial action, whilst I agree with my friend Dr Scouler, with whom I visited them, that they are proofs of elevation. Mr Trimmer, after noticing the ready reception of the diluvial theory of Buckland, remarks, that "the interest excited by these new and striking facts (i.e. proofs of change of level) had now diverted the current of geological speculation into an opposite direction from that in which it had lately flowed, and from the one extreme of having generalized too hastily on diluvial phenomena, geologists began to run into the other, of endeavouring to exclude diluvial action from the list of geological agencies,—to expunge the very name from geological nomenclature,—to forget all the evidence which had been collected of the passage of large bodies of water over the land, and, in every mass of transported gravel in which marine shells of existing species were discovered, to see a raised beach, or a marine formation, of gradual accumulation, regardless of the proofs which, in many cases, existed of such deposits being due to the sudden and transient action of the sea." It is impossible to examine the diluvial deposits which I have formerly noticed, without remarking the evident effects of such sudden and transient action, so perfectly resembling those which we know must have been owing to similar causes. In the summer of 1818, I had an opportunity of observing the deposit caused by the eruption of the lake which had been formed by a glacier in the Valley of Bagne, and which was spread over the valleys of the Dranse and the Rhone before it was covered by vegetation or obliterated by cultivation. No word could so well express its appearance as diluvium, except that the occurrence of works of art formed a prominent feature, especially below the village of Martigny, where several houses were destroyed, and where beams, hewn stones, and fragments of furniture, were confusedly mixed with gravel and clay. At Greenock, in 1834, I witnessed the effects of an inundation, caused by the breaking down of the head of a reservoir, in which upwards of thirty lives were destroyed in its tract to the sea. It exhibited all the phenomena of diluvial action. The streets and walls were marked with furrows, masses of stone, and even of cast-iron, were mixed up with clay and gravel without regard to their gravity, whilst within the houses every thing was covered with a thick layer of fine silt exactly as in the diluvial caves. Were this covering, therefore, to occur in insulated patches, we might seek in similar causes for similar effects; but where could the lake have existed so vast as to have swept away nearly the whole of the alluvial covering of the great coal basin of Scotland from sea to sea, and lodged in one confused mass in some places hundreds of feet in thickness? It must, I apprehend, be sought for in some sudden geological action of a magnitude far surpassing any like event recorded in the short page of human history. The long continued action of submarine currents could not have been the cause of the beds in question, although I have no doubt that they often have given origin to coarse beds of gravel improperly termed diluvium.
not observe any such marks of violence as are indicated by extensive inclinations of the stratification, or by the fractures, erosions, and unstratified deposits which have been produced by diluvial agency, it is quite evident that some of these changes must have been sudden; and beds of testaceous animals have been entombed alive by the subsequent deposit of clay or sand from a considerable depth. This is particularly observable in the laminated clay in which marine remains are so frequently found in the basin of the Clyde. The upper parts seem quite destitute of them, and it is only when the excavations are made deep enough, such as in digging wells or coalpits, or in the lower beds of brick-works, that they may be ex-
pected to be found. In the brick-works near Glasgow, I am often told by the workman that they are not deep enough for shells.
Such sudden changes we know have in recent times taken place on the west coast of America, and in Cutch; and no doubt earthquakes have accompanied the ancient changes as well as those of a modern date. Fissures and dislocations are occasionally to be observed in the beds of sand and clay, produced probably by such causes. In an excavation made in the line of the Edinburgh and Leith railway, in cutting through a ridge of about 400 yards long, 100 yards broad and 10 yards high, the section, which was at right angles with its length, exhibited numerous rents traversing the beds, which could only have been produced by a sudden upheaving. A horizontal section would have represented the fissures as parallel with its length, whilst the cross one shews them radiating, as it were, from a centre. The inclination of the beds is too great to be ascribed to original inequalities in the mode of deposition. In some cases they form an angle of more than 60 degrees with the horizon. Some of them consist of fine and coarse sand or clay, and others of small fragments of coal. The section presented a beautiful miniature model of the stratification, fissures, slips, and faults of a coal-field. These beds are covered by another of gravel, which lies unconformable to them, and has evidently been deposited immediately after, filling from above some of the open fissures. It is impossible to account for these appearances, without supposing that they are the effect of a local upheaving.
Although, however, the changes in level might in some cases have been sudden and attended with earthquakes, it is probable that in others they have been slow and gradual, like those taking place in Sweden at the present day. Indeed, with the exception of the absence of works of art, nothing can more perfectly agree with the appearances of the ancient marine alluvial beds than Mr Lyell's description of similar, but more recent
ones in Sweden. I have often met with beds of shells imbedded in marly clay, which had received a violet colour from the decomposition of the common mussel (Mytilus edulis), exactly as described by him.*
The question as to the identity of the flora and fauna of the present period, with that of submergence, is an important one. It would perhaps be premature to say with certainty, whether they are identical or not. With regard to the vegetation no observations which have yet been made shew any difference between it and the existing race of plants. But too little has been done in this department to be of any value in settling the question. The same observation applies to the remains of birds and land animals; or to those of cetaceæ, crustacea, algæ, zoophytes, and other marine remains which have been found in these deposits. I have endeavoured to institute as rigorous a comparison as I could between the testacea of the two periods, and refer to the catalogues which I have appended to this paper for the results. It will be observed that, although the greatest proportion of the shells are identical with existing species, there is a certain proportion which differs from them.† Of those which are unknown, some may probably yet be discovered in a recent state; while others, in place of being specially different from their recent congeners, may be only varieties arising from the different circumstances under which they were placed. Still, as the per-centage of unknown of shells is greater than that of the newer pliocene of the Val di Noto in Sicily, it appears highly probable, therefore, that some change in the fauna must have taken place.
The organic remains belonging to these deposits have been termed Quaternery by M. Risso, and subfossil by other geologists. Professor Phillips includes the beds in which they are found amongst the post-tertiary and modern deposits,—although with some doubt; observing, that it is difficult to "discriminate
* Phil. Trans. 1835, p. 1.
† At the late meeting of the British Association at Newcastle, I had an opportunity of clearing up some points of interest respecting the unknown species of shells belonging to these deposits, and have to acknowledge the advantage I derived from the kind assistance of Messrs Adamson and Alder, and from my visits of the Museum of Natural History, which is arranged in a manner well worthy of the scientific reputation of that splendid city.
between the Sicilian tertiaries with 95 per cent. of existing species of shells and the conchiferous gravels and sands of Holderness and Lancashire, in which, among twenty species of shells now living in the German Ocean, one occurs which is not yet known. If the Lancashire shells are, like those of Specton, Udevalla, and the coasts of Devon and Calvados, raised beaches, and to be classed in the modern epoch, why are the Sicilian ranked as tertiary?"* It appears to me that Mr Lyell has solved the difficulty, by classing amongst the tertiary formations "all those geological monuments which cannot be proved to have originated since the earth was inhabited by man." The appearance of man on the surface of the earth is an event of such transcendent importance, as to justify its being used as the separating line of the recent or human period, and those which preceded it. Changes of level have occurred in every stage of the earth's history: those of which I have been treating must have taken place during that which immediately preceded the recent period, and, of course, the organic remains belonging to that division of the tertiary group, which he has named the newer pliocene. It is of great importance that every circumstance connected with this deposit should be carefully observed and recorded, as an accurate knowledge of it cannot fail to throw much light on that hitherto obscure branch of geology, the nature and origin of the different alluvial beds which compose the earthy covering of the more ancient formations; and, as it must be the object of the science to proceed from what is known to what is unknown, we cannot too minutely investigate that part of it which forms the first step in the descending series, in order that we may obtain firmer footing in prosecuting our researches into the more remote epochs of the history of the earth.†
* Treatise on Geology, p. 263.
† The catalogues which accompany the memoir consist of one of the recent marine shells in that portion of the British seas which extends from the Firth of Clyde to the north coast of Ireland, containing 276 species, of which five are new to the British Fauna; and another of the fossil shells described in the paper, chiefly found in the same geographical region, containing 180 species, of which about fourteen are unknown or extinct. These catalogues, with figures and descriptions of the unknown species, will we expect appear in a future Number, accompanied, it is hoped, by a notice from M. Deshayes on the subject.
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Citation: John van Wyhe, editor. 2002-. The Complete Work of Charles Darwin Online. (http://darwin-online.org.uk/)
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