[From BA handbook, 1896]
With Geological Map.
By G. W. LAMPLUGH, F.G.S
(By permission of the Director-General of H.M. Geological Survey).
THE literature devoted to the Geology of the Isle of Man is voluminous, and it will only be possible here to mention a few of the principal works. In 1814 Dr. J. F. Berger1 gave a general account of the structure of the Island, and he was followed in 1819 by Dr. J. MacCulloch,2 and in 1821 by Rev. J. S, Henslow.3
A quarter of a century later, the Rev. J. G. Cumming made those notable contributions4 which, by reason of their thoroughness and accuracy, must ever remain the leading classics of this literature.
Since that time the chief work has been that of Messrs. Harkness and Nicholson in 1866,5 of Mr. J. Home in 1874,6 and of Rev. J. Clifton Ward in 1880 ;7 while more recently there is the admirable series of papers of Prof. W. Boyd Dawkins8 and his assistants, Mr. B. Hobson9 and Mr. H. Bolton,10 on the solid geology; and the glacial deposits have, for the first time, been adequately described by Mr. P. F. Kendall.11 A description of certain "crush conglomerates" has also recently been published by Mr. W. W. Watts and the present writer.12
THE OLDER PALAEOZOIC SERIES The physical features of the island, which have been described in another portion of this Handbook, may be read in connection with its geological structure. Its framework is made up of a confused mass of sedimentary strata usually correlated with the Skiddaw Slates of the Lake District. This mass is traversed by innumerable small dykes of igneous material, and is also, in certain places, punctured by larger intrusive bosses of the same. The strata are alternately of slaty, flaggy and gritty character; but owing to the intense deformation which they have undergone there is still considerable doubt as to their sequence. Their palaeontology is very unsatisfactory. In some of the sandy layers, worm-casts of the Pakeochorda type, and other like markings, are rather plentiful ; the only other fossils known are those brought to light by the diligent search of Mr. Bolton, who obtained an impression of a supposed trilobite from a crush-conglomerate near Ramsey, and some fragments referred to Dictyonema sociale from an old slate quarry at Cronk Sumark, near Sulby.13 Some hope is thus thrown on the possibility of farther fossil-evidence being eventually obtained; but what we at present possess is not sufficient to determine more than approximately the age of the rocks; nor does the correlation of the series with the Skiddaw Slates of the Lake District, which seems to rest on a fairly satisfactory basis, afford us much assistance. The Lower Silurian (Ordovician) fossils which the rocks of the Lake District have yielded, are believed to occur chiefly, if not wholly, in the upper portion of the formation; and Mr Marr has recently shown14 that, in its downward range, the series almost certainly reaches well into the Cambrian period. And as all the evidence at present in hand tends to show that the Manx rocks are equivalent to the lower portion only of the Skiddaw slates of the Lake District, it appears probable that these rocks are not of later age than Cambrian.
Though the succession of the series is still doubtful, their arrangement in more or less definite belts can without much difficulty be demonstrated.15 The central ridge of the island, including all the highest summits, is composed of a dark-blue slate-rock, usually so greatly affected by shearing and strain-slip cleavage that the original bedding is almost, or altogether, obliterated. I have proposed to apply the term "Barrule Slates" to the strata of this character. These are flanked on either side, and probably likewise underlain, by flaggy grits, the thicker layers of which are traversed in all directions by quartz-strings, (the "Agneash Grits "), there being apparently a rather gradual passage from the slates to the grits. On the eastern side of the Island, these "Agneash Grits" give place to a strongly banded flaggy series of alternately slaty and sandy beds, with some coarsish gritty intercalations which occupy a wide area between the eastern coast and the central axis. To this series the term "Lonan Flags" may be fitly applied. On the west coast, between Peel and Niarbyl Point, there is a rather similar flaggy series ("Niarbyl Flags"), which includes some slightly calcareous bands; and it is probable that these flags of the east and west coasts represent the same series, though no actual proof of this relationship can yet be offered. The general strike of these rock-mass is from S. W. to N.E., and the dominant dips are towards S.E. on the south-eastern side of the central range, and towards N.W, on the Opposite side, thus forming an anticlinal axis having the direction of the longer axis of the Island. This anticline is, however, primarily connected with the cleavage structure, and cannot safely be taken as indicating the true arrangement of the bedding of the highly folded rock-masses in which it is developed, In fact, the general field-evidence suggests that the flaggy and gritty series which if the dips could be trusted would overlie the "Barrule Slates," more probably pass beneath the slates in the form of a plicated platform, and that there is a synclinal depression in the centre of the Island, from which the folds of the rocks spread outward in fan-like arrangement.
The complicated character of the folding is, often beautifully exposed in the cliff sections. Indeed, in such Sections it is rare that excessive disturbance of the bedding is not at once evident, and where, as in the cliff east of Spanish Head, the strata are apparently more regular, and the dips less steep, we may be almost confident that this apparent simplicity is misleading, and that the true structure is an overturned fold, such as is revealed on a small scale in a little bay on the south side of Maughold Head. In the absence of well-defined traceable horizons, the attempt to unravel with any certainty the complex arrangement of these rocks is almost hopeless, In some cases it is evident that the strata have been folded and refolded; and almost everywhere there has been a sliding, or "fluxion movement," of the layers one over the other, and a squeezing of the softer beds around the harder. Where large masses of grit and slate are in juxtaposition, the effect of this differential movement is highly intensified, and some very interesting studies in dynamic geology are thereby presented. One most striking feature is the occurrence, on an extensive scale, of "crush-conglomerate," due to the breaking up of the beds into fragments, and their rearrangement in the form of a friction-breccia, in which lenticular or more or less rounded bits of the harder bands are set, like pebbles, in a matrix composed of the crushed softer layers. In its extreme condition, this torn-up rock . cannot readily be distinguished from a water-borne conglomerate; but its true character is made evident by the numerous sections which reveal every stage in the progressive destruction of the original bedding. These crush-conglomerates are mainly found on the north-western flank of the Barrule Slates, being best developed in Sulby Glen, and between that valley and Ramsey. They may, in this area, be traced continuously for many miles, apparently on the same plane, and in places are some hundreds of feet in thickness.16
The rocks have, moreover, been greatly affected by later earth-movements after the production of this brecciation. The crush-conglomerate possesses a system of strain-slip cleavage distinct from the earlier planes of disruption, and it is traversed by igneous dykes which have suffered considerable deformation after their injection. At Langness, where the Carboniferous Basement Beds rest on the upturned edges of the Skiddaw rocks, it is clearly seen that all these stages of movement have occurred previous to the deposition of the Carboniferous strata. The major part of the intrusions of igneous material also appear to have been Pre-Carboniferous.
PRE-CARBONIFEROUS IGNEOUS ROCKs.The abundance and variety of these intrusions permeating the Skiddaw framework of the Island are remarkable. Notwithstanding some published statements to the contrary (based, no doubt, on the sill-like character of many of the dykes and the abnormal aspect of the crush-conglomerates), there do not appear to be any contemporaneous volcanic rocks among the Manx Skiddaw Series. The oldest intrusions are the diabasic rocks or "greenstones," which occur very abundantly in the form of small dykes, swelling locally into knots or bosses, usually lying in disconnected strips or lenticles along cleavage or bedding planes, and often so much altered by shearing and by mineral decomposition as to be scarcely distinguishable from the surrounding slates. They are especially numerous on the south-eastern side of the central axis, where they tend to group themselves into sets along irregular zones.
The more extensive granitic outbursts are of somewhat later date than these "greenstone " dykes Of these granites, that near the Dhoon Glen, in Maughold, is about half a square mile in extent; and the Foxdale mass, near the centre of the Island, is nearly the same size; while a much smaller granite boss is exposed at Oatlands, near Santon, and there are indications of the presence of another small patch, of approximately granitic nature, beneath a covering of glacial drift, on the north-eastern border of the parish of Onchan. Connected with the granites both of the Dhoon and of Foxdale are well-defined sets of elvan or micro-granite dykes, which present in both cases the mineral peculiarities of the parent-mass. These traverse the Island from coast to coast, participating in the general S. W. to N.E. strike, and they are seen occasionally to cut the older "greenstones," e.g., in an interesting cliff section at Traie-my-Ijainaigue, in the parish of Maughold. These granitic dykes, like their "greenstone" precursors, have acquired in some localities a schistose character during the later stages of the PreCarboniferous earth-movements.
Besides the older basic and the acid dykes, there are also a few of intermediate type, including diorites, lamprophyres and mica-traps, with probably also a much altered audesitin rock, certain of which may perhaps be of Carboniferous age, but lack of space forbids any detailed account of these. The Post. Carboniferous basaltic intrusions will be referred to on a later page.
Metamorphism of the Skiddaw Slates.The metamorphic phenomena of the Manx Skiddaw Series present many points of extreme interest, but have not yet been sufficiently studied. Pending further research, it may be briefly stated that there is around the granite masses a narrow aureole in which the usual effects of thermo-metamorphism are visible, but not very markedly so, while in certain localities the rocks have undergone a far more striking alteration at considerable distances from any known igneous outcrop, and sometimes where the field-evidence goes to disprove the supposition that an igneous mass might be concealed not far below the present surface. This type of alteration usually attains its extreme development in the vicinity of the great anticline of the cleavage. The slates in this position have occasionally been converted into garnetiferous mica-schists. Within the areas where metamorphism of this kind is prevalent, the development of new minerals seems to have been accentuated, if not indeed initiated, by the junction of the softer slaty rock with any hard mass,17 such as a gritty band, quartz-vein, or igneous dyke of previous consolidation. This is well seen in places in the new tram-line cuttings on Snaefell and in the Cornah Valley, while a more extreme type of alteration is revealed in Glen Darragh in the parish of German. The greenstone dykes in these areas frequently take the form of actinolite-schists. This kind of alteration must, I think, be in some degree due to the action of dynamic forces upon the rock-mass. Nevertheless the crush-conglomerates, wherein such forces must have been exerted with great violence, present only a slight degree of mineralogical alteration, and it would therefore appear as if the metamorphism were not produced by such forces alone and unaided. Possibly the dynamic action may have wrought the greater change only in the deeper-seated pal-ta of the massif after the temperature had been raised almost to the critical point by the repeated permeation of igneous material.
Mineral Veins.The Skiddaw rocks of the Island are traversed by valuable metalliferous lodes, yielding chiefly argentiferous galena, and zinc (blende), with a little copper.. pyrites. The great lead-mines of Foxdale and of Laxey have been among the most successful in the United Kingdom. The evidence at present in hand as to the age and origin of these metalliferous deposits is only sufficient to show that they are later than the granitic eruptions, and are probably Post-Carboniferous
THE CARBONIFEROUS SYSTEM.Between the period of the deposition of the Skiddaw Series and the early stages of the Carboniferous epoch, there is, so far as the sedimentary rocks are concerned, an absolute gap in the Isle of Man. If any later Silurian or Devonian rocks were deposited in this area, they had all been removed by erosion before the beginning of Carboniferous times (unless perchance any older fragment still remain hidden beneath the drift-plain). Elevation and denudation seem, in fact, by that time already to have brought the Island approximately to its present proportions and to the condition of an insulated area. The basal beds of the Carboniferous consist, in the south of the Island, of a coarse conglomerate of well-rounded quartzite pebbles (derived from the grits of the Skiddaw group),18 which rests in strong unconformability on the eroded edges of the older rocks, as is beautifully seen in the picturesque section of the Arches of Langness. This conglomerate was evidently formed along the beach-line of the period. It no doubt spread transgressively upward over the slopes of the old Island during the slow depression of the area beneath the sea in which the overlying Carboniferous Limestones were deposited. As now exposed, it is seen to fringe the little Carboniferous basin of 7 or 8 square miles in extent, stretching along the coast from Langness, across Derbyhaven to low-water mark off Ronaldsway, re-appearing at Cass-nyhawin, and running thence inland past Ballasalla, to Athol Bridge on the Silverburn, where it is cut off by a W.S.W. to E.N.E. fault which bounds the Carboniferous rocks. This fault strikes across to the sea at Kentraugh on the shores of Bay-ny-Carricky, but catches the Port St. Mary headland on the S.W. side of the bay, bringing in a small isolated patch of Carboniferous Limestone there, with the Basal Conglometate just exposed in Port St. Mary Harbour.19
The Limestone Series has been divided by Cumming into the~ Lower or Castletown Limestone, the Upper or Poolvash Limestone, and the Posidonia (Posidonomya) Shale, or Black Marble. The first of these he correlates with the Lower Scar Limestones of Prof. Phillips, and the second with the Upper Scar of that author. Characteristic Carboniferous Limestone fossils abound in both divisions, the Poolvash Limestone being especially rich in well-preserved shells, chiefly of the genera Goniatites, Productus, Spirijera, Orthis, Terebratula, &c.; while the Castletown beds yield, among other fossils, some fine Goniatites, Producti, and many large corals. Very little detailed work has yet been done in the palaeontology of these rocks, and in this branch our knowledge has made scarcely any advance since Cummings work was published.
The "Knoll-reefs" of Poylvaaish.The mode of occurrence of the Upper Limestone on the broad foreshore of Poylvaaish, deserves especial attention, as illustrating an obscure subject which has of late been frequently discussed. Massive knolls or hummocks, from 5 to 20 feet high, of pale somewhat crystalline limestone, crowded in places with well-preserved and uncrushed shells, stand out boldly amid flaggy and shaly beds containing a rather different fauna. The flaggy beds appear to lap round and over the hummocks, filling interstices in their cliff-like sides, and possibly in some instances also underlying them. Around the knolls, in places, fragments of the pale limestone occur abundantly in the darker flaggy beds, apparently the result of contemporaneous erosion. The general aspect of the knolls is, in fact, very like that of the bosses described by Mr. R. H. Tiddeman in the Carboniferous Limestone of the Skipton district of Yorkshire, by him considered to be old coral-reefs.20 Other geologists, however, have thought that these structures may be the result of the squeezing and folding which the rocks have undergone. At Poylvaaish the evidence is strongly in favour of Mr. Tiddemans view that the Knolls are original structures, like the recently described "Teppe Buttes" of Colorado, U.S. A.21 They seem to mark areas where there has been a rapid submarine consolidation of shelly banks, such as is known to take place locally beneath our modern seas, whereby certain portions were able to resist the currents of increased strength which afterwards invaded the area and destroyed the less coherent portions of the banks.
The above-mentioned overlapping flaggy beds form the lower part of the "Posidonia Shale," and dip seaward beneath the extremely interesting Volcanic Series next to be described. The uppermost portion of the flaggy limestone is to some extent interbedded with the Volcanic Ash; but the junction is somewhat obscure.
The Carboniferous Volcanic Series. These rocks occupy a narrow strip extending along the coast S. W. of Castletown, from the Stack of Scarlet to Poylvaaish Bay. They are magnificently exposed in a broad craggy fore~shore, while their seaward extension is indicated in the lower-water reef of Lheeah-rio, in Castletown Bay, half a mile from the shore. The abrupt transition at Scarlet Point from the gently undulating Castletown Limestones to the chaotic mass of volcanic agglomerate traversed by rugged dykes of augiteporphyrite,22 is an impressive scene for the geologist. It is probable that this sharp junction has been brought about by faulting, though the possibility of its being the edge of a volcanic vent must be borne in mind. An isolated mass of Carboniferous Limestone, containing bands of quartz-pebbles, is surrounded by the volcanic rocks; but whether brought up by the eruptive forces of the volcano or by normal faulting is not evident. Beyond it, a broad dyke of augiteprophyrite breaks through the tuffs and agglomerates, and is prolonged into the conspicuous outstanding Stack of Scarlet.
From this point we pass, at first westward and then northwestward, for two miles across ledges of fine tuff interspersed with coarser agglomerate, and traversed in places by dykelike ridges of vesicular lava. Westward these tuffs become of finer texture and well-bedded, and contain in a few places encrinite stems and other marine remains. Lenticular bands of limestone, some of which are fossiliferous, also make their appearance, and show that the eruption has been, in part at any rate, submarine. Sections of this character are continued as far as Poylvaaish, where, as already stated, the underlying Limestones again occupy the coast.
For the student of volcanic phenomena no finer display could be desired than is afforded by this strip of the Manx coastline, "for here he sees a small ancient volcano dissected and laid bare".23
Peel Sandstone and Conglornerate.The description of these rocks has been deferred to this place because of their having recently been claimed to be of Permian age,24 although by Cumming and the older writers they were regarded as underlying the base of the Carboniferous Limestone like the conglomerate of Langness, and by Home were compared to the Calciferous Sandstone Series of Scotland. They occur as a patch faulted into the Skiddaw rocks on the west side of the Island, in the vicinity of Peel, being well exposed for nearly two miles in the cliff sections, and obscurely seen inland, in one locality only, near Glenfaba Bridge one mile to the southward of Peel. They consist of a considerable thickness of red and sometimes mottled sandstones, with bands of calcareous conglomerate and lenticles of impure concretionary limestone. Fossils occur sparingly in the calcareous pebbles and concretions of the cliff-section at Lhoob-y-Reeast, being chiefly fragments of corals of Lower Carboniferous or Upper Devonian types, in poor state of preservation, referable to the genera Heliolites, Favosites, Cyatliophyllum (~), etc., with encrinite stems and traces of shells. I am inclined to think that some of these fossils are indigenous to the deposit, and not derivative. On the whole, the evidence seems to me to support the view of the earlier writers, that these rocks are not of hater date than the beginning of the Carboniferous Period. The strongest confirmation of this opinion has recently been furnished by the boring operations in search of coal in the north of the Island. These borings have revealed the presence of a series of sandstones and shales intercalated with the Carboniferous Limestone, and possessing, like the Peel Sandstone, a high dip (from 40 to 500), which are overlain by undoubted Permian and Triassic strata lithologically unlike the Peel rocks, with low dips (rarely exceeding 5°).
THE CONCEALED STRATA OF THE NORTHERN PLAIN. Until the above-mentioned borings were instituted, we possessed no definite information of the presence of rocks newer than the Carboniferous in the Isle of Man. It has now been ascertained, however, that beneath an unusual thickness of glacial deposits, at depths varying from 160 to 290 feet below sea-level, there exists a varied series of Permian and Triassic strata lying in strongly marked unconformability on the Carboniferous rocks.25 These Red Rocks appear to occupy a basin deepening towards the north-east. The uppermost division of this series consists of Triassic Sahiferous Mans, which have been penetrated to a depth of 382 feet at the Point of Ayre. These probably rest directly on the St. Bees Sandstone, of which a thickness of about 400 feet has been pierced at Rue Point (No, 4 Bore), 4 miles W.S.W. of the Point of Ayre. This St. Bees Sand-stone graduates downward into a thin Lower Marl Series, which, in turn, passes into Permian Sandstone of coarser grain than the St. Bees, containing in its lower layers small fragments of Carboniferous Limestone, and evidently representing the Brockram of the Whitehaven sections. The thickness of this "Brockram-Sandstone" in the Rue Point boring is under 30 feet; it rests on the eroded edges of the steeply-dipping Carboniferous rocks as above described.
THE BASALTIC (TERTIARY ?) DYKES While the concealed Triassic rocks are the newest solid sedimentary strata of the Island, there is a set of igneous dykes which in their petrographical characters and their general direction agree closely with the well-known Tertiary Basaltic Dykes of Scotland and the North of Ireland. These dykes of olivinedolerite usually strike for some point between west and north. They are especially abundant in the south of the Island, traversing all the Carboniferous rocks (including the Volcanic Series) as well as the Skiddaw Slates. Owing to their rapid weathering, they are rarely seen in the interior, but are found at intervals all along the western coastline as far north as Glen Meay, and along the whole extent of the eastern coast from Maughold Head to Port St. Mary. They frequently accompany the smaller mineral lodes of the Island.
THE GLACIAL DEPOSITS.From its isolated position in the basin of the Irish Sea, the Isle of Man forms an excellent glaciometer by which to gauge the flow of the Ice sheet in this latitude. Its drift-sections are of unrivalled extent and interest, and have been described by many observers, the most detailed and satisfactory account yet published being that of Mr. P. F. Kendall.26 In some parts of the northern plain the total thickness of the glacial deposits may be as much as 450 feet,27 consisting of alternations of sand and gravel with boulder-clay and sandy boulder-loam, the upper portion of which is magnificently exposed in cliff-sections on both sides of the Island. The drift which overspreads this low ground is largely made up of material foreign to the Island. In many places it contains worn and broken shells 28 and other relics of marine origin. In its present condition, however, it does not seem anywhere to be a truly marine deposit, but to represent the rearrangement of the material of a glacial sea-bottom during the encroachment of an ice-sheet which crept down over it from the north and north-west. The foreign boulders are chiefly from the neighbouring parts of Scotland and the Lake District, and many of these have evidently undergone marine transportation in the first instance, before their incorporation along with the other material of the sea-bottom in the morainic gatherings of the confluent glaciers.
That during the period of maximum glaciation the basin of the Irish Sea in this latitude was entirely filled up by an ice-sheet, which attained a thickness of from 1,000 to 2,000 feet above sea-level, and that this great mass had a general motion of flow from the north or north-northwest, sweeping southward round and along the flanks of the Island, is the only consistent explanation of the field-evidence over the whole Island. The general distribution of the drift deposits and of the boulders; the direction of the glacial strite to be found in almost every part, even near the summits of the highest hills; the character and arrangement of the late-glacial and post-glacial deposits - are among the indications which combine to force this conclusion upon the student of the phenomena. In the earlier stages of its growth this Irish Sea ice-sheet was no doubt the outcome of the glaciers which flowed down into the shallow sea from the surrounding mountainous ground; but afterwards its great increase was probably obtained directly through the precipitation of snow upon its surface, which formed a vast gathering-ground for the excessive snowfall cast down by the moisture-laden winds from the Atlantic. It is to be noted that, before the advent of the extraneous ice, the Island possessed local glaciers in most of its valleys, and that these, keeping pace with the invading sheet, grew into an ice-cap which accumulated deposits of local material in all the sheltered places among the hills, so that the foreign drift is confined to the flanks of the Island, except along one strip, where a lobe of ice (or possibly an englacial river) seems to have swept into the gap at St. Johns, and thence, not as might have been expected eastward along the central valley, but south-south-eastward along and across the high watershed at Archallagan. The transportation of the boulders of Foxdale Granite up the slopes of South Barrule to a height of over 800 feet above the parent mass in a distance of less than two miles, is a classic feature of Manx glacial geology which must here be cited, though want of space forbids further details.29
During the decadence of the ice-sheet many noteworthy features were wrought in the superficial geology of the Island. The evidence points to the early emergence of the high ground in the form of a nunatak, surrounded by temporary lakelets. To the drainage of these lakelets and of the bordering ice-fields, we may assign the production of the steep-walled notches and gullies to be found in a few places crossing narrow rocky ridges, or coursing along the contours of the mountain-slopes, in positions where under other circumstances no streams could flow. As more of the rock-surface emerged, and the old drainage channels re-asserted themselves, their flood-waters could attain a temperature well above freezing-point, and higher than any englacial river could possess, and these, impinging upon the retreating ice-front, seem to have hastened the melting back of the great glacier where they touched it. - Under such conditions, the combined effect of the north-flowing waters of the Dhoo, the Sulby and the Auldyn, appear to have brought about a rapid thawing back of the ice-front opposite their mouths; and the broad hollow known as the Curragh, now partly filled in with alluvial deposits, was produced in this quarter by the formation of the great semi-circle of hills and mounds of partly-stratified drift materialthe hills of Bride -and Orrisdalewhere the ice-front, being beyond reach of the land-waters, paused for a time in its recession.30 The broad terraced aprons of high-level drift-gravel which fringe these hills probably mark the places where the en-glacial drainage swept the morainic material forward into the lake-filled hollow; and similar bars and terraces of slaty gravel, derived from mountains, denote the effects of the land-. drainage on the opposite side of the hollow. From a peaty bed in one of these high-level platforms at Kirk Michael, we have recently obtained, among other plants, the Arctic species Salix herbacea and Carex cf. alpina, along with a little Arctic fresh-water crustacean .dpus (Lepidurus~ glacialis, which lives now only in the icy tarns of Spitz-bergen and Greenland. An interesting glimpse of the conditions immediately following the retreat of the ice is thus afforded.
POST-GLACIAL AND RECENT.Many changes have taken place in the area since the final disappearance of the ice-sheet, which can barely be referred to here. The streams have lost much of their former volume, so that in most cases they are unable to exercise any erosive power in the uppermost part of their courses. The temporary bodies of fresh water which persisted into historical times in the northern Curragh are now permanently dry, and the drainage of this area and of the Sulby River, has been diverted from the broad trench-like hollow of the Lhen Mooar, which was its ancient channel. The forests which sheltered the Irish Elk, and the bogs which engulfed these animals, have alike disappeared. An elevation of the Island to 10 or 15 feet above its former level, which seems, from the presence of chipped flints on the raised beach, to have taken place before or during. Neolithic times, has left a dry rock-shelf on outstanding parts of the sea-coast, and a raised beach of shingle in the bays on which the older portions of the coast-towns usually stand. In the extreme north this raised beach spreads out into the Ayre, a barren waste of shingle and blown sand, bordered by ancient cliffs of drift, which near the Point of Ayre has a breadth of 4 miles. There is an erroneous belief that the land is still gaining on the sea in this quarter, but it is clear that since the elevation of the beach the&e has been considerable loss, though in the higher drift-ground lying to the south of the Ayre the loss has been still greater. A general review of the Post-glacial phenomena discountenances the opinion which has been expressed that the time-interval since the Glacial Period may have been comparatively short. For the cycle of change which has been run since the close of that period, it seems to me that, by any standard of computation, a long stretch of time must be allowed.
1 Berger, Dr. J. F. Mineralogical Account of the Isle of Man. Trans. Geol. Soc. vol. II., p. 29. (18141.
2 MacCulloch, Dr. J. A Description of the Western Islands of Scotland, including the Isle of Man. 2 vols 8vo. Atlas 4to. Lond., 1819.
3 Henslow. Rev. Professor J. S. Supplementary Observations to Dr. Bergers Account of the Isle of Man. Trans. Geol. Soc. Vol. v., p. 482. (1821).
4 Cumming, Rev. J. G. On the Geology of the Isle of Man. Quart. Journ. Geol. Soc. Vol. II., pp. 317 and 335. (1846). On the Geology of the Calf of Man. Quart. Journ. Geol. Soc. Vol III., p. 179. (1847). Also, "The Isle of Man; Its History," &c. Svo. London, 1848. And "Guide to the Isle of Man." 8vo. London. 1861.
5 Harkness, Prof. R., and Nicholson, H, A. On the Lower Silurian Rocks of the isle of Man. Quart. Jour. Geol. Soc. Vol. XXII., p. 488. (1866).
6 Home, J. A Sketch of the Geology of the Isle of Man. Trans. Edin. Geol. Soc. Vol. II., pt. 3., p. 323. (1874;.
7 Ward, J. Clifton. Notes on the Geology of the Isle of Man, Geol. Mag. Dec. II., Vol. VII., p. 1. (1880).
8 Dawkins, Prof. W. Boyd. "On the Clay Slates and Phylljtea of the South of the Isle of Man," &c. Trans. Manchester Geol, Soc. Vol. XX., p. 53 (1888) "On the Conglomerates of the South of the Island." Vannin Lioar. Vol. I., pt. 1, p. 16. (1839). "Do the Porsulan, Carboniferous, and Triassic Rocks, and the New Saitfield of the North" (of the Isle of Man). Trans. Manch. Geol. Soc. Vol. XXII. p. 590, (1894).
9 Hobson, B. "On the Igneous Rocks of the South of the Isle of Man." Quart. Journ. Geol. Soc. Vol. XLVII., p. 432. (1891). Reprinted, with additions and corrections, in Yn Lioar Mannhsiagh. Vol. I., part 10, p. 337. (1892).
10 Bolton, H. "On a Trilobite from the Skiddaw Slate of the Isle of Man." Geol. Mag. Dec. 111. Vol. X., p. 29. (1893;. "Observations on the Skiddaw Slates of the North of the Isle of Man. Rep. Brit. Assoc. Nottingham, 1893, p. 770.
11 Kendall, P. F. "On the Glacial Geology of the Isle of Man." Yn Lioar Manninagh. Vol. I., Pt. 12, p. 397. (1894).
12 Lamplugh, G. W., and Watts, W. W "The Crush-Conglomerates of the Isle of Man" Quart. Joumn, Geol. Soc. Vol. LI., p. 563. (1895).
13 Bolton, H. Op. cit. See also discussion to paper on crush. Conglomerate in Quart. Journ. Geol. Soc. vol. LI., p. 597.
14 Marr, J. E. "Notes on the Skiddaw Slates." Geol. Mag. Dec. IV. Vol. 1, p. 122. (Mch. 1894).
15 See Annual Report of the Geological Survey for 1895. p. 5 (App. to 43rd Rep. of Departmt. of Science and Art).
16 For further information on this subject, see "The Crush.Conglomeratcs of the Isle of Man." Op. cit.
17 See Prof. W. B. Dawkins. Trans. Manch. Geol. Soc. Vol. XX., p. 54.
18 Prof. W. Boyd Dawkins. Vannin Lioar, Vol. I., part1, p. 16.
19 The existence of the Conglomerate at this place was noticed by MacCulloch (op. cit. p. 66), but has been overlooked by Cumming and the later writers. Its presence shows that the throw of the bounding fault, in spite of its stratigraphical importance, is probably not great.
20 R. H. Tiddeman. Proc~ Yorks. Geol. and Polytech. Soc. Vol. XI., p. 482 (1890) British Association Excursion Handbook. Leeds, 1890.
21 "Tepee Buttes" by G. K. Gilbert and F. P. Gulhiver. Bull. Geol. Soc. of America, vol. Vi., pp. 333342. (Mch. 1896.)
22 The stratigraphical and petrographical details of the Volcanic Series have been fully described by Mr. B. Hobson in the paper already referred to.
23 Rev. J. Clifton Ward. Op. cit. p. 5.
24 Prof. W. Boyd Dawkins. (Op. cit.) Trans. Manch. Geol. Soc. Vol. XXII., p. S90.
25 For full details see Prof. W. Boyd Dawkins.
Trans. Manch. Geol. Soc. Vol. XXII., p. 590, and vol. XXIII, p.
Some modifications of the interpretation therein given of the results obtained from the Bahlawhane (No. 2.) Bore will, I think, be necessary, especially as regards the supposed Permian rocks beneath the St. Bees Sandstone in this boring. The discussion of this question, however, while the work is still in progress would be premature. See also J. Todd. Yn Lioar Manninagh. Vol. III. pt. II., p. 65, 1896.
26 P. F. Kendall. Yn Lioar Manninagh. vol. I., pt. 12, p. 397.
27 Prof. W. B. Dawkins, Trans. Manch. Geol. Soc. Vol. XXII.
28 For lists, Ste., see P. F. Kendall. Op. cit.,
29 Noted by Cumming and all the later writers.
30 Mr. P. F. Kendall was the first to recognize t,he moralnic character of these hills, and the "kanie-delta" structure of the gravel platforms. Op. cit. pp.. 428 and 429.
THE BRITISH ASSOCIATION EXCURSION ROUTES
THE above general sketch of the Geology of the Island ha~ been written without especial reference to the excursion programme, that it might prove the more useful in case any member should desire to extend his researches beyond the limits of the excursions. The routes, however, will be found to cover the points of chief interest, and will, besides~ show the.broader outlines of the whole area.
Friday, 25th September, will be devoted to the investigation of the south of the Island, including the Carboniferous Limestone rocks of the Castletown district, the contemporaneous Volcanic Series of Scarlet Stack, the Oarboniferous Conglomerates of Langness, the striking unconformability at their base, and the underlying Skiddaw Series of that district.
The following day, 26th September, wi]l be spent on the northern portion of the Skiddaw massi! and will include the ascent (by electric tram) of Snaefell, the investigation of the curiously partial metamorphism of the slates on that mountain, and of the sections in Sulby Glen which reveal the breaking-up of the bedding and the production of Crush-conglomerates. If the day be clear, a good view will also be gained of the drift-plain of the north.
On Monday, 28th September, it is intended to visit the centre and west of the Island, when the sandstones of disputed age at Peel will be examined. Brief stops will be made en route at Crosby and Rockmount to see the igneous rocks of different type, intrusive into the Skiddaw Series at these places. On the return journey, the extensive lead-mines at Foxdale will be visited, and the granite boss of that locality investigated.
The leaders of the excursion will be glad to give any advice or information in their power to members who may wish to prolong their stay in the Island.