HTML Transcription © F.Coakley, 2011
[From Isle of Man, Cumming 1848]
The trap rocks of Scarlet.-Evidences of successive volcanic eruptions.-Great thickness of trappean beds.-Fossils of the traptuff.-The Posidonian schist interposed in it.-Probable extent and duration of the black marble quarries.-The economy of their working.-The Poolvash limestone.-Great abundance in it of the fossils of the Lower Scar limestone of Yorkshire.-Pleistocene beds at Strandhall.-Singular stalactitic concretions.
IF the hieroglyphics which we have just been endeavouring to decipher at the limekilns remind us that there has been a period when the great agent employed in giving its physical contour and character to this area was ice, we have a chapter which has been stereotyped in a frame of molten rocks hard by at the Stack of Scarlet, which declares that the intensity of volcanic fire has also been exerted on the same object.
The undulations which we have noticed as increasing in number and intensity towards this point are suddenly intercepted by a series of igneous trappean rocks of every character and description, from a light pumice and volcanic ash to solid columnar basalt.
First of all we fall in with a compact trap-dyke of five feet in width running nearly north and south magnetic and intersecting the limestone; thirty yards further to the westward is another large dyke, or assemblage of dykes, running N. 30° W., and the limestone appears thrown down violently towards it, and masses of it are entangled in the trap and metamorphosed. Proceeding eleven yards further to westward we come upon a mass of amygdaloid, and this abuts against an isolated patch of altered limestone which has been raised into a dome or boss rather more than thirty yards across; round this boss and enveloping it like the coats of an onion are beds of trap-tuff or volcanic ash, and upon these a mass of trap-breccia. All these beds appear to have been broken up by the protrusion of the basaltic mass terminated in the Stack, and it would seem as if through the openings thus caused in the earth's crust another accumulation of trappean mud or ash had been poured forth which rests across the upturned edges of the previously erupted beds. We may perhaps sometimes have seen a thick coat of ice on the surface of a canal broken up by the passage of a boat and piled in masses on either side, and then frozen in a second time, so that the fragments of the first freezing stood bristling up edgeways at every conceivable angle and presented fantastic groups of miniature sierras. Now something of a similar picture is set before us in these trappean beds at Scarlet Stack. The scenery is extremely wild and picturesque, though the scale is so limited in its extent; a miniature volcanic mountain with traces of separate convulsions and outpourings of volcanic products. But the volcano was subaqueous which afforded the materials accumulated here, or at any rate it was so near the sea that the mass of ashes and scoriae which were ejected fell at once into the waters, were borne along by the currents, and deposited in regular layers of stratification over the sea-bottom.
I have never been able to make out with certainty where the volcanic vent was that emitted the trappean materials first deposited, though I have conjectured that it was a prolonged chasm extending from the Stack of Scarlet into Poolvash Bay. We have but a mere strip of these rocks along the shore between high and low water to judge by, and the whole country has suffered so much from denudation that we cannot always be certain where the denser masses have originally been. That there was afterwards a great convulsion along an axis in this direction is very evident, and also that it was this which originated the more violent contortions of the strata and was connected with the formation of the basaltic pile of the Stack; and that still subsequently there was an outpouring of similar volcanic matter to that at first accumulated in this basin, but enveloping altered fragments of the former eruption as well as of carboniferous limestone by which a species of breccia was formed; this is all pretty plain as to the general statement, but it is not always so easy to determine of two contiguous masses of the trappean formation to which of these periods they belong. It appears also most probable, that at the period of the great convulsion just alluded to there was so -much heat evolved (perhaps with acid gases) as to alter considerably the character of the adjacent rocks. The metamorphosis is so complete in some instances as to render it difficult at first sight to determine of a piece of rock whether it is altered limestone or true trap. Such a result may arise from the circumstance which will presently be more particularly noticed, that at the period of the deposit of the volcanic ash at the bottom of the sea the ordinary calcareous deposit was also proceeding, and the resulting beds were a mixture of trappean with carbonaceous matter. Such a rock altered in various degrees must necessarily present appearances of a passage from true lime.. stone into true trap.
The view from the summit of the Stack of Scarlet is very striking. We are standing on a pile of basaltic columns, not so magnificent or distinct certainly as those of the Giant's Causeway in the north of Ireland, but exhibiting the same characteristics; the sail-clad sea is spread almost entirely around, and at high water completely isolates the mass. To the extreme west the Calf of Man with the Burrough and the Eye rock stretch far down into the Irish Sea, appearing from this point as it did from Langness, simply a prolongation of the Mull Hills. The bold front of Spanish Head rears itself aloft, and casts its black shadow athwart the waters of Poolvash Bay; then the precipitous Head of Brada to the north of Port Erin, from which (with the exception of the deep chasm in which lies Fleshwick Bay) we have a continuation of the insular chain to the north-east, including the more elevated points of Irey-na-Lhaa and South Barrule. The northern mountains of the island as seen from this point appear well clustered together, and form a fine background to Castletown and its bay, at the head of which the college facing in this direction is seen to great advantage. Sweeping round to the east, Derbyhaven with its white-washed cottages and herring-house, then the fort and ruined oratory on St. Michael's Isle, come into view, and quite round to the south-east we have the round tower on Langness.
The contortions of the limestone at Scarlet are well seen hence 1; the smooth surface of the beds, and their step-like face where opposed to the denuding action of the sea, are finely contrasted with the rugged character of the trap rocks, and the isolated mass of the crystalline altered limestone nearer to us. The very violent contortion of an apparently detached portion of the dark limestone enveloped in the trap-tuff, and jammed up against the outburst of basalt which terminates in the Stack, is particularly interesting, and catches the eye from this point when the rocks are laid bare at low water.
The fault before mentioned running N. 35° W. magnetic enables us to determine pretty nearly the thickness of the lower limestone series in this part of the basin.
I have measured accurately the thickness of this limestone from low water mark, spring tides, to the black shaly bed which appears just to underlie the Poolvash limestone, and this amounts to 129 feet. I cannot add more than 50 feet from the low water mark to the base of the limestone series, as the old red conglomerate at the south-western end of Langness runs out into Castletown Bay a great distance and at a low angle; so that we may put down in round numbers 180 feet for the dark limestones and shales at the Stack of Scarlet.
The upper portion of the isolated and altered patch of limestone nearest the Stack appears by the included fossils, as far as they can be made out, to belong to the lightcoloured Poolvash limestone, but the lower portion may readily be observed as being the same with the black beds to the northward of the protruded amygdaloid and trap which have isolated this limestone boss. It is very unfortunate that at this point (where, in consequence of the removal of the drift-gravel and boulder clay, we have a distinct view of the order of superposition in the limestone series) the rocks should have jëen so much altered from their ordinary character. It is the more unfortunate because the junction between the upper and lower series of limestones in this basin is everywhere covered up by the tertiary formations ; or wherever along this line of fault, running from Scarlet to Strandhall in the north-west of Poolvash Bay, they are brought up to view, they are both metamorphosed in such a manner as to render it somewhat difficult to determine whether the passage from one to the other was gradual or sudden. Such evidence as we have is in favour of the latter supposition. It is very plain that some decided change took place in the character of the sea-bottom of that period, either in consequence of gradual filling up, or, as seems equally probable, by volcanic elevation, which rendered it a fit area for the development of organized life to a greater extent and of a more diversified character than hitherto. To be fully convinced of this fact, let us just before parting with them observe carefully the fossils of the upper portion of the lower dark limestone group.
There is a great dearth of fossils here, and the few which we find are of large species:-viz. Caninia gigantea of Michelin2, Orthoceras gigantens, Nautilus complanatus and Goniatites Henslowii; the two latter were named originally from specimens found at this spot which are now in the Woodwardian Museum at Cambridge and are almost unique 3. The Brachiopodous Mollusks are extremely rare. We shall find the reverse to be the case when we get into the Poolvash limestone series. Let us start thither.
The summary which we must give of appearances at the Stack of Scarlet is, that there were certainly two eruptions or disturbances; the one producing cracks and faults running about S. 35° E. and N. 35°W. magnetic, with others at right angles to this direction; another producing cracks and faults running S. 15° E. and N. 15° W., with others at right angles to this direction. The latter was probably contemporary with the trap-dykes which intersect this area.
As we pass along the shore the great thickness of the trappean deposit seems to come out in a more striking manner, and presents scenery quite peculiar to itself, whose wildness and desolateness, though on so small a scale, it is impossible to realize from mere description. This is particularly the case at a spot familiarly known by the name of CromwelPs Walk, where the action of the sea, aided by the peculiar condition of these beds, which have a tendency to split up into rhomboidal masses, has wrought deep chasms; which after all do not discover the base of the formation4. It is however very interesting to mark the regularity of stratification in this trap-tuff or trappean ash, and the evidences it presents of a quiet deposit of the ash along with the ordinary carboniferous limestone of this area. We begin to see this first in a deep gully 100 yards northwestward of Cromwell's Walk, where there is a thin bluishcoloured bed low down in the tuff, which appears in every respect like a mixture of limestone and trap. If we follow the rise of this bed towards the north-west, we come to another denuded recess just under the stone shed which has been built on a prominent point of the tuff. In this we have, rising on a boss, a thick bed of black schistose limestone which is completely enveloped in the trappean beds and appears to dip under them in all directions; but as the boss is much broken and disturbed, we cannot be quite certain of this latter statement, more particularly as there is another bed of similar limestone I believe higher up in the series, as well as a thin band of the same rock, consisting mostly of black cherty nodules of a still more recent date,
My opinion at present is, that this black schistose limestone, which here appears low down in the trap-tuff, is the same as the Posidonian schist of Poolvash, though, from the circumstance of there being, as it would seem, three beds of this black limestone and the whole district much disturbed, it is hardly right to speak confidently. Amongst the disturbed trappean beds for instance, close upon the Stack of Scarlet, there is a contorted fragment of black schistose limestone, but it is quite impossible to fix the bed to which it belongs.
With some little trouble a small dyke may be made out intersecting all these trappean beds between high and low water, and its protrusion seems to have contorted the neighbouring beds; it runs hence N. 46° W. in a line for the Carrig rock in Port St. Mary Bay, and is probably the same as that which appears in the schist at the mouth of the Colby river near Kentraugh.
The trappean beds present a very singular brecciated appearance at a point where the shore begins to take a more northerly direction. It is a species of conglomerate rock, of which the inclosed boulders seem to be altered limestone. Here also we find masses of the black schist enveloped in the trappean beds and quite cberty.
Along the high water mark the strata have been much dislocated, and there is evidently the continuation of the line of fault from the Stack of Scarlet. But between high and low water the beds are very regular, and dip at a low angle in a direction for Port St. Mary N. 56° W. magnetic.
On the shore near the pile of stones erected by the Trigonometrical Survey, we have rather a large development of the trappean limestone beds, in which sometimes the trappean ash is the prevailing ingredient, at others the carbonate of lime. But the most interesting circumstance is, that we meet with organic remains regularly imbedded not only in the limestone, but in the trappean ash; they are chiefly corals and crinoidea, and are the newest of the palæozoic fossils occurring on the Isle of Man; they are rather abundant than otherwise, though the eye does not readily catch the particular beds in which they occur.
It is very readily seen that the black schistose beds are lenticular, and that the black mud of which they are composed was deposited in hollows and a shallow sea; indeed, from the manner in which the great Posidonian schist bed at Poolvash, which is wrought as a marble quarry, thins out round the bosses of limestone which appear just at. high water mark, it seems probable that the configuration of the sea, shore in this immediate locality was the same at that particular period as it is now.
There is a ruined workshop of the marble-cutters at a point where the shore turns northward to form the inlet which is generally called 'par excellence' Poolvash' 5, and it is here that the great black schistose limestone bed rapidly attains, as it dips into the sea, a thickness which has not yet been pierced, though at the back of the building a few yards to the south we find it only a foot thick, and seeming to die out amongst the trappean beds inland. It is an important point for the geologist to observe and to trace the continuity of this thin bed, having the trap-tuff below and above it, as it is the only evidence we have of the reality of the interpolation of the great Posidonian schist bed in the trappean formation. The circumstance of our observing all along the coast of this bay the black schist reposing at once on the limestone 6, would lead at first to the belief that the passage from the one to the other was direct and uninterrupted. But a happy denudation of the beds near this workmen's shed shows, that after some disturbance of the limestone the trap-tuff was deposited in hollows and bays which it filled up, and,that then the mud of the Posidonian schist was thrown down so as to overlap the line of junction of the trap-tuff and limestone and thus overspread both formations. Perhaps a closer examination may show, that just before the deposit of the Posidonian schist there was a second slight disturbance of the sea-bottom, and hence the rapid thickening to seaward of this bed.
In consequence of the extensive denudation which has taken place over this area, it is impossible to determine the thickness of the accumulating trap-tuff superior to the great Posidonian schist bed. As measured near Scarlet Head it was certainly not less in that locality than 60 feet, and it includes, as we have noticed, other beds of black cherty limestone and trappeo-limestone beds, and beds containing organic remains. There are also overspreading masses of a trap-breccia which contains lumps of the altered subjacent rocks, and there are trap-dykes intersecting all these beds which probably overflowed their surface and added to their thickness, though we have no distinct evidence of the passage of these dykes into overlying trap, so far as I have hitherto observed them.
This immediate locality is much intersected with these trap-dykes, to the protrusion of which I attribute the great contortions and the mammillated appearance of the beds of Posidonian schist7. There is a small dyke, one foot wide, -close under the marble-mason's ruined workshop just mentioned; to the northward of that a few yards, another 6 feet wide; and thirty yards still further north, one of 21 feet in width. The general direction of these is about N. 15' W. and S. 15° E. I presume that these are a continuation of the dykes observed at the Stackof Scarlet disappearing under the drift in this direction. Again, at the mouth of the stream from Balladoole, where the surface of the Posidonian schist is remarkably studded with bosses, we have a dyke which seems first to run hence N.15° W. magnetic for some little distance, and then turns N. 35° W., and is probably the most southerly of the three which we meet with in the little creek opposite to which the road from Balladoole comes out upon the shore. I conjecture this to be a branch of the great Knockrushen dyke, and thus a continuation of the more southerlyof the two notable dykeswhich intersect Langness 8.
The Posidonian schist is so important a bed in an economical point of view, that I have dwelt rather largely upon it, in order that its true character and probable extent may be known. Since the days when Bishop.Wilson caused to be quarried here the steps of St. Paul's Cathedral in London, a great inroad has been made upon the workable portion of it. The great depth at which it is in some places buried under the trap-tuff, the contortion and cracking which it has experienced in others, and its alteration for some feet where in contact with the trap- dykes; again, its thinning out inland, and the circumstance of the thicker beds only lying below high water mark,-all combine to make the exhaustion of the quarry as a remunerative investment a very possible thing, though very many years must elapse at the present rate of working ere this can take place. From the facility with which it is wrought into chimneypieces, tomb-stones, steps, &c., it is evident that if it were better known in England a large demand would probably arise. It does not however take a natural polish in consequence of its soft character. A kind of black varnish is put upon the objects which are wrought out of it, and in this way they are made to look not much inferior to the best Derbyshire black marble.
The labours of the quarriers in getting at this black marble have shown us another locality in which the groovings and striations of the glacial period may be well examined.
At the point where the more northerly of the two largest trap-dykes just noticed disappears under the drift and boulder series, a large portion of the latter has been removed, and on the surface of the Posidonian schist thus laid bare the glacial marks are very finely developed, with their bearings corresponding to those which we have observed at the limekilns near Scarlet Stack.
The different layers (or lifts, as the quarriers call them) of the Posidonian schist bed vary both in their lithological texture and in organic contents. The finest and most compact layer, which is worked for ornamental purposes, is characterized by an abundance of the Posidonia and the relics of tree-ferns, which we must necessarily regard with interest as indicating an approach, though still at a considerable distance, towards the coal formation of Great Britain. Another layer is little better than a soft shale charged largely with sulphuret of iron, and in this we have preserved (converted into that sulphuret) the remains chiefly of cephalopods, Goniatites and Orthocerata. There is a gentle rill in the eastern corner of this little creek which bursts out from under the drift-gravel near one of the trap-dykes, and the cattle coming down to drink there, trample about in this shale bed and break it up, and the tides then wash out the fossils and cast them ashore. In consequence of their metallic lustre and electrotypic appearance, they have been much sought after by those who are acquainted with the locality, and have become rather scarce. It may be well to note, that all the beds of the Posidonian schist are more or less charged with iron pyrites.
Continuing our sea-side ramble north-westward, we meet with a succession of creeks in which we find the hollows occupied by thin beds of the Posidonian schist, but the ridges are composed of a pale grey limestone almost entirely made up of fossils. These belong to the upper limestone series, which from the locality I have termed the Poolvash limestone. The colour seems owing almost entirely to the abundance of the organisms contained in the rock, and throughout the entire mass no trace of a shale bed appears.
The manner in which this patch of the newest limestone in the very centre of the basin has been preserved and exhibited to our view, is somewhat singular.
The original deposition of the beds of this Poolvash limestone seems to have been in a wide but Dot very deep bay, in which a line drawn from Spanish Head to Kirk Santon Head would perhaps unite the extreme horns, though, in consequence of the denudation of the tilted edges both on the east and west side, we have no data by which to establish this satisfactorily.
When the whole of this area was broken up by the convulsion which originated the trap-dykes undulations and bosses, the hill above Balladoole near Poolvash seems to have been elevated somewhat more than the neighbouring portion in a dome shape, and in the elevation it cracked along its south-western and north-western sides. Thus, when the denuding action subsequently took place, which we have always presumed to have come from the northeast and round towards the south, in consequence of their dip towards that quarter, the Poolvash beds on the Balladoole side were preserved.
We may perhaps illustrate the changes which have passed over this southern area since the Old Red Sandstone or Devonian period in the following manner.
Suppose a freshwater lough fed by a large river, but just accessible to the sea at about the ordinary half-tides. A hard frost sets on at the time of high water of the highest spring tide, and coats the lough with ice; on the ebb of the tide the water falls a few feet, and the ice sinks down upon the sides of the lough. In this condition a second coat of ice is formed at the ordinary level of the lough, and the first icy coating sticks upon all sides around it. When the tide flows again, the incoming waters force up the ice in the centre of the lough, and through the cracks thus formed the salt water gushes up, and forming a mixture with the fresh water of the river, overflows the broken beds of the first and second freezing ; but as the tide this time does not rise so high as before, supposing another coat of ice now formed of this mixture of salt and fresh water, though it will overspread all the ice of the second freezing and partially of the first, it will still leave a portion of the first freezing sticking up around its edges. A fourth sheet of freshwater ice is again formed on the recess of the tide, which is again contorted and broken up at the next high water, when another and fifth crust is frozen of mixed salt and fresh water, the proportions of each, as in the third freezing, varying according to the distance of any particular locality from the mouth of the lough. The tide ebbs and flows again, but this last time, in consequence of a violent storm producing a heavy ground-swell upon the sea, the force of the incoming tide is such as to produce great undulations and domes in the centre of the lough. A thaw commences in the interior of the country, and the augmented volume of the river consequent upon it sweeps along over the surface of the lough and erodes the beds, which being tilted up are more directly opposed to its violence, whilst those which present a smooth surface to the current are preserved. At the same time the hollows are filled up with gravel and detritus brought down from the uplands on the melting of the snow and the overflow of the river upon its banks.
In the above illustration, the first freezing will represent the older lowest dark limestone of Ronaldsway and Port St. Mary; the second is the Poolvash limestone; the third the lower trap-tuff; the fourth the Posidonian schist bed; the fifth is the upper trap-tuff; and the alluvial deposit after the denudation may represent the position of the boulder formation and drift-gravel series overlying the undulating and broken beds in the lower portion of this southern area of the Isle of Man.
The crack which we have just noticed as occurring on the south-west side of the Balladoole hill is best seen at a point where the sea-road suddenly descends to the shore after passing a small cluster of cottages four hundred yards west-ward of the road to Balladoole. The lift seems to have been sufficient to bring up the black beds of the lower limestone to view.
At this point between high and low water there is a spring which seems to communicate with an underground pool, filled from the sea at high water, and which continues to run as a salt stream several hours after the ebb of the tide 9.
The whole of the coast hence to Strandhall is so cut up with dykes and metamorphosed, that it is impossible to make out any order in the beds, though it is generally evident that in proceeding north-westward, we are descending again into the lower series 10.
The very great alteration which has taken place in the limestone here would seem to indicate that this was the grand focus of disturbance at the period of the trap-dykes, and this is further confirmed by the circumstance that the majority of the dykes which stretch over the area seem to converge towards this locality as a centre.
I had the great gratification of submitting a portion of this altered limestone to that eminent continental geologist Baron Leopold Von Buch, and after a very little examination, he pronounced it pure dolomite. A chemical analysis also of the same rock, by my valued friend George Kemp, Esq., M.D., determined its magnesian character. The establishing this species of metamorphism in connexion with trap-rock is highly interesting and important 11.
When we reach Strandhall, the limestone has recovered its ordinary character, and the lower beds are exhibited between high and low water nearly horizontal, and charged abundantly with its characteristic fossils.
There is an interesting phænomenon connected with the tertiary sands, which, as we have the opportunity, we may as well study at this point, though the same is developed on a much grander scale in the north of the island.
As might be naturally anticipated, many of the springs of this neighbourhood, passing through and over the beds of limestone or washing the boulder clay, are highly charged with carbonate of lime. A spring of this character bursting forth from under the drift-gravel near some cottages on the sea-shore at the mouth of the Strandhall streamlet, has cemented the pleistocene sands of this locality in a very singular manner, forming hard, sonorous, stalactitic-looking masses. These are often tabular, and pierced with a series of long tubes varying in bore from that of a straw to two or even three inches diameter. At other times they are like long tapering icicles with a stone attached to the thicker end. It would seem that the water forcing its way through the pleistocene sands interposed between two layers of loam, and carrying with it particles of the carbonaceous clay thence derived, has a tendency to form concretionary masses on the lee side of any obstacle, (as for instance a pebble bedded in the sand,) and pipes or cavities where it has a freer course. The fragments of shells in the sand assist towards this concretionary structure, and perhaps it is owing to their great abundance that in the north of the island, though the actual quantity of lime in the boulder clay is hardly more than six per cent., we have large masses formed in every gully after excessive rains by the percolation of the waters through the alternating beds of sand and loam.
At Strandhall we have also a modern raised beach, cemented by the carbonated water, and a lovely bed of moss of some extent is being converted into travertine by the same cause. We can easily select specimens whose upper portion is all alive, green and flourishing, whilst the lower is fixed and rigid in its coating of stone, which preserves for ever the delicate outline of the growth of other days,
1 See the view, " Castletown from the Stack of Scarlet."
2 I am indebted for this identification to Count Keyserling.
3. There is also a large fucoid or perhaps zoophyte (Calamopora inflata?) which is distributed very extensively over two of the beds.
4 It is at this point certainly not less than 50 feet thick.
5 The bay of death, from the Manx Poyll, a pool, and Baase, death.
6 See section to ground plan in Plate V.
7 See Plate V.
8 See Plate 11.
9 This is the best locality for obtaining a series of the Poolvash fossils. They are so abundant as really to make up the substance of the rock. Within an area of 100 yards, almost every species noted in the Scar limestone of Yorkshire may be found.
10 If the tray-tuff and Posidonian schist extended thus far (which is highly probable), they have been entirely denuded.
11 Geologists seem to have been misled many years ago, when their science was in its infancy (a mere branch of mineralogy), by specimens of rock taken from this neighbourhood. Some early geological maps which I have seen, lay down a broad hand of the maguesian limestone formation enveloping palaeozoic formations in the Isle of Man. There is dolomite indeed, but it is metamorphic limestone of the carboniferous aera.
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