the valley, as its excavation has destroyed the continuity of the beds. Now, it is generally acknowledged that these valleys were formed by the last great diluvian action; therefore we have a convenient and natural division of extinct volcanoes into ante and post diluvian.

For the sake of illustration, let us suppose fig. 59. to

represent a chain of hills, in which the outliers a, b, and c are composed of the same volcanic rock. It is evident, we think, from their situation and dip, that they were once continuous, and that the catastrophe which excavated the valleys destroyed the intervening portions of the bed: therefore the valley is antediluvian. But if we find the cone d and its contiguous valley to be covered with a bed of lava, there will be no danger in ascribing to it a postdiluvian origin.

It is not uncommon to find volcanic rocks, or, at least, rocks which so much resemble those which are now produced by volcanoes, that few geologists, reasoning by analogy, doubt their igneous origin, alternating with, or interposed between, stratified deposits. The rocks of this class are generally known by the name of trap, and consist of greenstone, basalt, porphyry, &c. The rocks of the Pic du Midi de Bigorre (Schubert), for instance, particularly the limestones, are generally superposed by trap. At Christiania, in Norway, slate and grauwacke are covered by a bed of porphyry, not less than 1600 ft. in thickness; " and in another part of the country, at Holmestrand, the same mass of porphyry, covering beds of sandstone, is seen to pass in the lower part, by almost insensible gradations, into a hard, fine-grained, black basalt, containing brilliant crystals of augite." (Bakewell's Geology.) In the Island of Skye, red sandstone is traversed by a great number of trap veins, and in some places is superposed by this rock. The alternation of rocks observed in Lamlash is sandstone, trap, conglomerate, and clinkstone, the whole of which is intersected by a vein of spheroidal trap. (Macculloch's Western Isles.)

Active Volcanoes may be divided into aerial and subaqueous, or those which have their craters exposed to the action of the air, and those which are under the water. There must be a considerable difference between the phenomena which attend these classes; namely, in proportion to the density of the obstructing medium. At present, however, our knowledge of the phenomena which attend subaqueous activity is exceedingly circumscribed: we shall, however, separately consider the phenomena which precede and accompany the activity of these two classes of active volcanoes.

The points of enquiry that most forcibly strike the attention, when thinking upon the volcanic agent, are the phenomena which precede and attend its activity, the geographical position of volcanic mountains, and the character of the substances they eject. We are then, by a natural train of thought, led to enquire what can be the cause of these destructive phenomena, and what the methods by which volcanic mountains are formed and destroyed. These are the subjects which will chiefly demand our attention in the following pages; and will, we hope, develope all the most important and striking facts connected with the subject.

The Phenomena which precede volcanic Action. — In every age of the world there have been some who, separating themselves in part from the common amusements of mankind, have devoted their leisure to the investigation of natural phenomena. These have searched into the secrets of Almighty Intelligence, and gazed in admiration upon the beauty of his creatures, and the contrivance displayed in their formation. But the more solemn and destructive of his agents, in their moments of sublime excitement, have drawn the attention of those who were unimpressed by Nature in her periods of beautiful repose. The accuracy of this remark will be admitted by those who have never witnessed a more violent disturbance than a passing thunder-storm; but how much more fully would it be received by those who have gazed on their native hills when torn by the impetuous tide of subterranean fire! It is the importance which all men attribute to unfrequent phenomena, and particularly when of a fearful character, whether they are seen, or known by tradition, that has furnished us with an ample fund of information concerning the many volcanic mountains with which every quarter of the globe abounds.

*We have preferred the term aerial to sub-aerial, because the subaqueous may be said to be under the air as well as those exposed to the action of the atmosphere.

From these observations it appears that the phenomena which precede volcanic eruptions of the same class, universally present a remarkable similarity of character, though they greatly vary in power. The energy chiefly depends upon the force required to open a passage for the liquefied mass, and the attendant gaseous fluids. Hence it is that the phenomena which result from the activity of a subaqueous volcano are probably very different from those which attend an aerial eruption. Particular attention, therefore, must always be paid to the situation of the vent, and the density of the fluid resisting the volcanic energies. On this account it is necessary that we should in this and the following section take a separate view of these classes.

If the vent of a habitual volcano, whether aerial or submarine, be much obstructed by the accumulation of a large quantity of the lava resulting from a previous eruption, greater power will be required to force the heated matter through the crust of the earth, than if no such obstruction were presented. And if it be necessary to form an entirely new vent, a still greater force must brought into action, and that in proportion to the solidity, position, and weight of the superposed mass. There is not, we believe, a single instance within the range of historical records, in which a new vent has been formed in a country never before the seat of volcanic action. It is not uncommon to hear of the eruption of volcanoes that have been supposed extinct, nor is it improbable that entirely new vents are sometimes formed in the neighbourhood of those situations where the same power has before exerted its energies. In both these cases the great obstruction presented to the volcanic agent by superposed rocks has been removed by former eruptions, and therefore but little increase of power is required to force a passage for the melted materials. We might,

perhaps, deduce from this fact, either that the volcanic energy has decreased, or that there is such a connection in the subterranean abyss, that the weakest portion of the earth's crust is broken, which is a volcanic vent: but we forbear to theorise.

Having premised these general observations, we may now enquire into the character of those phenomena which precede volcanic eruption. The most invariable of these is earthquake. The same night that Lima was destroyed by earthquake, four new volcanic vents were formed in the Andes. In the year 447, the earth was convulsed for six months, almost without intermission, from the Black to the Red Sea; and on the 20th of May, 520, Antioch was destroyed, and 250,000 of the inhabitants buried in its ruins. Soon after the earthquake at

Lisbon in 1755, various parts of the world suffered under the effect of a similar cause, and within a few succeeding years there happened some of the most violent eruptions that ever afflicted the world. In 1759 the American continent was dreadfully agitated, and Jorullo was in a condition of violent eruption, attended with some most remarkable phenomena. In 1760, Vesuvius opened at once fifteen fissures, and from each vomited immense volumes of lava; and during the same year Kattlagiaa, in Iceland, broke out with intense paroxysmal violence, attended with such electric phenomena as were never before witnessed. Thirty days after the destruction of the city of Caraccas, the volcano of St. Vincent became active; and at the moment it broke forth, a subterranean noise was heard, and the earth was shaken for nearly 2200 square leagues. From these, and numerous other similar facts which we might have selected, it will be evident that earthquake of most alarming character frequently precedes volcanic eruption; but it is only when the activity is very intense that the earth is thus severely shaken. We should, perhaps, be warranted by facts, in stating that the violence of the earthquake is always a criterion to the intensity of the subsequent eruption; but, however this may be, we do not remember an instance in which volcanic action has not been preceded by earthquake. But earthquake has frequently been unattended with eruption, which may have resulted either from a want of sufficient energy to form a vent, or from the escape of the gases generated by the subterranean heat.

The violence of an earthquake seldom lasts more than a minute; but successive shocks are sometimes felt at very short intervals. During the agitation immense chasms are frequently formed, through which flames, torrents of water, or dense volumes of gaseous fluid, are thrown. The effects, therefore, of earthquakes, independent of the volcanic action which usually follows, are of a most extensive and dreadful character, and little inferior to the actual ejection of lava. A more appalling description of the consequences of an earthquake cannot be given than a relation of the circumstances that followed this awful catastrophe in 1638, as observed by Kircher. The father was on his way to Euphemia; but so violent was the agitation of the water, and so dreadful the noises which seemed to issue from it, that it was found impossible to proceed, and he landed with his companions at Lopizicum. "Here," he says, "scenes of ruin appeared everywhere around me; but my attention was quickly turned from more remote to contiguous danger, by a deep rumbling sound, which every moment grew louder. The place where we

stood shook dreadfully. After some time the violent paroxysm ceased. I stood up, and turning my eyes to Euphemia, saw only a frightful black cloud. We waited till it passed away, when nothing but a dismal and putrid lake was to be seen where the city once stood."

If such are the effects of earthquakes, it is not surprising that the inhabitants of those countries in which they have been felt, should be seized with an extreme terror at the least agitation of the earth. Captain Basil Hall states, in his interesting Voyage to Chili and Peru, that when paying a visit at Valparaiso, the whole party was suddenly thrown into a state of violent alarm, and rushed into the street, crying, Misericordia! misericordia! The whole population seemed to have been aroused by the same feeling; the streets were filled with people in a state of wild confusion and alarm. The cause of this instantaneous terror was, our traveller afterwards found, a violent shock of earthquake, though he was perfectly unconscious of the least motion. But Humboldt describes the feeling experienced on the coast of Peru in a far different manner. "From our infancy," he says, "the idea of certain contrasts fixes itself in our mind; water appears to us an element that moves, earth a motionless and inert mass. These ideas are the effects of daily experience; they are connected with every thing that is transmitted to us by the senses. When a shock is felt, when the earth is shaken on its old foundations, which we had deemed so stable, one instant is sufficient to destroy long illusions. It is like awakening from a dream; but a painful awakening. We feel that we have been deceived by the apparent calm of nature; we become attentive to the least noise; we mistrust, for the first time, a soil on which we had so long placed our feet with confidence. If the shocks be repeated, if they become frequent during successive days, the uncertainty quickly disappears. In 1784, the inhabitants of Mexico were accustomed to hear the thunders roll beneath their feet, as we are to witness the vivid flash in the region of the clouds. Confidence easily springs up in the human mind; and we end by accustoming ourselves, on the coast of Peru, to the undulations of the ground, like the sailor to the tossing of the ship caused by the motion of the waves." (Pers. Nar, vol. iii. p. 321.)

The shocks of an earthquake are invariably most violent in volcanic countries, but not in those parts which are nearest to the seat of volcanic action. The effects, however, are sometimes very extensive. During the earthquake at Lisbon in 1755, the waters in every part of Europe were agitated,