HTML Transcription © F.Coakley, 2011
[From Isle of Man, Cumming 1848]
Lithological character of the Isle of Man.- Granite Bubbles.- Great extent of schistose formations. -The Isle of Man existing as such in the Devonian period.- No disturbance between the Old Red conglomerate and Carboniferous limestone.- The lower and upper Limestone series.- Eruption of Trap rocks and interpolation in Carboniferous beds.- Great gap between the Carboniferous and Glacial deposits.- The Glacial epoch.- Subsidence and emergence of the Island.- Its present condition.
CONSIDERING the Isle of Man lithologically, it may be stated as consisting to the amount of three-fourths of it of a series of schists mantling apparently round bosses of granite, these granitic domes or nuclei being arranged in a rather irregular curved line running in a general direction from S.W. to N.E. There is no appearance of the granite having been pushed up in a solid state through the schists, nor again of its having overflowed the surface1 in the manner of basaltic rocks; but it seems to have risen up in a semifluid condition, in gigantic bubbles (if we may so speak) of molten matter, forcing itself in amongst the schists wherever they gave way.
These granitic bosses appear at the surface in two localities, in consequence most probably of the schists which enveloped them having suffered denudation.
Both these localities are on the south-eastern side of the great range of mountains which divide the island, the one near the head of the Dhoon river betwixt Laxey and Ramsey2 , and the other on the eastern side of South Barrule, on the road between Castletown and St. John's Vale3 . The schists in contact with the granite have been completely metamorphosed, and as we recede from these nuclei, pass regularly through the stages of a gneissose rock and mica-schist into clay and grauwacke-schist. Of the geological age of the schists we have no good criterion, the few undetermined fucoids or corallines in the newer portion being insufficient guides; they are probably, as far as developed, lower Silurian. Their texture is, generally speaking, softer than the Cambrian or Snowdonian rocks, and the slaty cleavage (if it exist at all) seems very imperfectly developed.
There is little reason to doubt that these schists were deposited in a deep sea. At Spanish Head, where they are nearly horizontal, we have seen that they are more than 300 feet in vertical thickness 4 , of a peculiar fibrous kind, not apparently metamorphosed, and yet giving not a trace of organized life, and these rocks form but a small portion of the entire series. Now, as every particle of these sedimentary rocks must have been derived from the destruction and degradation of pre-existing igneous rocks (excepting of course such as may be attributed to animal secretions from the waters of the primæval ocean), we must soon come to the conclusion that an enormous period was requisite for the aqueous deposition of the schistose series alone. Where the continent or land was, the degradation of which furnished the materials for this series, is entirely a matter of speculation.
It is evident however that there was an elevation of the consolidated sea-bottom of the Silurian age at the commencement of the middle palæozoic period, as the old red sandstone and conglomerate of the island rest on the upturned edges of the older schists.
Of the existence of some portion also of these schists above the level of the sea, so as to form an island, or series of islands, at the time of the Old Red Sandstone formation, we have seen evidence at several points along the edge of the southern basin of the island, the conglomerate occurring as a mass of small white quartz pebbles in a carbonaceous paste only a few feet in thickness5 , though further down towards the centre of the basin it attains a thickness of from fifty to sixty feet, and in all cases is unconformable to the schists, though passing conformably upwards into the dark limestones and shales; and it is evident that whatever cause elevated the schists, throwing them off to the S.E. by S. and N.W. by N. of the central ridge, has given to the island its general form.
Hitherto no boulders of the central granite have been found in the old red conglomerate, which is negative evidence against the hypothesis of its having at that time been brought to the surface. Still it may have been (and most probably was) the elevating agent, rising up in its characteristic dome shape, and metamorphosing the schists by its molten contact and subsequent cooling. The secondary elevations on the island seem due to an outburst of porphyries at a subsequent period 6 .
There is no evidence of any disturbance having taken place between the deposit of the old red sandstone and the mountain limestone; the former passes into the latter with a most easy gradation by an abstraction of the larger quartz pebbles, and the substitution of, first, a brown arenaceous, and then of a dark calcareous matrix for the ferruginous paste of the lower portion.
The fossils too of the carboniferous series set on (so to speak) before the quartz pebbles of the old red conglomerate have ceased. This we have seen to be the case in the south of the island at least, where alone the passage can be regularly observed 7 .
In the neighbourhood of Peel, which is the only other locality where the old red sandstone is discovered, the beds are of an increased thickness, attaining about 300 feet, and are in the character chiefly of a workable sandstone. Here the overlying limestone is not seen, having in fact been denuded from the elevated beds ; but there is no doubt of its position not far out at sea, as indicated in the geological section across the island 8 , as boulders of it occur plentifully along the shore, and have been collected at various times for burning into lime. There appears to have been a small patch of it also at one time on the edge of the cliff near Craig Mallin, which has been entirely converted into lime, in the kilns of which the ruins still exist. The old red sandstone in this locality, just under the presumed position of the limestone, is extremely calcareous, and effervesces largely with acids, yet contains its own characteristic fossils, as, for instance, Favosites polymorpha.
The carboniferous series, as developed in the south of the island, is divided into two portions in its lowest members; it consists of thick beds of dark limestone, alternating with thin bands of shale; the lifts of limestone not being equally calcareous, or alike convertible into lime when burnt, and it is remarkable that the most fossiliferous are the least suited to that end.
On a comparison of the fossils of this division with those of the carboniferous series in other parts of the British Isles, we find them remarkably agreeing with the lower Northumbrian type, or still more closely with the series developed in the neighbourhood of Hook Point in the south of Ireland; they may very well be compared also with the Kendal beds. Scarlet, the western horn of Castletown Bay, is however still alone in its glory of possessing the beautiful fossil which was first noticed there, the Goniatites Henslowii 9 , of which the original is in the Woodwardian Museum at Cambridge.
The upper division of the carboniferous series, as seen in the south of the island, indicates, by its different and extremely abundant fauna, that a change took place in the physical condition of the basin in which the deposit took place, probably in consequence either of the filling up or elevation of the then sea-bottom.
The black carbonaceous mud which previously was deposited in this area seems not to have been favourable to organized existence, or the sea may have been too deep for the more abundant species of the lower scar limestone. Certain it is that the two series of dark and light-coloured limestone differ as much in their contained fauna as in their lithological appearance. They have comparatively few species in common, and those which are common are mostly such as have a great vertical range.
The light-coloured limestones again seem separately divisible into (so to speak) zones of life; and thus we see, even within the very limited area of this basin, that, as in the present day, so also in the palwozoic period, there were certain ranges of depth within which each animated species was confined, and that whenever, from any cause the sea bottom was elevated or depressed, certain species died out, and others came in to take their place10 . , There were other subsequent changes in the physical condition of this area within the carboniferous period of a still more remarkable nature. A violent convulsion (which may be traced in its effects more particularly along a line running from the Stack of Scarlet through Poolvash) crumpled up the strata into a series of folds 11 , and formed a number of troughs or smaller basins for the reception of a new and peculiar deposit. There was at first a large outpouring of trap, which, where it has flowed over the limestone, has greatly metamorphosed it; in some cases indeed transforming it into pure dolomite. Whilst, on the one hand, the more violent eruption seems to have been but of short continuance, it is evident also that the vent (wherever it might be) was kept open, and emitted for a lengthened period volcanic ash, which was carried by the currents and deposited quietly in different parts of this area.
We have seen indications indeed that the deposit went on so quietly, or was poured out only at such intervals, as not very greatly to interfere with the development of organized life 12 . We find fossils imbedded as regularly in the beds of volcanic ash as in the limestone beds. We find also a very interesting local deposit of black carbonaceous mud (very similar to that forming the earlier shales of the basin) going on at the same time, and mingled with the volcanic products, the prevalence of one or other in any particular locality depending, it would seem, on the relative distance of that locality from the sources of the respective ingredients there deposited. We may easily explain the appearances, by supposing a river whose waters were charged with a carbonaceous silt, having its embouebure in the neighbourhood, and thus mixing its contents with the quiet outpouring of a subaqueous volcanic vent at no great distance. At one period, indeed, the carbonaceous deposit seems to have entirely prevailed, perhaps the volcanic action entirely ceased, gathering strength for a subsequent eruption. The bed then formed has its own lithological character and fossils. It is the Posidonian schist or black Poolvash marble so largely used for economic purposes13 .
I have termed it Posidonian schist, from the circumstance of its containing the Posidonia in great abundance, and as its characteristic fossil. But it is also otherwise remarkable. It contains, on the one hand, Favosites Gothlandica, hardly hitherto considered a carboniferous fossil; and on the other hand, we find in it the first and only traces of coal-plants met with on the island. I have in my possession a beautiful cone of Lepidostrobus ornatus. Like all the other shale beds, this abounds in sulphuret of iron, and in one particular layer the contained fossils have become converted into that mineral : they are exquisitely beautiful, and give us the idea that they are some of nature's electrotypes. Every line and every curve of the original has been preserved with the closest exactness, as we perceive by a comparison with the corresponding species in the limestone beds. And as the surface of many of them presents the appearance of burnished copper coins, the illusion is complete.
This quiet and regular deposit was afterwards suddenly interrupted. The volcanic action was again exhibited with renewed violence, as at the first. The lower beds of the first eruption, together with the beds of volcanic ash, of mixed trappæan ash and calcareous deposits, and Posidonian schists, were contorted, broken up, reduced to a fragmentary condition and enveloped in the outpoured deposits. There results a trap-breccia, in which the fragments of the older beds seem to have been considerably influenced by heat. The Posidonian schist has become cherty, the limestone highly crystalline, and in some cases hardly distinguishable from amygdaloid.
It is an extremely interesting question, as to whether the trap-dykes which stretch across several portions of this basin in a direction a little to the south of east and north of west, were or were not the accompaniments of this eruption. In the only spot (Poolvash Bay) where they can be seen in connection with it, they seem to merge into the breccia, and the impression consequently is that it was the overflow of the dykes which assisted in forming that breccia, and that the convulsion and contortion of the inferior beds was contemporaneous with the formation of the cracks from which the trap poured forth. There is good evidence to show that whenever the trap did flow forth, forming the dykes, it did not merely find vent through pre-existing cracks, but that the eruption also was the cause of those cracks which it filled; and it seems to have forced itself in also between the beds of conglomerate and the tough superior limestone. Should any clear evidence hereafter arise showing that the trap-dykes are posterior to the trapbreccia, it must still be impossible to fix their exact date, as they may range through the whole of the mesozoic or secondary, and a large portion of the kainozoic or tertiary periods; the next deposits superior to the trap-breccia being those of the pleistocene formation.
In this interval however must at any rate be fixed the protrusion of those porphyritic masses which, as we before said, seem to have formed the secondary elevations on the island, and perhaps contributed a lift to the central chain. This period seems to be fixed in the following manner. If we examine (as an instance) the ridge running from Rock Mount 14 near St. John's towards Cronk Urley, at both which places the porphyry is discovered, we shall find that it was the elevation of this ridge which gave the high angle to the old red sandstone of reel (to which also the limestone is conformable). But the pleistocene formation at Peel appears to rest quietly on the up-turned edges of the old red sandstone.
Again, the great fault running from Perwick Bay through Port St. Mary, Strandhall and Athol Bridge, in a direction nearly N.E. by N., cutting off at once all the carboniferous series to the N.W. of this line, seems closely connected with these porphyries, which are developed almost continuously along the fault. Since this fault took place a great denuding force has swept over the island, and has planed down both sides of the fault to the same level, and the boulder-clay formation lies evidently undisturbed continuously across the line of disturbance15 . And we have similar evidence on Langness, at Cosbnahawin, and the whole way to Santon Head.
It seems not unlikely that. the same denuding action which, as we have just stated, swept away so large a portion of the carboniferous series in the southern basin, reducing to the same level the beds on each side of the fault, laid bare the granitic boss on the eastern side of South Barrule, for the boulders of that granite appear in the boulder clay, but not in any previous formation.
We thus arrive at the conclusion, that, if we except the low extended area of the tertiary formation in the north of the island, its present physical appearance was attained in great part in the secondary and earlier tertiary periods. Whether in the entire interval between the carboniferous and glacial deposits it was above the waters of the sea, and therefore not receiving on its surface the beds of the Permian, Triassic, Liassic, Oolitic and Cretaceous series, or whether after having received some or all, and having been elevated above the sea-level it was denuded of all of them in succession down to the Carboniferous, and including a portion of it, is quite uncertain. It is evident that either may have been the case, yet neither of these views is without its difficulties16 .
The period of the boulder-clay formation on the island manifestly commenced with a state of atmospheric conditions very different to that existing in the carboniferous epoch or those now existing.
Those conditions seem to belong to a severe climate17 . It appears impossible to exclude the agency of ice in the greater part of that formation, though how far this may have been aided in its effects by extraordinary currents and waves of translation, originated perhaps by the upheaval of mountain-chains or extensive tracts of land above the ocean, is still a question sub judice. The facts bearing upon the question have presented themselves in the course of our journeys, and I am not without hope that the Isle of Man itself may be found hereafter to afford the key to the unlocking this mystery.
Very distinct evidence, as we have seen, is presented in the south of the island, that vast masses of clay, sand, gravel and fragments of rocks must have been forced along by powerful currents in definite directions. Underneath the boulder-clay formation there, the rocks of the limestone series are grooved and scratched in a remarkable manner. .The lines are not always continuous, but seem struck out as if by some sharp body brought in contact, suddenly pushed forward and then elevated again18 . The fragments of rock also in the boulder-clay formation are themselves scratched and grooved, and when they can be determined as belonging to the island not much rounded. Indeed there is every indication that they were not rolled but pushed along whilst held tight in some matrix; and what matrix supplying all the requisite conditions can we so readily conjecture as ice ? The effects of that agent in the present day on the shores of our Arctic and Subarctic seas and rivers19 so closely correspond with the appearances presented to the geological inquirer as belonging to the period of the boulder clay or pleistocene formation 20 , that we can hardly resist the argument for the identity of the agency in both cases.
The lower portion of the boulder deposit is the more loamy. Perhaps this may argue that it was originated in a muddy and deep sea, and the included fossils point to the same hypothesis. Yet the ploughing-up by icebergs of a seabottom, consisting of limestones and shales, tilted so as to present a series of basset edges to the drifting currents, must have contributed largely to the materials of this formation. And it has been noticed, as remarkably confirmatory of this view, that the boulder clay to the leeward of any particular rock relatively to the drifting current has the predominant colour and mineral contents of that rock; to the leeward for instance of the basset edge of the old red sandstone it has a reddish tinge, of the limestone and shales a dark dingy blue.
It will readily be conceded, that in such a climate and insular locality, the loftier mountains might generate glaciers, bringing down large accumulations of detrital matter, with angular and scratched fragments of rocks. As yet however no distinct evidence has presented itself of such glaciers having existed on the Isle of Man. The small elevation and extent of its mountains above the then sea-level would by some perhaps be considered hardly favourable to their development, though modern researches have shown that in islands within the arctic regions glaciers descend even to the sea-level21 . But the icy waves (especially if aided by storms) must have acted powerfully in the degradation of the shales, whilst masses of gravel and sand frozen into coast-ice would be carried onwards by the currents, which hurried through the different channels, and being arrested in their course by any more elevated object, would become packed, and form heaps of gravel, sand and clay on the deliquescence of the ice.
The formation of long ellipsoidal hillocks, whose major axis (so to speak) is in the direction of the general drifting current (as shown by the subjacent scratched rocks), is a very remarkable fact, and has been well-studied in the south of the island; whether they were so formed originally beneath the sea, or have attained the shape through the action of currents at a period of gradual elevation, it may not at present be safe to say. That after a long continuance of depression there was for some time again a gradual elevation of the sea-bottom seems pretty clear, and that the sea itself subsequently became of a less muddy character 22 is also evident; the upper portion of the boulder deposit consisting largely of rolled pebbles, gravel and sand, generally in waved layers, with very little clay. The contained rocks too in the upper portion, as we have seen, are more generally foreign than in the lower. We should be prepared to expect this, as presuming the boulder-clay series at the commencement of its formation to have been originated in the manner we have described, in at first a gradually deepening sea and without any extraordinary action of denuding waves ; it would follow, that after a time, when the sea-bottom became well-covered by this deposit, the further degradation of the inferior rocks by the ploughing action of the icebergs would cease.
In speaking of the commencement of the boulder-clay formation, as presenting to us no problems requiring necessarily a violent diluvial action for its solution, it is not intended that no such action existed at any period of the deposit. On the contrary, there are phænomena which point to the probability at least, that enormous waves with vast carrying force must have swept over the surface of the island at a later period of the formation. The general appearance of its eastern, as compared with its western side, described by Swedish naturalists under the term stoss seite or weathered side, indicates in some measure that fact, and also the direction of such action. But the evidence which tends most powerfully to the establishment of such a view, is to be read in the phmnornena presented to us on the western side of South Barrule. We have noticed there23 , on its western side, and even within a hurndred feet of its summit, large boulders of the same granite which is developed on its eastern side more than 600 feet below the summit. No simple carrying action of icebergs can have transported these blocks up the very steep eastern face of the mountain and so over to the other side, but we can imagine the extraordinary action of great waves acting on masses of ice charged with these granitic blocks, and bearing them to a considerable elevation above the then sea-level. We must either grant this, or suppose an elevation of the mountain chain to the westward of the granitic boss since the deposit of the blocks on the top and western side of South. Barrule, but of such elevation no independent evidence has been as yet discovered.
There is some reason, as I said, for concluding a gradual elevation again of the island towards the close of the boulder-clay deposit, and that the singular low rounded hills of that formation, which are observable both in the south of the island, and in the north on a line from Point Cranstal to Blue Head, are due to the beating about of the waves and the action of currents at such period. It appears however that when this gradual elevation had proceeded to some extent, there was a long-continued rest, during which the great platform of drift-gravel was deposited, in part formed by the degradation of the upraised masses of the previous boulder-clay deposit. In this drift there is little or no clay; coarse sands alternate with beds of gravel, and occasionally there are some large foreign boulders in it, especially on the surface.
In some parts of the island (but more especially on the north-western coast between Peel and Kirk Michael) there are appearances as if the boulder clay had been worn down and its surface swept clean before the deposit of the driftgravel was formed upon it. In other cases the gravel has filled up hollows in the boulder clay, not in horizontal layers, but in layers which are concentric with the bounding surfaces of these depressions, indicating a shallow sea.
On the surface of this drift-gravel, as we have just noticed, we often fall in with large boulders, sometimes single, at other times several of them together. They are also occasionally met with on the tops of the low hills of the boulder deposit, and on the eastern slopes of the mountains: they have been most probably dropped on the melting of drift-ice which has grounded.
The elevating process seems again to have set on after a time, and during that elevation there was considerable denudation of the drift-gravel. The great depression now occupied by the Curragh in the north of the island, seems to have been formed at this period; the valleys of St. John, Baldwin and Spring Valley in the centre, and the long valleys in the south, are evidently coeval and originated by like causes. They have assumed their present form, as is very readily perceived, in consequence of the particular arrangement of the subjacent palwozoic rocks, forming natural breakwaters in particular localities, and preventing the removal of the gravel by the beating of the waves at the period of elevation. In the unprotected places, the denudation has proceeded down to the boulder clay, which, from being in its lower portion of a rather tougher texture, has resisted the denuding action longer, though in some instances the denudation has proceeded to the surface of the older rocks.
It will perhaps be always impossible to determine the extent to which the elevation of the sea-bottom continued.
My own conviction is, that the greater portion of the area now occupied by the Irish Sea became dry land, and formed extensive plains occupied by many freshwater lakes. There is no reasonable ground for doubting that the Isle of Man became connected by such means with England, Scotland and Ireland. Over these plains roamed the stately Megaceros, and in these lakes he was frequently mired24 .
At the same time England would be united with the continent of Europe, and opportunity would be given for the emigration into the British Isles of the various tribes of animals which appear to have inhabited them at this period. Professor E. Forbes has most elaborately worked out the same result from considerations of the flora of the British Isles, as compared with foreign types25.
What ages may have elapsed with such a condition of land and water in this portion of the northern hemisphere ! The inland lakes became filled up with alluvium and peat by the ordinary and slow operations which we see now going on, and vast forests of oak, pine, ash and birch grew up and covered the surface of the country.
We have however further evidence that a depression of this area again took place. The forests were overthrown perhaps by the incursion of the sea, and covered by marine deposits. The different races of animals then existing were perhaps in part destroyed, the remainder betook themselves to the higher grounds and the mountains, and became isolated.
Perhaps amongst them the Megaceros Hibernicus may be included, though we have not as yet any distinct evidence of his existence after the growth of the great forests. Further immigration from the continent of Europe was then stopped.
But again the elevatory process commenced, a process which may be slowly carried on up to the present time. The submerged forests have again in part re-appeared above the waves of the sea. The Curragh and the lakes it contained, as also those in the south of the island of which we have historical records, have been one after the other drained; land has been reclaimed from the sea in the openings of some of the alluvial valleys, though the work of destruction still proceeds, aided by the ordinary atmospheric operations of wind, rain and frost at more exposed portions of the tertiary formation, where it presents cliffs to the action of the breakers26 .
1 I believe that in the streamlet near the Foxdale mines, where the granite appears to overlie the schist, it is simply a case of intrusion along the line of contact.
2 See the last chapter, p. 233, and Plate I.
3 See Chapter XII. p. 176, and Plate I., general section across the island.
4 See Chapter XI. supra, p. 147.
5 See Chapter V. supra, p. 44.
6 As in the case at Rock Mount: see general section across the Island, Plate I.
7 See p. 44, supra.
8 See Plate I.
9 Named in the Dean of Westminster's Bridgewater Treatise Ammonites Henslowii.
10 See List of Carboniferous Fossils, Appendix Q.
11. See Plate VII. sections 2 and 3.
12 See Chap. X. supra, p. 129.
13 See Chap. X. supra, p. 130,
14 See Plate I. General section across the Island.
15 See Chap. XI. p. 143, supra, and Plate VII, section 1, with comparison of Plates II. and III.
16 The former hypothesis would be most consistent with the views which I have expressed at p. 118, Chap. IX. supra.
17 See list of Pleistocene fossils, Appendix R.
18 See Chap. IX. p. 115, supra.
19 As in the instance adduced, p. 117, supra.
20 See p. 113, supra.
21See 'Recherches sur les Glaciers, les Glaces Flotantes, les Dépbts Erratiques, &c.,' by Mons. Jules Granges. Paris, 1846. See also p. 2%, supra.
22 See the Memoir of Professor E. Forbes in the first volume of the Geological Survey of Great Britain, pp. 383 and 385.
23 See Chap. X11. p. 177, supra,
24 See Chap. VII. supra, p. 81.
25 See his valuable paper on the Flora of the Isle of Man, Appendix S.
26 For a report of the Meteorology of the Island, see Appendix, Note T.
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