Read Ebook: The Ancient Volcanoes of Great Britain Volume 1 (of 2) by Geikie Archibald

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CHAP.

INDEX . . . . . . . . . . . . . . . . . . . . . . . 245

THE SCHOOL OF MILETUS

The city naturally held a leading place politically as well as commercially. Empire in our sense was alien to the instincts of the Greek race; but Miletus was for centuries recognised as the foremost member of a great commercial and political league, the political character of the league becoming more defined, as first the Lydian and then the Persian monarchy became an aggressive neighbour on its borders.

It is interesting to find that the man who was thus the first philosopher, the first observer who took a metaphysical, non-temporal, analytical view of the world, and so became the predecessor of all those votaries of 'other-world' ways of thinking,--whether as academic idealist, or 'budge doctor of the Stoic fur,' or Christian ascetic or what not, whose ways are such a puzzle to the 'hard-headed practical man,'--was himself one of the shrewdest men of his day, so shrewd that by common consent he was placed foremost in antiquity among the Seven Sages, or seven shrewd men, whose practical wisdom became a world's tradition, enshrined in anecdote and crystallised in proverb.

The chief record that we possess of the philosophic teaching of Thales is contained in an interesting notice of earlier philosophies by Aristotle, the main part of which as regards Thales runs as follows:

This philosopher is said to have been younger by one generation than Thales, but to have been intimate with him. He, like Thales, was a native of Miletus, and while we do not hear of him as a person, like Thales, of political eminence and activity, he was certainly the equal, if not the superior, of Thales in mathematical and scientific ability. He is said to have either invented or at least made known to Greece the construction of the sun-dial. He was associated with Hecataeus in the construction of the earliest geographical charts or maps; he devoted himself with some success to the science of astronomy. His familiarity with the abstractions of mathematics perhaps accounts for the more abstract form, in which he expressed his idea of the principle of all things.

Then, again, as to the origin of man, he seems to have in like manner taught a theory of development from lower forms of life. In his view the first living creatures must have come into being in moisture . As time went on, and these forms of life reached their fuller possibilities, they came to be transferred to the dry land, casting off their old nature like a husk or bark. More particularly he insists that man must have developed out of other and lower forms of life, because of his exceptional need, under present conditions, of care and nursing in his earlier years. Had he come into being at once as a human creature he could never have survived.

THE SCHOOL OF MILETUS

The fundamental principle or fact of being Heraclitus formulated in the famous dictum, 'All things pass.' In the eternal flux or flow of being consi cone and surrounding bedded tuffs 71

MAPS

BOOK I

GENERAL PRINCIPLES AND METHODS OF INVESTIGATION

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 AEgean had their volcanoes, and the Greeks who dwelt among them watched their glowing 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.

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.

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 this kind 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 large measure 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.

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.

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 . 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.

There is yet another respect in which the volcanic geology of Britain possesses a special value. Popular imagination has long been prone to see signs 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.

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 answer this 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.

The Nature and Causes of Volcanic Action--Modern Volcanoes.

A volcano is a conical or dome-shaped hill or mountain, consisting of materials which have been erupted from an orifice leading down from the surface into the heated interior of the earth. Among modern and recent volcanoes three types may be recognized. In the first and most familiar of these, the lavas and ashes ejected from the central vent have gathered around it by successive eruptions, until they have built up a central cone like those of Etna and Vesuvius. As this cone grows in height and diameter, lateral or parasitic cones are formed on its flanks, and may become themselves the chief actively erupting vents. This type of volcano, which has been so long well known from its Mediterranean examples, was until recently believed by geologists to be the normal, or indeed the only, phase of volcanic energy on the face of the earth.

A modification of this type is to be found in a few regions where fragmentary discharges are small in amount and where the eruptions are almost wholly confined to the emission of tolerably liquid lava. A vast dome with gently sloping declivities may in this way be formed, as in the Sandwich Islands and in certain parts of Iceland.

The second type of volcano is at the present day extensively developed only in Iceland, but in Tertiary time it appears to have had a wide range over the globe, for stupendous memorials of it are preserved in North-Western Europe, in Western America, and in India. It is distinguished by the formation of numerous parallel fissures from which the lava gushes forth, either with or without the formation of small cinder-cones along the lines of the chasms.

Without entering here into a detailed inquiry regarding the nature and causes of Volcanic Action, we may with advantage consider briefly the two main factors on which this action appears to depend.

From all the evidence obtainable it is certain that the temperature of the earth's interior must be high. The rate of increase of this temperature downward from the surface differs from place to place; but an increase is always observed. At a depth of a few miles, every known substance must be much hotter than its melting point at the surface. But at the great pressures within the earth, actual liquefaction is no doubt prevented, and the nucleus remains solid, though at a temperature at which, but for the pressure, it would be like so much molten iron.

Any cause which will diminish the pressure may allow the intensely hot material within the globe to pass into the liquid state. There is one known cause which will bring about this result. The downward increment of temperature proves that our planet is continually losing heat. As the outer crust is comparatively cool, and does not become sensibly hotter by the uprise of heat from within, the hot nucleus must cool faster than the crust is doing. Now cooling involves contraction. The hot interior is contracting faster than the cooler shell which encloses it, and that shell is thus forced to subside. In its descent it has to adjust itself to a constantly diminishing diameter. It can do so only by plication or by rupture.

When the terrestrial crust, under the strain of contraction, is compressed into folds, the relief thus obtained is not distributed uniformly over the whole surface of the planet. From an early geological period it appears to have followed certain lines. How these came to be at first determined we cannot tell. But it is certain that they have served again and again, during successive periods of terrestrial readjustment. These lines of relief coincide, on the whole, with the axes of our continents. The land-areas of the globe may be regarded as owing their existence above sea-level to this result of terrestrial contraction. The crust underneath them has been repeatedly wrinkled, fractured and thrust upward by the vast oceanic subsidence around them. The long mountain-chains are thus, so to speak, the crests of the waves into which the crust has from time to time been thrown.

Again, the great lines of fracture in the crust of the earth probably lie in large measure within the land-areas, or at least parallel with their axes and close to their borders. Where the disposition of the chief ruptures and of the predominant plications can be examined, these leading structural features are found to be, on the whole, coincident. In the British Islands, for instance, the prevalent trend of the axes of folding from early Palaeozoic to Tertiary time has been from south-west to north-east. How profoundly this direction of earth-movement has affected the structure of the region is shown by any ordinary map, in the long hill-ranges of the land and in the long inlets of the sea. A geological map makes the dependence of the scenery upon the building of the rocks still more striking. Not only have these rocks been plicated into endless foldings, the axes of which traverse the British Islands with a north-easterly trend: they have likewise been dislocated by many gigantic ruptures, which tend on the whole to follow the same direction. The line of the Great Glen, the southern front of the Highlands, and the northern boundary of the Southern Uplands of Scotland, are conspicuous examples of the position and effect of some of the greater fractures in the structure of this country.

The ridging up of any part of the terrestrial crust will afford some relief from pressure to the parts of the interior immediately underneath. If, as is probable, the material of the earth's interior is at the melting point proper for the pressure at each depth, then any diminution of the pressure may allow the intensely heated substance to pass into the liquid state. It would be along the lines of terrestrial uplift that this relief would be given. It is there that active volcanoes are found. The molten material is forced upward under these upraised ridges by the subsidence of the surrounding regions. And where by rupture of the crust this material can make its way to the surface, we may conceive that it will be ejected as lava or as stones and ashes.

Viewed in a broad way, such appears to be the mechanism involved in the formation and distribution of volcanoes over the surface of the earth. But obviously this explanation only carries us so far in the elucidation of volcanic action. If the molten magma flowed out merely in virtue of the influence of terrestrial contraction, it might do so for the most part tranquilly, though it would probably be affected by occasional sudden snaps, as the crust yielded to accumulations of pressure. Human experience has no record of the actual elevation of a mountain-chain. We may believe that if such an event were to happen suddenly or rapidly, it would be attended with gigantic catastrophes over the surface of the globe. We can hardly conceive what would be the scale of a volcanic eruption attending upon so colossal a disturbance of the terrestrial crust. But the eruptions which have taken place within the memory of man have been the accompaniments of no such disturbance. Although they have been many in number and sometimes powerful in effect, they have seldom been attended with any marked displacement of the surrounding parts of the terrestrial crust. Contraction is, of course, continuously and regularly in progress, and we may suppose that the consequent subsidence, though it results in intermittent wrinkling and uplifting of the terrestrial ridges, may also be more or less persistent in the regions lying outside these ridges. There will thus be a constant pressure of the molten magma into the roots of volcanoes, and a persistent tendency for the magma to issue at the surface at every available rent or orifice. The energy and duration of outflow, if they depended wholly upon the effects of contraction, would thus vary with the rate of subsidence of the sinking areas, probably assuming generally a feeble development, but sometimes bursting into fountains of molten rock hundreds of feet in height, like those observed from time to time in Hawaii.

The deep-seated magma from which lavas ascend contains various vapours and gases which, under the enormous pressure within and beneath the terrestrial crust, are absorbed or dissolved in it. So great is the tension of these gaseous constituents, that when from any cause the pressure on the magma is suddenly relieved, they are liberated with explosive violence.

A volcanic paroxysm is thus immediately the effect of the rapid escape of these imprisoned gases and vapours. With such energy does the explosion sometimes take place, that the ascending column of molten lava is blown into the finest impalpable dust, which may load the air around a volcano for many days before it falls to the ground, or may be borne in the upper regions of the atmosphere round the globe.

The proportion of dissolved gases varies in different lavas, while the lavas themselves differ in the degree of their liquidity. Some flow out tranquilly like molten iron, others issue in a pasty condition and rapidly congeal into scoriae and clinkers. Thus within the magma itself the amount of explosive energy is far from being always the same.

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