CHAPTER XXXIII

Vast lapse of time between the close of the Palæozoic and beginning of the Tertiary Volcanic Eruptions--Prolonged Volcanic Quiescence--Progress of Investigation among the Tertiary Volcanic Series of Britain.

From the evidence which has been led in the foregoing chapters it is clear that during the later stages of the Palæozoic period there was a gradual enfeeblement of volcanic vigour over the area of the British Isles. When the last puys of the Permian series became extinct a remarkable volcanic quiescence settled down on the region. This interval of rest lasted throughout the whole of the long succession of the Mesozoic ages. Though the geological record of this section of geological time is singularly complete in Britain, not a single vestige has yet been found in it of any contemporaneous eruption. And what is true of this country is, on the whole, true of the entire European continent. With some trifling exceptions there were no volcanoes in Europe, so far as we know, during the enormous lapse of time between the last of the Palæozoic and the earliest of the Tertiary eruptions.

When the geologist attempts to form an estimate of the chronological value of this interval of time he is soon lost in bewilderment over its obvious vastness, and the impossibility of discovering any standards of measurement by which to reckon its duration. On the one hand, he sees that it lasted long enough to admit of the gradual elaboration of many thousands of feet of various sedimentary deposits, which, from their remarkable diversities of character, were evidently accumulated, on the whole, with extreme slowness and amidst many geographical vicissitudes. On the other hand, he perceives that the interval sufficed to bring about an entire change in the fauna and flora of the globe. Indeed, the more he investigates the details of this biological transformation, the more he is impressed with the length of time that it must have required. For it is not merely one complete change, but a multifold succession of changes. The stratigraphical records of the long array of geological periods over which it was spread show that the biological evolution advanced through a vast series of species, genera and orders which one by one appeared and disappeared.

The ages that elapsed between the final dying out of the Palæozoic volcanoes and the outburst of those of Tertiary time were so protracted that many revolutions of the geography of Europe were comprised within them. Land and sea changed places again and again. First came the singular topography of the Trias, which prolonged and accentuated the characteristics of the closing Palæozoic ages. Next arose the more genial climate and more varied geography of the Jurassic period, when comparatively shallow seas overspread the site of most of the European continent, and tracts of old land stretched away to the west and north. Another crowded succession of changes in the disposition of land and sea filled the long Cretaceous period, at the close of which a more rapid and complete transformation in European geography took place.

Yet during all these transitions and vicissitudes, so far as we know, volcanic energy remained quiescent throughout Western Europe. It was not until some time after the great terrestrial movements that raised so much of the Cretaceous sea-floor into land, and laid the foundations of the modern continent, that the subterranean fires once more awoke to vigorous action.

The renewal of eruptions in the early ages of Tertiary time was as widespread as it was energetic. Over many regions of the European continent volcanoes broke out either in new areas or on old sites. For the most part they appeared as scattered puys or as Vesuvian vents, generally not of the first magnitude, like those of Central France, Hungary, Würtemberg and Italy. But in the north-west they assumed more colossal proportions, and took the form of fissure-eruptions by which many thousands of square miles of country were deluged with lava. From the South of Antrim all along the West of Scotland to the north of the Inner Hebrides remains of these basalt-floods form striking features in the existing scenery. The same kind of rocks reappear in the Faroe Islands and in Iceland, so that an enormous tract of North-western Europe, much of it now submerged under the sea, was the scene of activity of the Tertiary volcanoes. In entering, therefore, upon a consideration of the British Tertiary volcanic rocks, we are brought face to face with the records of the most stupendous succession of volcanic phenomena in the whole geological history of Europe. Fortunately these records have been fully preserved in the British Isles, so that ample materials remain there for the elucidation of this last and most marvellous of all the volcanic epochs in the evolution of the continent.

As the remains of the Tertiary series of volcanic eruptions are the youngest of all the volcanic records of Britain, they are naturally the freshest and most abundantly preserved. They consequently reveal with singular clearness multitudes of volcanic phenomena that are less distinctly recognizable, or not to be found at all, among the Palæozoic systems. Hence they will be discussed in greater detail in the following chapters.

As a consequence of their greater freshness and wider extent, and largely also because of the way in which they have been exposed along many leagues of picturesque sea-cliffs in the North of Ireland and the West of Scotland, they attracted attention at an earlier time than the less obvious volcanic memorials of older ages. The gradual development of opinion regarding the nature and history of volcanic rocks is thus in no small measure bound up with the progress of observation and inference in regard to the Tertiary volcanic series. I shall therefore begin this narrative by offering a rapid sketch of the history of inquiry respecting the Tertiary volcanic areas of the British Isles.

The basaltic cliffs of Antrim and the Inner Hebrides had attracted the notice of passing travellers, and their striking scenery had become more or less familiar to the reading public, before any attention was paid to their remarkable geological structure and history. In particular, the wonders of the Giant's Causeway and the Antrim coast had already begun to draw pilgrims, even from distant countries, at a time when geology had not come into existence. The scientific tourist of those days who might care to look at rocks was, in most cases, a mineralogist, for whom their structural relations and origin were subjects that lay outside of the range of his knowledge or habits of thought. In the year 1772 Sir Joseph Banks, together with Solander and a party, visited Staffa and brought back the earliest account of the marvels of that isle as they appeared to the sober eyes of science. His narrative was communicated to Pennant, together with a number of drawings of the cliffs and of Fingal's Cave. These were inserted by that geographer in his _Second Tour_, published in 1774, and from their careful measurements of the basaltic pillars and their delineation of the basaltic structure, are of special interest in the history of volcanic geology.

An intelligent appreciation of some of the geological interest of the region is to be found in the writings of Whitehurst,[125] who gave a good account of the basalt-cliffs of Antrim, and regarded the basaltic rocks as the results of successive outflows of lava from some centre now submerged beneath the Atlantic. More important are the observations contained in two letters of Abraham Mills.[126] This writer had been struck with the dykes on the north coast of Ireland, and was led to examine also those in some of the nearer Scottish islands. He believed them to be of truly volcanic origin, and spoke of them as veins of lava. A few years later, Faujas St. Fond made his well-known pilgrimage to the Western Isles. Familiar with the volcanic rocks of Central France, he at once recognized the volcanic origin of the basalts of Mull, Staffa and the adjoining islands.[127] His account of the journey, published in Paris in 1797, may be taken as the beginning of the voluminous geological literature which has since gathered round the subject. Three years afterwards (1800) appeared Jameson's _Outline of the Mineralogy of the Scottish Isles_. Fresh from the teaching of Werner at Freiberg, the future distinguished Professor of Natural History in the Edinburgh University naturally saw everything in the peculiar Wernerian light. He gave the first detailed enumeration of some of the eruptive rocks of the Hebrides, but of course ridiculed the idea of their igneous origin. Having heard of a reported "crater of a volcano" near Portree, he ironically expressed a hope that "there may be still sufficient heat to revive the spirits of some forlorn fire-philosopher, as he wanders through this cold, bleak country."[128]

[Footnote 125: _Inquiry into the Original State and Formation of the Earth_, 2nd edit. 1786.]

[Footnote 126: _Philosophical Transactions for 1790._]

[Footnote 127: _Voyage en Angleterre, en Écosse et aux Îles Hébrides._ Paris, 1797.]

[Footnote 128: It will be shown in a later chapter that there is a remarkably perfect volcanic vent near Portree, but the supposed crater referred to by Jameson was probably some little corry among the sheets of basalt.]

The advent of Jameson to Edinburgh gave a fresh impetus to the warfare of the Plutonists and Neptunists, for he brought to the ranks of the latter a mineralogical skill such as none of their Scottish opponents could boast. The igneous origin of basalt, which the Plutonists stoutly maintained, was as strongly denied by the other side. For some years one of the most telling arguments against the followers of Hutton was derived from the alleged occurrence of fossil shells in the basalt of the north coast of Ireland. Kirwan[129] quoted with evident satisfaction Richardson's observation of "shells in the basalts of Ballycastle," and Richardson[130] himself, though the true explanation, that the supposed basalt is only Lias shale altered by basalt, had been stated in 1802 by Playfair,[131] continued for ten years afterwards to reiterate his belief in the aqueous origin of basalt. Thus the Tertiary volcanic rocks furnished effective weapons to the combatants on both sides. The dispute regarding the black fossiliferous rocks of Portrush had the effect of drawing special attention to the geology of the North of Ireland. Among the more noted geologists who were led to examine them, particular reference must be made to Conybeare and Buckland, who, in the year 1813, studied the interesting coast-sections of Antrim. The report of their observations gives an excellent summary of the arguments for the truly igneous origin of basalt, and a statement of opinion in favour of the view that the bedded basalts are the products of submarine volcanoes. Berger also about the same time described in fuller detail the geology of the Antrim district, and showed the rocks of the basalt-plateau to be younger than the Chalk. He likewise made a study of the basalt-dykes of the North of Ireland, and was the first to point out their prevalent north-westerly direction. The memoirs of these geologists[132] may justly be regarded, to quote the words of Portlock, as "the first effectual step made in Irish geology." Portlock's own description is still the most complete summary of the geology of that interesting region.[133]

[Footnote 129: _Geological Essays_, 1799, p. 252, _footnote_.]

[Footnote 130: Richardson lived on the Antrim coast, and had daily opportunities of examining the admirable rock-sections there exposed. It was he who found the shells in supposed basalt, and led the geologists of his day astray on this subject. He made a clever but irrelevant reply to Playfair's plain statement of facts (_Trans. Roy. Irish Acad._ vol. ix. 1803, p. 481). His elaborate attack on "the Volcanic Theory" will be found in _Trans. Roy. Irish Acad._ vol. x. (1806), pp. 35-107. Though lively enough as a specimen of controversial writing, it forms, when seriously considered, rather a melancholy chapter in geological literature.]

[Footnote 131: _Illustrations of the Huttonian Theory_, § 252.]

[Footnote 132: They are contained in the third volume of the _Transactions of the Geological Society_.]

[Footnote 133: "Report on the Geology of the County of Londonderry and parts of Tyrone and Fermanagh," _Mem. Geol. Survey_, 1843.]

While such advances were being made in the knowledge of the structure of the volcanic rocks of the North of Ireland, the geologist had already appeared who was the first to attempt a systematic examination of the Western Islands, and whose published descriptions are still a chief source of information regarding the geology of this extensive region. Dr. Macculloch seems to have made his first explorations among the Hebrides some time previous to the year 1814, for in that year he published some remarks on specimens from that district transmitted to the Geological Society.[134] For several years in succession he devoted himself with great energy and enthusiasm to the self-imposed task of geologically examining and mapping in a generalized way all the islands that lie to the westward of Scotland, from the remote St. Kilda even as far as the Isle of Man. From time to time, notices of parts of his work were given in the _Transactions of the Geological Society_. But eventually in 1819 he embodied the whole in his _Description of the Western Islands of Scotland, including the Isle of Man_.

[Footnote 134: _Trans. Geol. Soc._ vol. ii. 1814.]

This great classic marks a notable epoch in British geology. Properly to estimate its value, we should try to realize what was the state of the science in this country at the time of its appearance. So laborious a collection of facts, and so courageous a resolution to avoid theorizing about them, gave to his volumes an altogether unique character. His descriptions were at once adopted as part of the familiar literature of geology. His sections and sketches were reproduced in endless treatises and text-books. Few single works of descriptive geology have ever done so much to advance the progress of the science in this country. With regard to the special subject of the present memoir, Macculloch showed that the basalts and other eruptive rocks of the Inner Hebrides pierce and overlie the Secondary strata of these islands, and must therefore be of younger date. But though he distinguished the three great series of "trap-rocks," "syenites" and "hypersthene-rocks" or "augite-rocks," and indicated approximately their respective areas, he did not attempt to unravel their relations to each other. Nor did he venture upon any speculations as to the probable conditions under which these rocks were produced. He claimed that those who might follow him would find a great deal which he had not described, but little that he had not examined. Subsequent observers have noted many important facts, of which, had he observed them, he would at once have seen the meaning, and which he certainly would not have passed over in silence. But as a first broad outline of the subject, Macculloch's work possesses a great value, which is not lessened by the subsequent discovery of details that escaped his notice, and of important geological relations which he failed to detect.

It has already been pointed out that some of the earliest and ablest observations among the volcanic rocks of this country, especially in Scotland, were made by foreigners. Students who had repaired from abroad to Edinburgh for education sometimes caught the geological enthusiasm, then so marked in that city, and made numerous journeys through the country in search of further knowledge of Scottish rocks and minerals. In other instances, geologists of established reputation, attracted by the interest which the published accounts of the geology of Scotland had excited, were led to visit the country and to record their impressions of its rock-structure. Of the first class of observers the two most noted were Ami Boué and L. A. Necker; of the second, special acknowledgment is due to Faujas St. Fond and to Von Oyenhausen and Von Dechen.

The labours of Boué[135] have already been referred to in connection with the literature of the Scottish Old Red Sandstone (vol. i. p. 269). In his treatment of the Tertiary Volcanic series of Scotland he appears to have relied mainly on the then recently published volumes of Macculloch.

[Footnote 135: _Essai géologique sur l'Écosse._ Paris, 1820.]

L. A. Necker, as the grandson of the illustrious De Saussure, had strong claims on the friendly assistance of the School of Geology at Edinburgh when he went thither in 1806, at the age of twenty, to prosecute his studies. He was equally well received by the Plutonists and Neptunists, and devoted some time to the exploration of the geology not only of the Lowlands, but of the Highlands and the Inner Hebrides. Most of his observations appear to have been made in the year 1807, but it was not until fourteen years afterwards that he published the account of them.[136] The geological part of this work must be admitted to be somewhat disappointing. The author's caution not to commit himself to either side of the geological controversy then waging makes his descriptions and explanations rather colourless. He adds little to what was previously known. Even as regards the origin of the basalts of the Western Islands, he could not make up his mind whether or not to regard them as volcanic, but contented himself by referring them to "the trappean formation." Yet these islands had so fascinated him that eventually he returned to them as his adopted home, passed the last twenty years of his life among them, and died and was buried there. Besides his _Voyage_, he published in French an account of the dykes of the Island of Arran.[137]

[Footnote 136: _Voyage en Écosse et aux Îles Hébrides._ See also biographical notice of L. A. Necker, by Principal J. D. Forbes, _Proc. Roy. Soc. Edin._ v. (1862), p. 53.]

[Footnote 137: _Trans. Roy. Soc. Edin._ vol. xiv. (1840), p. 667.]

Among the foreign geologists who have been drawn to the Scottish mountains and islands by the interest of their Tertiary volcanic rocks, I have already spoken of Faujas St. Fond. Much more important, however, were the observations made some thirty years later by two German men of science, Von Oyenhausen and Von Dechen. Their careful descriptions of the geology of Skye, Eigg and Arran added new materials to the knowledge already acquired by native geologists.[138] To some of the more interesting parts of their work reference will be made in later pages.

[Footnote 138: Karsten's _Archiv_ (1829), vol. i. p. 56.]

The numerous trap-dykes of Northumberland, Durham and Northern Yorkshire at an early date attracted the attention of geologists. As far back as 1817, they had been the subject of a memoir by N. J. Winch,[139] who gave an account of their effects on the adjacent rocks. More important were the subsequent papers on the same subject by Sedgwick, who, discussing the lithological characters, probable origin and geological age of the dykes, pointed out that while the Cleveland dyke was undoubtedly younger than a large part of the Jurassic rocks, there was no direct evidence to determine whether dykes farther north were earlier or later than the time of the Magnesian Limestone.[140] Subsequent accounts of the dykes of the same region were given by Buddle,[141] M. Forster,[142] N. Wood,[143] H. T. M. Witham,[144] Tate [145] and others, while in more recent years important additions to our knowledge of these dykes and of their effects have been made by Sir J. Lowthian Bell[146] and Mr. J. J. H. Teall.[147]

[Footnote 139: _Trans. Geol. Soc._ vol. iv. (1817), p. 21. See also Tilloch's _Phil. Mag._ vols. xlix. and l.]

[Footnote 140: _Cambridge Phil. Trans._ vol. ii. (1827), pp. 21, 139.]

[Footnote 141: _Trans. Nat. Hist. Soc. Northumberland_, i. (1831), p. 9.]

[Footnote 142: _Op. cit._ i. p. 44.]

[Footnote 143: _Op. cit._ i. pp. 305, 306, 308, 309.]

[Footnote 144: _Op. cit._ ii. (1838), p. 343.]

[Footnote 145: _Trans. Northumberland and Durham_, ii. (1868), p. 30.]

[Footnote 146: _Proc. Roy. Soc._ xxiii. (1875), p. 543.]

[Footnote 147: _Quart. Journ. Geol. Soc._ xl. (1884), p. 209.]

The geological age of the great series of Tertiary volcanic rocks has only been determined district by district, and at wide intervals. That some part of the Antrim basalts is younger than the Chalk of that region was clearly shown by Berger, Conybeare and Buckland. Portlock, however, referred to the occurrence of detached blocks of basalt which he supposed to be immersed in the Chalk near Portrush, and which inclined him to believe that "the basaltic flows commenced at a remote period of the Cretaceous system."[148] Macculloch showed that the corresponding basaltic plateaux of the Inner Hebrides were certainly younger than the Oolitic rocks of that region. But no nearer approximation to their date had yet been made when in the year 1850 the Duke of Argyll announced the discovery of strata containing fossiliferous chalk-flints and dicotyledonous leaves, lying between the bedded basalts of Ardtun Head, in the Isle of Mull.[149] In the following year these fossil leaves were described by Edward Forbes, who regarded them as decidedly Tertiary, and most probably Miocene. This was the first palæontological evidence for the determination of the geological age of any portion of the basalt-plateaux, and it indicated that the basalts of the south-west of Mull were of older Tertiary date. Taken also in connection with the occurrence of lignite-beds between the basalts of Antrim, it suggested that these volcanic plateaux were not due to submarine eruptions, as the earlier geologists had supposed, but were rather the result of the subærial outpouring of lava at successive intervals, during which terrestrial vegetation sprang up upon the older outflows.

[Footnote 148: _Report on the Geology of Londonderry_, p. 93. There can be no doubt that this was an error of observation. The Antrim basalts are all certainly younger than the Chalk. The supposed "lumps of basalt" were probably the ends of veins intruded into the Chalk, and perhaps partially disconnected from the main parts of the veins. Such apparently detached masses of intrusive rock are not infrequent occurrence in connection with the Tertiary intrusive sills. An example will be found represented in Fig. 321.]

[Footnote 149: _Brit. Assoc. Report_, 1850, Sections, p. 70; and _Quart. Jour. Geol. Soc._ vii. (1851), p. 87.]

While Forbes brought forward palæontological proofs of the Tertiary age of the volcanic rocks of the south-west of Mull, he at the same time laid before the Geological Society a paper on the Estuary Beds and the Oxford Clay of Loch Staffin, in Skye, wherein, while admitting the existence of appearances which might be regarded as favourable to the view that the intercalated basalts of that region were of much later date than the Oolitic strata between which they might have been intrusively injected, he stated his own belief that they were really contemporaneous with the associated stratified rocks, and thus marked an outbreak of volcanic energy at the close of the Middle Oolitic period.[150] The Duke of Argyll, in the paper which he on the same occasion communicated to the Geological Society, adopted this view of the probable age of most of the basalts of the Western Islands. He looked upon the Tertiary volcanic rocks of Mull as occupying a restricted area, the great mass of the basalt of that island, like that of Skye, being regarded by him as probably not later than some part of the Secondary period.

[Footnote 150: _Quart. Journ. Geol. Soc._ vol. vii. (1851), p. 104.]

It must be granted that the appearances of contemporaneous intercalation of the basalt among the Secondary strata are singularly deceptive. When, several years after the announcement of the Tertiary age of the basalts of Ardtun, I began my geological work in the Inner Hebrides, I was led to the same conclusion as Edward Forbes, and expressed it in an early paper.[151] All over the north of Skye I traced what appeared to be evidence of the contemporaneous interstratification of basalts with the Jurassic rocks and I concluded (though with some reservation) that the whole of the vast basaltic plateaux of that island were not younger than some late part of the Jurassic period. In that same paper the attention of geologists was called to the probable connection of the great system of east-and-west dykes traversing Scotland and the North of England, with the basalt-plateaux of the Inner Hebrides, and as I believed the latter to be probably of the age of the Oolitic rocks, I assigned the dykes to the same period in geological history. But subsequent explorations enabled me to correct the mistake into which, with other geologists, I had fallen regarding the age of the volcanic phenomena of the Western Islands. In 1867 I showed that instead of being confined to a mere corner of Mull, the Tertiary basalts, with younger associated trachytic or granitic rocks, covered nearly the whole of that island, and that in all likelihood the long chain of basaltic masses, extending from the North of Ireland along the west coast of Scotland to the Faroe Islands, and beyond these to Iceland, was all erupted during the Tertiary period. At the same time I drew special attention to the system of east-and-west dykes as proofs of the vigour of volcanic action at that period, and I furnished evidence that this action was prolonged through a vast interval of time, during which great subærial denudation of the older lavas took place before the outflow of the younger.[152] Later in the same year, in an address to the Geological Section of the British Association, I reiterated these views, and more particularly emphasized the importance of the system of dykes, which in my opinion was possibly the most striking manifestation of the vigour of Tertiary volcanic action.[153] In 1871, after further explorations in the field, I gave a detailed account of the structure which had led to the mistake as to the age of the Tertiary volcanic rocks of the Western Islands; and in a description of the island of Eigg, I brought forward data to show the enormous duration of the Tertiary volcanic period in the west of Britain.[154]

[Footnote 151: "On the Chronology of the Trap-rocks of Scotland," _Trans. Roy. Soc. Edin._ xxii. (1861), p. 649.]

[Footnote 152: _Proc. Roy. Soc. Edin._ vi. (1867), p. 71.]

[Footnote 153: _Brit. Assoc. Report_ (Dundee), 1867, Sections, p. 49.]

[Footnote 154: _Quart. Journ. Geol. Soc._ xxvii. (1871), p. 279.]

Three years later Mr. J. W. Judd read before the Geological Society a paper "On the Ancient Volcanoes of the Highlands."[155] The most novel feature of this paper was the announcement that the author had recognized the basal wrecks of five great central volcanoes in the Western Islands, among which that of Mull was inferred by him to have been at least 14,500 feet high. He was led to the conclusion that the volcanic period in these regions was divisible into three sections--the first marked by the outburst of acid rocks (felspathic lavas and ashes, connected with deeper and more central granitic masses); the second by the extrusion of basic lavas and tuffs (the basaltic plateaux); the third by the appearance of small sporadic volcanic cones ("felspathic, basaltic, or intermediate in composition") after the great central cones had become extinct. It will be seen in the following pages that these conclusions of Professor Judd are not supported by a more detailed study of the region.

[Footnote 155: _Quart. Journ. Geol. Soc._ xxx. (1874), p. 220.]

In the year 1879, during a traverse of some portions of the volcanic region of Wyoming, Montana and Utah, I was vividly impressed by the identity of structure between the basaltic plateaux of these territories and the youngest volcanic areas of Britain. It then appeared to me that some of the puzzling features in the Tertiary volcanic series of the Inner Hebrides might be explained by the structures so admirably displayed in these lava-fields of the Far West.[156] Riding over the great basalt-plains of the Snake River and looking at the sections cut by the river through the thick series of horizontal basalt-beds, I appreciated for the first time the significance of Baron von Richthofen's views regarding "massive" or "fissure" eruptions, as contradistinguished from those of great central cones of the type of Etna or Vesuvius, and I gathered so many suggestions from my examination of these American regions that I renewed with increased interest the investigation of the Tertiary volcanic tracts of Britain. At last, after another interval of nine years, during which my weeks of leisure were given to the task, I was able to complete a discussion of the whole history of Tertiary volcanic action in this country, which was communicated to the Royal Society of Edinburgh in the early summer of 1888.[157] Since that time I have continued the research, and have from time to time communicated my results to the Geological Society. These various memoirs are combined with hitherto unpublished details in the following account of the British Tertiary Volcanic Rocks.

[Footnote 156: _Geological Essays at Home and Abroad_ (1882), pp. 271, 274; _Nature_, November 1880.]

[Footnote 157: _Trans. Roy. Soc. Edin._ vol. xxxv. part ii. (1888), pp. 23-184.]

Professor Judd has also prosecuted the investigation of the petrography of the rocks, and has published his observations in the _Quarterly Journal of the Geological Society_.[158] To these papers by him more detailed reference will be made in later Chapters.

[Footnote 158: _Quart. Journ. Geol. Soc._ vols. xlv. (1889), xlvi. (1890), xlix. (1893). In the first of these volumes Professor Judd offered a detailed criticism of my views as to the order of succession and history of the volcanic rocks of the Inner Hebrides. Subsequent investigation having entirely confirmed my main conclusions, it is not necessary to enter here upon matters of controversy. Reference, however, will be made in subsequent Chapters to some of the points in dispute.]

In describing the geological history of a great series of rocks, chronological order is usually the most convenient method of treatment. Where, however, the rocks are of volcanic origin, and do not always precisely indicate their relative age, and where moreover the same kinds of rock may appear on widely-separated geological horizons, it is not always possible or desirable to adhere to the strict order of sequence. With this necessary latitude, I propose to follow the chronological succession from the older to the newer portions of the series. I shall treat first of the system of dykes, by which so large a part of Scotland and of the north of England and Ireland is traversed. Many of the dykes are undoubtedly among the youngest members of the volcanic series, and in no case has their age been as yet determined except relatively to the antiquity of the rocks which they traverse. They must, of course, be posterior to these rocks, and hence it would be quite logical to reserve them for discussion at the very end of the whole volcanic phenomena. My reason for taking them at the beginning will be apparent in the sequel. After the dykes, I shall describe the great volcanic plateaux which, in spite of vast denudation, still survive in extensive fragments in Antrim, the Inner Hebrides and the Faroe Islands. The eruptive bosses of basic rocks that have broken through the plateaux will next be discussed. An account will then be given of the protrusions of acid rocks which have disrupted these basic bosses. The last chapters will contain a sketch of the subsidences and dislocations which the basalt-plateaux have suffered, and of the denudation to which they have been subjected.

As has been explained in Chapter iii., the volcanic cycle of any district, during a given geological period, embraces the whole range of erupted products from the beginning to the end of a complete series of eruptions. Reference was made in Book I. to the remarkable variation in the character of the lavas successively poured out from the same volcanic reservoir during the continuance of a single cycle, and it was pointed out that Richthofen's law generally holds good that while the first eruptions may be of a basic or average and intermediate nature, those of succeeding intervals become progressively more acid, but are often found to return again at the close to thoroughly basic compounds.

This law is well illustrated by the volcanic history of Tertiary time in Britain. We shall find that the earliest eruptions of which the relative date is known consisted generally of basic lavas (dolerites and basalts), but including also more sparingly andesites, trachytes and rhyolites; that the oldest intrusive masses consisted of bosses, sills and dykes of dolerite and gabbro; that these intrusions were followed by others of a much more acid character--felsites, pitchstones, quartz-porphyries or rhyolites, granophyres and granites; that the latest lava is a somewhat acid rock, being a vitreous form of dacite; and that the most recent volcanic products of all are dykes of a thoroughly basic nature, like some of the earlier intruded masses.