Volcanoes: Past and Present Part 10

SUCCESSION OF VOLCANIC ERUPTIONS.

(_c._) _First Stage._--The earliest eruptions of lava in the North-east of Ireland belonged to the highly acid varieties, consisting of quartz-trachyte with tridymite.[1] This rock rises to the surface at Tardree and Brown Dod hills and Templepatrick. It consists of a light-greyish felsitic paste enclosing grains of smoke-quartz, crystals of sanidine, plagioclase and biot.i.te, with a little magnet.i.te and apat.i.te. It is a rock of peculiar interest from the fact that it is almost unique in the British Islands, and has its petrological counterpart rather amongst the volcanic hills of the Siebengebirge than elsewhere. It is generally consolidated with the columnar structure.

[Ill.u.s.tration: Fig. 29.--Part of the section shown in the quarry at Templepatrick, showing the superposition of the basalt (_d_) to the trachyte (_b_), with the intervening bed of flint gravel (_c_). All these rocks are seen to rest upon an eroded surface of the Chalk formation (_a_).]

The trachyte appears to have been extruded from one or more vents in a viscous condition, the princ.i.p.al vent being probably situated under Tardree mountain, where the rock occurs in greatest ma.s.s, and it probably arose as a dome-shaped ma.s.s, with a somewhat extended margin, above the floor of Chalk which formed the surface of the ground.[2]

(Fig. 27.) At Templepatrick the columnar trachyte may be observed resting on the Chalk, or upon a layer of flint gravel interposed between the two rocks, and which has been thrust out of position by a later intrusion of basalt coming in from the side.[3] It is to be observed, however, that the trachytic lavas nowhere appear cropping out along with the sheets of basalt around the escarpments overlooking the sea, or inland; showing that they did not spread very far from their vents of eruption; a fact ill.u.s.trating the lower viscosity, or fluidity, of the acid lavas as compared with those of the basic type.

(_d._) _Second Stage._--After an interval, probably of long duration, a second eruption of volcanic matter took place over the entire area; but now the acid lavas of the first stage are replaced by basic lavas. Now, for the first time, vast ma.s.ses of basalt and dolerite are extruded both from vents of eruption and fissures; and, owing to their extreme viscosity, spread themselves far and wide until they reach the margin of some uprising ground of old Palaeozoic or Metamorphic rocks by which the volcanic plain is almost surrounded. The great lava sheets thus produced are generally more or less amorphous, vesicular and amygdaloidal, often exhibiting the globular concentric structure, and weathering rapidly to a kind of ferruginous sand or clay under the influence of the atmosphere. Successive extrusions of these lavas produce successive beds, which are piled one over the other in some places to a depth of 600 feet; and at the close of the stage, when the volcanic forces had for the time exhausted themselves, the whole of the North-east of Ireland must have presented an aspect not unlike that of one of those great tracts of similar lava in the region of Idaho and the Snake River in Western America, described in a previous chapter.

(_e._) _Third Stage (Inter-volcanic)._--The third stage may be described as inter-volcanic. Owing to the formation of a basin, probably not deep, and with gently sloping sides, a large lake was formed over the centre of the area above described. Its floor was basalt, and the streams from the surrounding uplands carried down leaves and stems of trees, strewing them over its bed. Occasionally eruptions of ash took place from small vents, forming the ash-beds with plants found at Ballypallidy, Glenarm, and along the coast as at Carrick-a-raide. The streams also brought down sand and gravel from the uprising domes of trachyte, and deposited them over the lake-bed along with the erupted ashes.[4] The epoch we are now referring to was one of economic importance; as, towards its close, there was an extensive deposition of pisolitic iron-ore over the floor of the lake, sometimes to the depth of two or three feet. This ore has been extensively worked in recent years.

[Ill.u.s.tration: Fig. 30.--Cliff section above the Giant's Causeway, coast of Co. Antrim, showing successive tiers of basaltic lava, with intervening bands of bole.]

(_f._) _Fourth Stage (Volcanic)._--The last stage described was brought to a termination by a second outburst of basic lavas on a scale probably even grander than the preceding. These lavas consisting of basalt and dolerite, with their varieties, and extruded from vents and fissures, spread themselves in all directions over the pre-existing lake deposits or the older sheets of augitic lava, and probably entirely buried the trachytic hills. These later sheets solidified into more solid ma.s.ses than those of the second stage. They form successive terraces with columnar structure, each terrace differing from that above and below it in the size and length of the columns, and separated by thin bands of "bole" (decomposed lava), often reddish in colour, clearly defining the limits of the successive lava-flows. Nowhere throughout the entire volcanic area are these successive terraces so finely laid open to view as along the north coast of Antrim, where the lofty mural cliffs, worn back into successive bays with intervening headlands by the irresistible force of the Atlantic waves, present to the spectator a vertical section from 300 to 400 feet in height, in which the successive tiers of columnar basalt, separated by thin bands of bole, are seen to rise one above the other from the water's edge to the summit of the cliff, as shown in Fig. 30. Here, also, at the western extremity of the line of cliffs we find that remarkable group of vertical basaltic columns, stretching from the base of the cliff into the Atlantic, and known far and wide by the name of "The Giant's Causeway," the upper ends of the columns forming a tolerably level surface, gently sloping seawards, and having very much the aspect of an artificial tesselated pavement on a huge scale. A portion of the Causeway, with the cliff in the background, is shown in the figure (Fig. 31). The columns are remarkable for their symmetry, being generally hexagonal, though occasionally they are pentagons, and each column is horizontally traversed by joints of the ball-and-socket form, thus dividing them into distinct courses of natural masonry. These are very well shown in the accompanying view of the remarkable basaltic pillars known as "The Chimneys," which stand up from the margin of the headland adjoining the Causeway, monuments of past denudation, as they originally formed individuals amongst the group belonging to one of the terraces in the adjoining coast.[5] (Fig. 32).

[Ill.u.s.tration: Fig. 31.--The Giant's Causeway, formed of basaltic columns in a vertical position, and of pentagonal or hexagonal section; above the Causeway is seen a portion of the cliff composed of tiers of lava with intervening bands of bole, etc.--(From a photograph.)]

[Ill.u.s.tration: Fig. 32.--"The Chimneys," columns of basalt on slope of cliff overlooking the Atlantic, north coast of Co. Antrim. The horizontal segments, or cup-and-ball joints, of the columns are well shown in this figure. (From a photograph.)]

(_g._) _Original Thickness of the Antrim Lavas._--It is impossible to determine with certainty what may have been the original thickness of the acc.u.mulated sheets of basic lavas with their a.s.sociated beds of ash and bole. The greatest known thickness of the lower zone of lavas is, as I have already stated, about 600 feet. The intermediate beds of ash and bole sometimes attain a thickness of 40 feet, and the upper group of basalt about 400 feet; these together would const.i.tute a series of over 1,000 feet in thickness. But this amount, great as it is, is undoubtedly below the original maximum, as the uppermost sheets have been removed by denuding agencies, we know not to what extent. Nor is it of any great importance. Sufficient remains to enable us to form a just conception of the magnitude both as regards thickness and extent of the erupted matter of the Miocene period over the North-east of Ireland and adjoining submerged tracts, and of the magnitude of the volcanic operations necessary for the production of such ma.s.ses.

(_h._) _Volcanic Necks._--As already remarked, no craters of eruption survive throughout the volcanic region of the North-east of Ireland, owing to the enormous extent of the denudation which this region has undergone since the Miocene Epoch; but the old "necks" of such craters--in other words, the pipes filled with either solid basalt, or basalt and ashes--are still to be found at intervals over the whole area. Owing to the greater solidity of the lava which filled up these "necks" over the plateau-basaltic sheets which surround them, they appear as bosses or hills rising above the general level of the ground.

One of these bosses of highly columnar basalt occurs between Portrush and Bushmills, not far from Dunluce Castle, another at Scawt Hill, near Glenarm, and a third at Carmoney Hill above Belfast Lough. But by far the most prominent of these old solidified vents of eruption is that of Sleamish, a conspicuous mountain which rises above the general level of the plateau near Ballymena, and attains an elevation of 1,437 feet above the sea. Seen from the west, the mountain has the appearance of a round-topped cone; but on examination it is found to be in reality a huge d.y.k.e, breaking off abruptly towards the north-west, in which direction it reaches its greatest height, then sloping downwards towards the east. This form suggests that Sleamish is in reality one of the fissure-vents of eruption rather than the neck of an old volcano. The rock of which it is formed consists of exceedingly ma.s.sive, coa.r.s.ely-crystalline dolerite, rich in olivine, and divided into large quadrangular blocks by parallel joint planes. Its junction with the plateau-basalt from which it rises can nowhere be seen; but at the nearest point where the two rocks are traceable the plateau-basalt appears to be somewhat indurated; breaking with a splintery fracture and a sharp ring under the hammer, suggesting that the lava of Sleamish had been extruded through the horizontal sheets, and had considerably indurated the portions in contact with, or in proximity to, it.[6]

Amongst the vents filled with ash and agglomerate, the most remarkable is that of Carrick-a-raide, near Ballycastle. It forms this rocky island and a portion of the adjoining coast, where the beds of ash are finely displayed; consisting of fragments and bombs of basalt, with pieces of chalk, flint, and peperino, which is irregularly bedded. These ash-beds attain a thickness of about 120 feet just below the road to Ballycastle, but rapidly tail out in both directions from the locality of the vent.

Just below the ash-beds, the white chalk with flints may be seen extending down into the sea-bed. Nowhere in Antrim is there such a display of volcanic ash and agglomerate as at this spot.[7]

(_i._) _d.y.k.es: Conditions under which they were Erupted._--No one can visit the geological sections in Co. Antrim and the adjoining districts of Down, Armagh, Derry, and Tyrone, without being struck by the great number and variety of the igneous d.y.k.es by which the rocks are traversed. The great majority of these d.y.k.es are basaltic, and they are found traversing all the formations, including the Cretaceous and Tertiary basaltic sheets. The Carlingford and Mourne Mountains are seamed with such d.y.k.es, and they are splendidly laid open to view along the coast south of Newcastle in Co. Down, as also along the Antrim coast from Belfast to Larne. The fine old castle of Carrickfergus has its foundations on one of those d.y.k.e-like intrusions, but one of greater size than ordinary. All the d.y.k.es here referred to are not, however, of the same age, as is conclusively proved by sections amongst the Mourne Mountains where cliffs of Lower Silurian strata, superimposed on the intrusive granite of the district, exhibit two sets of basaltic d.y.k.es--one (the older) abruptly terminated at the granite margin, the other and newer penetrating the granite and Silurian rocks alike. It is not improbable that the older d.y.k.es belong to the Carboniferous or Permian age, while the newer are with equal probability of Tertiary age.

Sir A. Geikie has shown that the Tertiary d.y.k.es of the North of Ireland are representatives of others occurring at intervals over the North of England, and Central and Western Scotland, all pointing towards the central region of volcanic activity; or in a parallel direction thereto, approximating to the N.W. in Ireland, the Island of Islay, and East Argyles.h.i.+re, but in the centre of Scotland generally ranging from east to west.[8] The area affected by the d.y.k.es of undoubted Tertiary age Geikie estimates at no less than 40,000 square miles--a territory greater than either Scotland or Ireland, and equal to more than a third of the total land-surface of the British Isles;[9] and he regards them as posterior "to the rest of the geological structures of the regions which they traverse." It is clear that the d.y.k.es referred to belong to one great system of eruption or intrusion; and they may be regarded as the manifestation of the final effort of internal forces over this region of the British Isles. They testify to the existence of a continuous _magma_ (or sh.e.l.l) of augitic lava beneath the crust; and as the aggregate horizontal extent of all these d.y.k.es, or of the fissures which they fill, must be very considerable, it is clear that the crust through which they have been extruded has received an accession of horizontal s.p.a.ce, and has been fissured by forces acting from beneath, as the late Mr. Hopkins, of Cambridge, had explained on mechanical grounds in his elaborate essay many years ago.[10] This view occurred to myself when examining the region of the North-east of Ireland, but I was not then aware that it had been dealt with on mathematical principles by so eminent a mathematician. The bulging of the crust is a necessary consequence of the absence of plication of the strata due to the extrusion of this enormous quant.i.ty of molten lava; and the intrusion of thousands of d.y.k.es over the North-east of Ireland, unaccompanied by foldings of the strata, must have added a horizontal s.p.a.ce of several thousand feet to that region.[11]

[1] A peculiar form of crystalline quartz first recognized in this rock by a distinguished German petrologist, the late Prof. A. von Lasaulx, who visited the district in 1876.

[2] Sir A. Geikie has disputed the correctness of the view, which I advocated as far back as 1874, that the trachytic lavas of Antrim are the earliest products of volcanic action; but at the time he wrote his paper on the volcanic history of these islands, it was not known that pebbles of this trachyte are largely distributed amongst the ash-beds which occur in the very midst of the overlying basaltic sheets, as I shall have to explain later on. This discovery puts the question at rest as regards the relations of the two sets of rocks.

[3] This remarkable section at the chalk quarries of Templepatrick the author has figured and described in the _Physical Geology and Geography of Ireland_, p. 99, 2nd edit. (1891), where the reader will find the subject discussed more fully than can be done here.

[4] These pebbles were first noticed by Mr. McHenry, of the Irish Geological Survey, in 1890.

[5] The vertical position of the columns of the Giant's Causeway is rather enigmatical. The Causeway cannot be a d.y.k.e, as has often been supposed, otherwise the columns would have been horizontal, _i.e._, at right angles to the sides of the d.y.k.e. Mr. R. G. Symes, of the Geological Survey, has suggested that the Causeway columns have been vertically lowered between two lines of fault, and that originally they formed a portion of the tier of beautiful columns seen in the cliff above, and known as "The Organ."

[6] Sleamish and several other of the Antrim vents are described by Sir A. Geikie in the monograph already referred to, _loc. cit._, p. 101, _et seq._ Also in the _Expl. Memoirs of the Geological Survey of Ireland_.

[7] A diagrammatised section of the Carrick-a-raide volcanic neck is given by Sir A. Geikie, _loc. cit._, p. 105.

[8] Geikie, _loc. cit._, p. 29, _et seq._

[9] P. 32. The view that the crust of the earth has been horizontally extended by the intrusion of d.y.k.es is noticed by McCulloch in reference to the d.y.k.es of Skye.

[10] Hopkins, _Cambridge Phil. Trans._, vol. vi. p. 1 (1836).

[11] As suggested in my Presidential Address to Section C. of the British a.s.sociation at Belfast, 1874.

CHAPTER III.

ISLAND OF MULL AND ADJOINING COAST.

The Island of Mull, with the adjoining districts of Morvern and Ardnamurchan, forms the more southern of the two chief centres of Tertiary volcanic eruptions in the West of Scotland, that of Skye being the more northern. These districts have been the subject of critical and detailed study by several geologists, from McCulloch down to the present day; and amongst the more recent, Sir Archibald Geikie and Professor Judd hold the chief place. Unfortunately, the interpretation of the volcanic phenomena by these two accomplished observers has led them to very different conclusions as regards several important points in the volcanic history of these groups of islands; as, for example, regarding the relative ages of the plateau-basalts and the acid rocks, such as the trachytes and granophyres; again as regards the presence of distinct centres of eruption; and also as regards the relations of the gabbros of Skye to the basaltic sheets. Such being the case, it would appear the height of rashness on the part of the writer, especially in the absence of a detailed examination of the sections over the whole region, to venture on a statement of opinion regarding the points at issue; and he must, therefore, content himself with a brief account of the phenomena as gathered from a perusal of the writings of these and other observers,[1] guided also to some extent by the a.n.a.logous phenomena presented by the volcanic region of the North-east of Ireland.

(_a._) _General Features._--As in the case of the Antrim district, the Island of Mull and adjoining tracts present us with the spectacle of a vast acc.u.mulation of basaltic lava-flows, piled layer upon layer, with intervening beds of bole and tuff, up to a thickness, according to Geikie, of about 3,500 feet. At the grand headland of Gribon, on the west coast, the basaltic sheets are seen to rise in one sheer sweep to a height of 1,600 feet, and then to stretch away with a slight easterly dip under Ben More at a distance of some eight miles. This mountain, the upper part of which is formed of beds of ashes, reaches an elevation of 3,169 feet, so that the acc.u.mulated thickness of the beds of basalt under the higher part of the mountain must be at least equal to the amount stated above--that is, twice as great as the representative ma.s.ses of Antrim. The base of the volcanic series is seen at Carsaig and Gribon to rest on Cretaceous and Jura.s.sic rocks, like those of Antrim; hence the Tertiary age is fully established by the evidence of superposition. This was further confirmed by the discovery by the Duke of Argyll,[2] some years ago (1850), of bands of flint-gravel and tuff, with dicotyledonous leaves amongst the basalts of Ardtun Head. The bas.e.m.e.nt beds of tuff and gravel contain, besides pebbles of flint and chalk, others of sanidine trachyte, showing that highly acid lavas had been extruded and consolidated before the first eruption of the plateau-basalts; another point of a.n.a.logy between the volcanic phenomenon of Antrim and the Inner Hebrides. These great sheets of augitic lava extend over the whole of the northern tract of Mull, the Isles of Ulva and Staffa, and for a distance of several miles inwards from the northern sh.o.r.e of the Sound of Mull, covering the wild moorlands of Morvern and Ardnamurchan, where they terminate in escarpments and outlying ma.s.ses, indicating an originally much more extended range than at the present day. The summits of Ben More and its neighbouring height, Ben Buy, are formed of beds of ash and tuff. The volcanic plateau is, according to Judd, abruptly terminated along the southern side by a large vault, bringing the basalt in contact with Palaeozoic rocks.[3]

(_b._) _Granophyres._--The greater part of the tract lying to the south of Loch na Keal, which almost divides Mull into two islands, and extending southwards and eastwards to the sh.o.r.es of the Firth of Lorn and the Sound of Mull, is formed of a peculiar group of acid (or highly silicated) rocks, cla.s.sed under the general term of "Granophyres." These rocks approach towards true granites in one direction, and through quartz-porphyry and felsite to rhyolite in another--probably depending upon the conditions of cooling and consolidation. In their mode of weathering and general appearance on a large scale, they present a marked contrast to the basic lavas with which they are in contact from the coast of L. na Keal to that of L. Buy. The nature of this contact, whether indicating the priority of the granophyres to the plateau-basalts or otherwise, is a matter of dispute between the two observers above named; but the circ.u.mstantial account given by Sir A.

Geikie,[4] accompanied by drawings of special sections showing this contact, appears to prove that the granophyre is the newer of the two ma.s.ses of volcanic rock, and that it has been intruded amongst the basaltic-lavas at a late period in the volcanic history of these islands. A copy of one of these sketches is here given (Fig. 33), according to which the felsite is shown to penetrate the basaltic sheets at Alt na Searmoin in Mull; other sections seen at Cruach Torr an Lochain, and on the south side of Beinn Fada, appear to lead to similar conclusions. These rocks are penetrated by numerous basaltic d.y.k.es.

[Ill.u.s.tration: Fig. 33.--Section at Alt na Searmoin, Mull, to show the intrusion of felsite (or granophyre) (_b_) into basalt and dolerite (_a_) of the plateau-basalt series.--(Geikie.)]

(_c._) _Representative Rocks of Mourne and Carlingford, Ireland._--a.s.suming Sir A. Geikie's view to be correct, it is possible that we may have in the granite and quartz-porphyries of Mourne and Carlingford representatives of the granites, granophyres, and other acid rocks of the later period of Mull. The granite of Mourne is peculiar in structure, and differs from the ordinary type of that rock in which the silica forms the ground ma.s.s. In the case of the granite of the Mourne Mountains, the rock consists of a crystalline granular aggregate of orthoclase, albite, smoke-quartz, and mica; it is also full of drusy cavities, in which the various minerals crystallise out in very perfect form. As far as regards direct evidence, the age of this rock can only be stated to be post-Carboniferous, and earlier than certain Tertiary basaltic d.y.k.es by which it is traversed. The granophyres of Mull are traversed by similar d.y.k.es, which are representatives of the very latest stage of volcanic action in the British Islands. The author is therefore inclined to concur with Sir A. Geikie in a.s.signing to the granite of the Mourne Mountains, and the representative felsitic rocks of the Carlingford Mountains, a Tertiary age--in which case the a.n.a.logy between the volcanic phenomena of the Inner Hebrides and of the North-east of Ireland would seem to be complete.[5]

[1] Geikie, _Proc. Roy. Soc. Edinburgh_ (1867); _Brit. a.s.soc. Rep._ (Dundee, 1867); "Tertiary Volcanic Rocks of the British Isles," _Quart.

Journ. Geol. Soc._, vol. xxvii. p. 279; also, "History of Volcanic Action in British Isles," _Trans. Roy. Soc. Edin._ (1888); Judd, "On the Ancient Volcanoes of the Highlands," etc., _Quart. Journ. Geol. Soc._, vol. x.x.x. p. 233; and _Volcanoes_, p. 139.

[2] _Brit. a.s.soc. Rep._ for 1850, p. 70.

[3] Judd, _Quart. Jour. Geol. Soc._, vol. x.x.x. p. 242.

[4] _History of Volcanic Action, etc._, _loc. cit._ p. 153, _et seq._ The "Granophyres" of Geikie come under the head of "Felsites," pa.s.sing into "granite" in one direction and quartz-trachyte in another, according to Judd; the proportion of silica from 69 to 75 per cent.--_Quart. Jour. Geol. Soc._, vol. x.x.x. p. 235.

[5] This view the author has expressed in a recent edition of _The Physical Geology of Ireland_, p. 177 (1891).

CHAPTER IV.

ISLE OF SKYE.

This is the largest and most important of all the Tertiary volcanic districts, but owing to the extensive denudation to which, in common with other Tertiary volcanic regions of the British Isles, it has been subjected, its present limits are very restricted comparatively to its original extent. Not only is this evident from the manner in which the basaltic sheets terminate along the sea-coast in grand mural cliffs, as opposite "Macleod's Maidens," and at the entrance to Lough Bracadale on the western coast, but the evidence is, according to Sir A. Geikie, still more striking along the eastern coast; showing that the Jura.s.sic, and other older rocks there visible, were originally buried deep under the basaltic sheets which have been stripped from off that part of the country. These great plateau-basalts occupy about three-fourths of the entire island along the western and northern areas, rising into terraced mountains over 2,000 feet in height, and are deeply furrowed by glens and arms of the sea, along which the general structure of the tableland is laid open, sometimes for leagues at a time.

It is towards the south-eastern part of the island that the most interesting and important phenomena are centred; for here we meet with representatives of the acid (or highly silicated) group of rocks, and of remarkable beds of gabbro, which have long attracted the attention of petrologists. These latter beds, throughout a considerable distance round the flanks of the Cuillin Hills, are interposed between the acid rocks and the plateau-basalts; but towards the north, on approaching Lough Sligahan, the acid rocks, consisting of granophyres, quartz-porphyries, and hornblendic-granit.i.tes, are in direct contact with the plateau-basalts; and, according to the very circ.u.mstantial account of Sir A. Geikie, are intrusive into them; not only sending veins into the basaltic sheets, but also producing a marked alteration in their structure where they approach the newer intrusive ma.s.s. Equally circ.u.mstantial is the same author's account of the relations of the granophyres to the gabbros,[1] as seen at Meall Dearg and the western border of the Cuillin Hills--where the former rock may be seen to send numerous veins into the latter. Not only is this so, but the granophyre is frequently seen to truncate, and abruptly terminate some of the basaltic d.y.k.es by which the basic sheets are traversed--as in the neighbourhood of Beinn na Dubhaic. All these phenomena strongly remind us of the conditions of similar rocks amongst the mountains of Mourne and Carlingford in Ireland; where, at Barnaveve, the syenite (or hornblendic quartz-felsite) is seen to break through the ma.s.ses of olivine gabbro, and send numerous veins into this latter rock.[2]

The interpretation here briefly sketched differs widely from that arrived at by Professor Judd. The granitoid ma.s.ses of the Red Mountains (Beinn Dearg) and the neighbouring heights are, in his view, the roots of the great volcano from which were erupted the various lavas; the earlier eruptions producing the acid lavas, to be followed by the gabbros, and these by the plateau-basaltic sheets, which stretch away towards the north and west into several peninsulas. Thus he holds that "the rocks of basic composition were ejected subsequently to those of the acid variety," and appeals to various sections in confirmation of this view.[3] To reconcile these views is at present impossible; but as the controversy between these two observers is probably not yet closed, there is room for hope that the true interpretation of the relations of these rocks to each other will ere long be fully established.

[1] Geikie, _loc. cit._, p. 161, etc.