CHAPTER I
Earliest Knowledge of Volcanoes—Their Influence on Mythology and Superstition—Part taken by Volcanic Rocks in Scenery—Progress of the Denudation of Volcanoes—Value of the Records of former Volcanoes as illustrating Modern Volcanic Action—Favourable Position of Britain for the Study of this Subject.
Earliest Knowledge of Volcanoes—Their Influence on Mythology and Superstition—Part taken by Volcanic Rocks in Scenery—Progress of the Denudation of Volcanoes—Value of the Records of former Volcanoes as illustrating Modern Volcanic Action—Favourable Position of Britain for the Study of this Subject.
Among the influences which affected the infancy of mankind, the most potent were those of environment. Whatever in outer nature stimulated or repressed courage, inventiveness, endurance, whatever tended to harden or to weaken the bodily faculties, whatever appealed to the imagination or excited the fancy, became a powerful factor in human development.
Thus, in the dawn of civilization, the frequent recurrence of earthquakes and volcanic eruptions throughout the basin of the Mediterranean could not but have a marked effect on the peoples that dwelt by the borders of that sea. While every part of the region was from time to time shaken by underground commotion, there were certain places that became specially noteworthy for the wonder and terror of their catastrophes. When, after successive convulsions, vast clouds of black smoke rose from a mountain and overspread the sky, when the brightness of noon was rapidly replaced by the darkness of midnight, when the air grew thick with stifling dust and a rain of stones and ashes fell from it on all the surrounding country, when streams of what looked like liquid fire poured forth and desolated gardens, vineyards, fields and villages—then did men feel sure that the gods were angry. The contrast between the peacefulness and beauty of the ordinary landscape and the hideous warfare of the elements at these times of volcanic fury could not but powerfully impress the imagination and give a colour to early human conceptions of nature and religion.
It was not only in one limited district that these manifestations of underground convulsion showed themselves. The islands of the Ægean had their volcanoes, and the Greeks who dwelt among them watched theirglowing fires by night and their clouds of steam by day, culminating now and then in a stupendous explosion, like that which, in prehistoric time, destroyed the island of Santorin. As the islanders voyaged eastward they would see, on the coast of Asia Minor, the black bristling lavas of the "Burnt Country," perhaps even then flowing from their rugged heaps of cinders. Or when, more adventurously still, they sailed westward into the Tyrrhenian waters, they beheld the snowy cone of Etna, with its dark canopy of smoke and the lurid nocturnal gleam of its fires; while from time to time they witnessed there on a still more stupendous scale the horrors of a great volcanic eruption.
From all sides, therefore, the early Greek voyagers would carry back to the mother-country marvellous tales of convulsion and disaster. They would tell how the sky rapidly darkened even in the blaze of mid-day, how the land was smothered with dust and stones, how over the sea there spread such a covering of ashes that the oarsmen could hardly drive their vessels onward, how red-hot stones, whirling high overhead, rained down on sails and deck, and crushed or burnt whatever they fell upon, and how, as the earth shook and the sea rose in sudden waves and the mountain gave forth an appalling din of constant explosion, it verily seemed that the end of the world had come.
To the actual horrors of such scenes there could hardly fail to be added the usual embellishments of travellers' tales. Thus, in the end, the volcanoes of the Mediterranean basin came to play a not unimportant part in Hellenic mythology. They seemed to stand up as everlasting memorials of the victory of Zeus over the giants and monsters of an earlier time. And as the lively Greek beheld Mount Etna in eruption, his imagination readily pictured the imprisoned Titan buried under the burning roots of the mountain, breathing forth fire and smoke, and convulsing the country far and near, as he turned himself on his uneasy pallet.
When in later centuries the scientific spirit began to displace the popular and mythological interpretation of natural phenomena, the existence of volcanoes and their extraordinary phenomena offered a fruitful field for speculation and conjecture. As men journeyed outward from the Mediterranean cradle of civilization, they met with volcanic manifestations in many other parts of the world. When they eventually penetrated into the Far East, they encountered volcanoes on a colossal scale and in astonishing abundance. When they had discovered the New World they learnt that, in that hemisphere also, "burning mountains" were numerous and of gigantic dimensions. Gradually it was ascertained that vast lines of volcanic activity encircle the globe. By slow degrees the volcano was recognized to be as normal a part of the mechanism of our planet as the rivers that flow on the terrestrial surface. And now at last men devote themselves to the task of critically watching the operations of volcanoes with as much enthusiasm as they display in the investigation of any other department of nature. They feel that their knowledge of the earth extends to little beyond its mere outer skin, and that the mystery which still hangs over the vast interior of theplanet can only, if ever, be dispelled by the patient study of these vents of communication between the interior and the surface.
If, however, we desire to form some adequate idea of the part which volcanic action has played in the past history of the earth, we should be misled were we to confine our attention to the phenomena of the eruptions of the present day. An attentive examination of any modern volcano will convince us that of some of the most startling features of an eruption no enduring memorial remains. The convulsive earthquakes that accompany a great volcanic paroxysm, unless where they actually fissure the ground, leave little or no trace behind them. Lamentably destructive as they are to human life and property, the havoc which they work is mostly superficial. In a year or two the ruins have been cleared away, the earth-falls have been healed over, the prostrated trees have been removed, and, save in the memories and chronicles of the inhabitants, no record of the catastrophe may survive. The clouds of dust and showers of ashes which destroyed the crops and crushed in the roofs of houses soon disappear from the air, and the covering which they leave over the surface of a district gradually mingles with the soil. Vegetation eventually regains its place, and the landscape becomes again as smiling as before.
Even where the materials thrown out from the crater accumulate in much greater mass, where thick deposits of ashes or solid sheets of lava bury the old land-surface, the look of barren desolation, though in some cases it may endure for long centuries, may in others vanish in a few years. The surface-features of the district are altered indeed, but the new topography soon ceases to look new. Another generation of inhabitants loses recollection of the old landmarks, and can hardly realize that what has become so familiar to itself differs so much from what was familiar to its fathers.
But even when the volcanic covering, thus thrown athwart a wide tract of country, has been concealed under a new growth of soil and vegetation, it still remains a prey to the ceaseless processes of decay and degradation which everywhere affect the surface of the land. No feature of a modern volcano is more impressive than the lesson which it conveys of the reality and potency of this continual waste. The northern slopes of Vesuvius, for example, are trenched with deep ravines, which in the course of centuries have been dug out of the lavas and tuffs of Monte Somma by rain and melted snow. Year by year these chasms are growing deeper and wider, while the ridges between them are becoming narrower. In some cases, indeed, the intervening ridges have been reduced to sharp crests which are split up and lowered by the unceasing influence of the weather. The slopes of such a volcanic cone have been aptly compared to a half-opened umbrella. It requires little effort of imagination to picture a time, by no means remote in a geological sense, when, unless renovated by the effects of fresh eruptions, the cone will have been so levelled with the surrounding country that the peasants of the future will trail their vines and build their cots over the site of the old volcano, in happy ignorance of what has been the history of the ground beneath their feet.
What is here predicted as probable or certain in the future has undoubtedly happened again and again in the past. Over many districts of Europe and Western America extinct volcanoes may be seen in every stage of decay. The youngest may still show, perfect and bare of vegetation, their cones and their craters, with the streams of lava that escaped from them. Those of older date have been worn down into mere low rounded hills, or the whole cone has been cleared away, and there is only left the hard core of material that solidified in the funnel below the surface. The lava-sheets have been cut through by streams, and now remain in mere scattered patches capping detached hills, which only a trained eye can recognize as relics of a once continuous level sheet of solid rock.
By this resistless degradation, a volcanic district is step by step stripped of every trace of its original surface. All that the eruptions did to change the face of the landscape may be entirely obliterated. Cones and craters, ashes and lavas, may be gradually effaced. And yet enough may be left to enable a geologist to make sure that volcanic action was once rife there. As the volcano marks a channel of direct communication between the interior of the earth and the atmosphere outside, there are subterranean as well as superficial manifestations of its activity, and while the latter are removed by denudation, the former are one by one brought into light. The progress of denudation is a process of dissection, whereby every detail in the structure of a volcano is successively cut down and laid bare. But for this process, our knowledge of the mechanism and history of volcanic action would be much less full and definite than happily it is. In active volcanoes the internal and subterranean structure can only be conjectured; in those of ancient date, which have been deeply eroded, this underground structure is open to the closest examination.
By gathering together evidence of this nature over the surface of the globe, we learn that abundantly as still active volcanoes are distributed on that surface, they form but a small fraction of the total number of vents which have at various times been in eruption. In Italy, for example, while Vesuvius is active on the mainland, and Etna, Stromboli and Volcano display their vigour among the islands, there are scores of old volcanoes that have been silent and cold ever since the beginning of history, yet show by their cones of cinders and streams of bristling lava that they were energetic enough in their day. But the Italian volcanic region is only one of many to be found on the European Continent. If we travel eastward into Hungary, or northward into the Eifel, or into the heart of France, we encounter abundant cones and craters, many of them so fresh that, though there is no historical record of their activity, they look as if they had been in eruption only a few generations ago.
But when the geologist begins to search among rocks of still older date than these comparatively recent volcanic memorials, he meets with abundant relics of far earlier eruptions. And as he arranges the chronicles of the earth's history, he discovers that each section of the long cycle of geological ages has preserved its records of former volcanoes. In a research of thiskind he can best realize how much he owes to the process of denudation. The volcanic remains of former geological periods have in most cases been buried under younger deposits, and have sunk sometimes thousands of feet below the level of the sea. They have been dislocated and upheaved again during successive commotions of the terrestrial crust, and have at last been revealed by the gradual removal of the pile of material under which they had lain.
Hence we learn that the active volcanoes of the present time, which really embrace but a small part of the volcanic history of our planet, are the descendants of a long line of ancestors. Their distribution and activity should be considered not merely from the evidence they themselves supply, but in the light derived from a study of that ancestry. It is only when we take this broad view of the subject that we can be in a position to form some adequate conception of the nature and history of volcanoes in the geological evolution of the globe.
In this research it is obvious that the presently active volcano must be the basis and starting-point of inquiry. At that channel of communication between the unknown inside and the familiar outside of our globe, we can watch what takes place in times of quiescence or of activity. We can there study each successive phase of an eruption, measure temperatures, photograph passing phenomena, collect gases and vapours, register the fall of ashes or the flow of lavas, and gather a vast body of facts regarding the materials that are ejected from the interior, and the manner of their emission.
Indispensable as this information is for the comprehension of volcanic action, it obviously affords after all but a superficial glimpse of that action. We cannot see beyond the bottom of the crater. We cannot tell anything about the subterranean ducts, or how the molten and fragmental materials behave in them. All the underground mechanism of volcanoes is necessarily hidden from our eyes. But much of this concealed structure has been revealed in the case of ancient volcanic masses, which have been buried and afterwards upraised and laid bare by denudation.
In yet another important aspect modern volcanoes do not permit us to obtain full knowledge of the subject. The terrestrial vents, from which we derive our information, by no means represent all the existing points of direct connection between the interior and the exterior of the planet. We know that some volcanic eruptions occur under the sea, and doubtless vast numbers more take place there of which we know nothing. But the conditions under which these submarine discharges are effected, the behaviour of the outflowing lava under a body of oceanic water, and the part played by fragmentary materials in the explosions, can only be surmised. Now and then a submarine volcano pushes its summit above the sea-level, and allows its operations to be seen, but in so doing it becomes practically a terrestrial volcano, and the peculiar submarine phenomena are still effectually concealed from observation.
The volcanic records of former geological periods, however, are in largemeasure those of eruptions under the sea. In studying them we are permitted, as it were, to explore the sea-bottom. We can trace how sheets of coral and groves of crinoids were buried under showers of ashes and stones, and how the ooze and silt of the sea-floor were overspread with streams of lava. We are thus, in some degree, enabled to realize what must now happen over many parts of the bed of the existing ocean.
The geologist who undertakes an investigation into the history of volcanic action within the area of the British Isles during past time, with a view to the better comprehension of this department of terrestrial physics, finds himself in a situation of peculiar advantage. Probably no region on the face of the globe is better fitted than these islands to furnish a large and varied body of evidence regarding the progress of volcanic energy in former ages. This special fitness may be traced to four causes—1st, The remarkable completeness of the geological record in Britain; 2nd, The geographical position of the region on the oceanic border of a continent; 3rd, The singularly ample development to be found there of volcanic rocks belonging to a long succession of geological ages; and 4th, The extent to which this full chronicle of volcanic activity has been laid bare by denudation.
1. In the first place, the geological record of Britain is singularly complete. It has often been remarked how largely all the great periods of geological time are represented within the narrow confines of these islands. The gaps in the chronicle are comparatively few, and for the most part are not of great moment.
Thanks to the restricted area of the country and to the large number of observers, this remarkably full record of geological history has been studied with a minute care which has hardly been equalled in any other country. The detailed succession of all the formations has been so fully determined in Britain that the very names first applied here to them and to their subdivisions have in large measure passed into the familiar language of geology all over the globe. Every definite platform in the stratigraphical series has been more or less fully worked out. A basis has thus been laid for referring each incident in the geological history of the region to its proper relative date.
2. In the second place, the geographical position of Britain gives it a notable advantage in regard to the manifestations of volcanic energy. Rising from the margin of a great ocean-basin and extending along the edge of a continent, these islands have lain on that critical border-zone of the terrestrial surface, where volcanic action is apt to be most vigorous and continuous. It has long been remarked that volcanoes are generally placed not far from the sea. From the earliest geological periods the site of Britain, even when submerged below the sea, has never lain far from the land which supplied the vast accumulations of sediment that went to form the Palæozoic and later formations, while, on the other hand, it frequently formed part of the land of former geological periods. It was thus most favourably situated as a theatre for both terrestrial and submarine volcanic activity.
3. In the third place, this advantageous geographical position is found to have been attended with an altogether remarkable abundance and persistence of volcanic eruptions. No tract of equal size yet known on the face of the globe furnishes so ample a record of volcanic activity from the earliest geological periods down into Tertiary time. Every degree of energy may be signalized in that record, from colossal eruptions which piled up thousands of feet of rock down to the feeblest discharge of dust and stones. Every known type of volcano is represented—great central cones like Etna or Vesuvius, scattered groups of small cones like thepuysof France, and fissure- or dyke-eruptions like those of recent times in Iceland.
Moreover, the accurate manner in which the stratigraphy of the country has been established permits each successive era in the long volcanic history to be precisely determined, and allows us to follow the whole progress of that history stage by stage, from the beginning to the end.
These characteristics may be instructively represented on a map, such as that which accompanies the present volume (Map I.). The reader will there observe how repeatedly volcanic eruptions have taken place, not merely within the general area of the British Isles, but even within the same limited region of that area. The broad midland valley of Scotland has been especially the theatre for their display. From the early part of the Lower Silurian period, through the ages of the Old Red Sandstone, Carboniferous and Permian systems, hundreds of volcanic vents were active in that region, while in long subsequent time there came the fissure-eruptions of the Tertiary series.
4. In the fourth place, the geological revolutions of successive ages have made this long volcanic chronicle fully accessible to observation. Had the lavas and ashes of one period remained buried under the sedimentary accumulations of the next, their story would have been lost to us. We should only have been able to decipher the latest records which might happen to lie on the surface. Fortunately for the progress of geology, the endless vicissitudes of a continental border have brought up the very oldest rocks once more to the surface. All the later formations of the earth's crust have likewise been upraised and exposed to denudation during long cycles of time. In this manner, the rocky framework of the country has been laid bare, and each successive chapter of its geological history may be satisfactorily deciphered. The singularly complete volcanic chronicle, after being entombed under younger deposits, has been broken up and raised once more into view. The active vents of former periods have been dissected, submarine streams of lava have been uncovered, sheets of ashes that fell over the sea-bottom have been laid bare. The progress of denudation is specially favoured in such a variable and moist climate as that of Britain, and thus by the co-operation of underground and meteoric causes the marvellous volcanic records of this country have been laid open in minutest detail.
There is yet another respect in which the volcanic geology of Britain possesses a special value. Popular imagination has long been prone to seesigns of volcanic action in the more prominent rocky features of landscape. A bold crag, a deep and precipitous ravine, a chasm in the side of a mountain, have been unhesitatingly set down as proof of volcanic disturbance. Many a cauldron-shaped recess, like the corries of Scotland or the cwms of Wales, has been cited as an actual crater, with its encircling walls still standing almost complete.
The relics of former volcanoes in this country furnish ample proofs to dispel these common misconceptions. They show that not a single crater anywhere remains, save where it has been buried under lava; that no trace of the original cones has survived, except in a few doubtful cases where they may have been preserved under subsequent accumulations of material; that in the rugged tracts, where volcanic action has been thought to have been most rife, there may be not a vestige of it, while, on the other hand, where the uneducated eye would never suspect the presence of any remnant of volcanic energy, lavas and ashes may abound. We are thus presented with some of the most impressive contrasts in geological history, while, at the same time, this momentous lesson is borne in upon the mind, that the existing inequalities in the configuration of a landscape are generally due far less to the influence of subterranean force than to the action of the superficial agents which are ceaselessly carving the face of the land. Those rocks which from their hardness or structure are best able to withstand that destruction rise into prominence, while the softer material around them is worn away. Volcanic rocks are no exception to this rule, as the geological structure of Britain amply proves.
In the following chapters, forming Book I. of this work, I propose to begin by offering some general remarks regarding the nature and causes of volcanic action, so far as these are known to us. I shall then proceed to consider the character of the evidence that may be expected to be met with respecting the former prevalence of that action at any particular locality where volcanic disturbances have long since ceased. The most telling evidence of old volcanoes is naturally to be found in the materials which they have left behind them, and the reader's attention will be asked to the special characteristics of these materials, in so far as they give evidence of former volcanic activity.
As has been already remarked, many of the most prominent phenomena of a modern volcano are only of transient importance. The earthquakes and tremors, and the constant disengagement of steam and gases, that play so conspicuous a part in an eruption, may leave no sensible record behind them. But even the cones of ashes and lava, which are piled up into mountainous masses, have no true permanence: they are liable to ceaseless erosion by the meteoric agencies of waste, and every stage in their degradation may be traced. In successive examples we can follow them as they are cut down to the very core, until in the end they are entirely effaced.
We may well, therefore, ask at the outset by what more enduring records we may hope to detect the traces of former volcanic action. The following introductory chapters will be devoted to an attempt to answerthis question. I shall try to show the nature and relative importance of the records of ancient volcanoes; how these records, generally so fragmentary, may be pieced together so as to be made to furnish the history which they contain; how their relative chronology may be established; how their testimony may be supplemented in such wise that the position of long vanished seas, lands, rivers, and lakes may be ascertained; and how, after ages of geological revolution, volcanic rocks that have lain long buried under the surface now influence the scenery of the regions where they have once more been exposed to view.
From this groundwork of ascertained fact and reasonable inference, we shall enter in Book II. upon the story of the old volcanoes of the British Isles. It is usual to treat geological history in chronological order, beginning with the earliest ages. And this method, as on the whole the most convenient, will be adopted in the present work. At the same time, the plan so persistently followed by Lyell, of working backward from the present into the past, has some distinct advantages. The volcanic records of the later ages are much simpler and clearer than those of older times, and the student may, in some respects, profitably study the history of the Tertiary eruptions before he proceeds to make himself acquainted with the scantier chronicles of the eruptions of the Palæozoic periods. But as I wish to follow the gradual evolution of volcanic phenomena, and to show how volcanic energy has varied, waxing and waning through successive vast intervals of time, I will adhere to the chronological sequence.