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unmoved upper strata that “they form a bridge.” As the mountain-chains appear to be raised on fissures, the waters of the cavities may perhaps favour the direction of undulations parallel to them; occasionally, however, the waves of commotion intersect several chains almost perpendicularly.
FEELINGS ON FIRST EXPERIENCING AN EARTHQUAKE.
Before we leave the important phenomena which we have considered, not so much in their individual characteristics as in their general, physical, and geognostical relations, I would advert to the deep and peculiar impression left on the mind by the first earthquake which we experience, even where it is not attended by any subterranean noise. This impression is not, in my opinion, the result of a recollection of those fearful pictures of devastation presented to our imaginations by the historical narratives of the past, but is rather due to the sudden revelation of the delusive nature of the inherent faith, by which we had clung to a belief in the immobility of the solid parts of the Earth. We are accustomed, from early childhood, to draw a contrast between the mobility of water and the immobility of the soil on which we tread; and this feeling is confirmed by the evidence of our senses. When, therefore, we suddenly feel the ground move beneath us, a mysterious and natural force with which we are previously unacquainted is revealed to us as an active disturbance of stability. A moment destroys the illusion of a whole life; our deceptive faith in the repose of nature vanishes, and we feel transported, as it were, into a realm of unknown destructive forces. Every sound—the faintest motion in the air-arrests our attention, and we no longer trust the ground on which we stand. Animals, especially dogs and swine, participate in the same anxious disquietude ; and even the crocodiles of the Orinoco, which are at other times as dull as our little lizards, leave the trembling bed of the river, and run with loud cries into the adjacent forest.
To man the earthquake conveys an idea of some universal and unlimited danger. We may flee from the crater of a volcano in active eruption, or from the dwelling whose destruction is threatened by the approach of the lava stream ; but in an earthquake, direct our flight whithersoever we will,
we still feel as if we trod upon the very focus of destruction. This condition of the mind is not of long duration, although it takes its origin in the deepest recesses of our nature; and when a series of faint shocks succeed one another, the inhabitants of the country soon lose every trace of fear. On the coast of Peru, where rain and hail are unknown no less than the rolling thunder and the flashing lightning, these luminous explosions of the atmosphere are replaced by the subterranean noises which accompany earthquakes. Long habit and the very prevalent opinion that dangerous shocks are to be apprehended only two or three times in the course of a century, cause faint oscillations of the soil to be regarded in Lima with scarcely more attention than a hail-storm in the temperate zone.
VOLCANOES. A volcano, properly so called, exists only where a permanent connexion is established between the interior of the Earth and the atmosphere, and the reaction of the interior on the surface then continues during long periods of time. It may be interrupted for centuries, as in the case of Vesuvius, and then manifest itself with renewed activity. In the time of Nero, men were disposed to rank Etna among the volcanic mountains which were gradually becoming extinct; and subsequently Ælian3 even maintained that mariners could no longer see the sinking summit of the mountain from so great a distance at sea. Where these evidences—these old scaffoldings of eruption, I might almost say—still exist, the volcano arises from a crater of elevation, while a high rocky wall encloses, like an amphitheatre, the isolated conical mount, and forms around it a kind of casing of highly elevated strata. Occasionally not a trace of this enclosure is visible, and the volcano, which is not always conical, rises immediately from the neighbouring plateau in an elongated form, as in the case of Pichincha, at the foot of which lies the city of Quito.
The vast range of subject in the Kosmos forces us to omit many interesting topics ; the internal heat of the earth, for example, and its varied electro-magnetic phenomena, with the polar streams of light in the Aurora-borealis. In connexion with earthquakes and volcanoes, the limitation of our space compels the omission of the subject of gaseous emanations from the earth, such as those of sulphuretted-hydrogen, and carbonic acid (mofettes) ; of mud volcanoes (salses! ; of hot springs, &c.
2 The first historical eruption is that of 79 A.D., by which Herculaneum and Pompeii were destroyed.
3 A teacher of rhetoric at Rome in the reign of the emperor Adrian. He wrote fourteen books of various history; he compiled treatises on animals; and is the alleged author of a work on Greek Tactics.
As the nature of rocks, or the mixture of simple minerals into granite, gneiss, and mica slate, or into trachyte, basalt, and dolerite, is independent of existing climates, and is the same under the most varied latitudes of the Earth ; so also we find everywhere in inorganic nature that the same laws of configuration regulate the reciprocal superposition of the strata of the Earth's crust, cause them to penetrate one another in the form of veins, and elevate them by the agency of elastic forces. This constant recurrence of the same phenomena is most strikingly manifested in volcanoes. When the mariner, amid the islands of some distant archipelago, is no longer guided by the light of the same stars with which he had been familiar in his native latitude, and sees himself surrounded by palms and other forms of an exotic vegetation, he still can trace, reflected in the individual characteristics of the landscape, the forms of Vesuvius, of the dome-shaped summits of Auvergne, the craters of elevation in the Canaries and Azores, or the fissures of eruption in Iceland. A glance at the satellite of our planet will impart a wider generalization to this analogy of configuration. By means of the charts that have been drawn in accordance with the observations made with large telescopes, we may recognise in the Moon, where water and air are both absent, vast craters of elevation, surrounding or supporting conical mountains, thus affording incontrovertible evidence of the effects produced by the reaction of the interior on the surface, favoured by the influence of a feebler force of gravitation.
Although volcanoes are justly termed in many languages “ fire emitting mountains," mountains of this kind are not formed by the gradual accumulation of ejected currents of lava; but their origin seems rather to be a general consequence of the sudden elevation of soft masses of trachyte or labradoritic augite. The amount of the elevating force is manifested by the elevation of the volcano, which varies from the inconsiderable height of a hill to that of a cone above 19,000 feet in height. It has appeared to me that relations of height have a great influence on the occurrence of eruptions, which are more frequent in low than in elevated volcanoes.
Whilst the small volcano Stromboli? has
* Ancient Strongyik
been incessantly active since the Homeric ages, and has served as a beacon-light to guide the mariner in the Tyrrhenian Sea, loftier volcanoes have been characterized by long intervals of quiet. Thus we see that a whole century often intervenes between the eruptions of most of the colossi which crown the summits of the Cordilleras of the Andes.
These relations between the absolute height and the frequency of volcanic eruptions, as far as they are externally perceptible, are intimately connected with the consideration of the local conditions under which lava currents are erupted. Eruptions from the crater are very unusual in many mountains; they generally occur from lateral fissures, wherever the sides of the upheaved mountain are least able, from their configuration and position, to offer any resistance. Cones of eruption are sometimes uplifted on these fissures; the larger ones, which are erroneously termed new volcanoes, are ranged together in a line, marking the direction of a fissure, which is soon reclosed, whilst the smaller ones are grouped together, covering a whole district with their dome-like or hive-shaped forms.
When volcanoes are not isolated in a plain, but surrounded, as in the double chain of the Andes of Quito, by a table land, having an elevation of from nine to thirteen thousand feet, this circumstance may probably explain the cause why no lava streams are formed during the most dreadful eruptions of ignited scoriae, accompanied by detonations heard at a distance of more than a hundred miles. The height of the cone of cinders, and the size and form of the crater, are elements of configuration which yield an especial and individual character to volcanoes, although the cone of cinders and the crater are both wholly independent of the dimensions of the mountain. Vesuvius is more than three times lower than the Peak of Teneriffe; its cone of cinders rises to onethird of the height of the whole mountain, whilst the cone of cinders of the Peak is only o of its altitude. Amongst all the volcanoes that I have seen in the two hemispheres, the conical form of Cotopaxi is the most beautifully regular. A sudden fusion of the snow at its cone of cinders announces the proximity of the eruption. Before the smoke is visible in the rarified strata of air surrounding the summit and the opening of the crater, the walls of the cone of cinders are sometimes in a state of glowing heat, when the whole mountain presents an appearance of the most fearful and portentous blackness. The crater which, with
few exceptions, occupies a summit of the volcano, forms a deep cauldronlike valley, which is often accessible, and whose bottom is subject to constant alterations. The greater or lesser depth of the crater is in many volcanoes likewise a sign of the near or distant occurrence of an eruption. Long narrow fissures, from which vapours issue forth, or small roundish hollows filled with molten masses, alternately open and close in the cauldron-like valley; the bottom rises and sinks, eminences of scoriae and cones of eruption are formed, rising sometimes far over the walls of the crater, and continuing for years together to impart to the volcano a peculiar character, and then suddenly fall together and disappear during a new eruption. The openings of these cones of eruption, which rise from the bottom of the crater, must not, as is too often done, be confounded with the crater which incloses them. If this be inaccessible from extreme depth and from the perpendicular descent, as in the case of the volcano of RucuPichincha, which is 15,920 feet in height, the traveller may look from the edge on the summit of the mountains which rise in the sulphurous atmosphere of the valley at his feet; and I have never beheld a grander or more remarkable picture than that presented by this volcano.
Volcanoes which, like the chain of the Andes, lift their summits high above the boundaries of the region of perpetual snow, present peculiar phenomena. The masses of snow, by their sudden fusion during eruptions, occasion not only the most fearful inundations and torrents of water, in which smoking scoriae are borne along on thick masses of ice, but they likewise exercise a constant action whilst the volcano is in a state of perfect repose, by infiltration into the fissures of the trachytic rock; cavities which are either on the declivity or at the foot of the mountain, are gradually converted into subterranean reservoirs of water, which communicate by numerous narrow openings with mountain streams, as we see exemplified in the highlands of Quito. The fishes of these rivulets multiply especially in the obscurity of the hollows, and when the shocks of earthquakes which precede all eruptions in the Andes have violently shaken the whole mass of the volcano, these subterranean caverns are suddenly opened, and water, fishes, and tuffaceous mud are all ejected together. It was through this singular phenomenon that the inhabitants