of an elevated temperature beneath the surface of the ground, more intense at some points than at others, seems not so certain. The boracic acid is found in combination with ammonia, as well as free, and Dr. Daubeny remarks that its presence in the steam may arise from the aqueous vapour passing over this substance, and carrying it upwards in mechanical suspension, as steam, by experiment, has been found capable of effecting.*

By employing the heat of the superabundant vapour, the water collected in artificial ponds is sufficiently evaporated to dispense with fuel, and the boracic acid obtained at small cost. These lagunes furnish about 1,650,000 lbs. of boracic acid annually, sufficient, when purified and mixed with soda, forming borax, nearly for the supply of Europe.-Daubeny, "Volcanos," p. 156.

CHAPTER XXII.

EARTHQUAKES.- CONNEXION OF VOLCANOS AND EARTHQUAKES.-EXTENT OF EARTHQUAKES.-MOVEMENT OF THE EARTH-WAVE DURING EARTHQUAKES. SEA-WAVES PRODUCED DURING EARTHQUAKES.-COMPLICATED TRANSMISSION OF EARTHQUAKE WAVES.— UNEQUAL TRANSMISSION OF EARTHQUAKES. LOCALLY EXTENDED RANGE OF EARTHQUAKES. EARTHQUAKES TRAVERSING MOUNTAIN CHAINS. FISSURES PRODUCED DURING EARTHQUAKES.-SETTLEMENT OF UNCONSOLIDATED BEDS ADJOINING HARD ROCKS DURING EARTHQUAKES.— BREAKING OF GREAT SEAWAVE, OF EARTHQUAKES, ON COASTS.-EFFECTS OF EARTHQUAKES ON LAKES AND RIVERS.-FLAME AND VAPOUR FROM EARTHQUAKE FISSURES. -SOUNDS ACCOMPANYING EARTHQUAKES.-ELEVATION AND DEPRESSION OF LAND DURING EARTHQAKES.-COAST OF CHILI RAISED DURING EARTHQUAKES.-EFFECTS OF EARTHQUAKES IN THE RUNN OF CUTCH.

IT has been seen that prior to, and sometimes during volcanic eruptions, the country in the vicinity has been disturbed by vibrations, as if from time to time certain resistances to volcanic forces were suddenly overcome. The rending of rocks by fissures, such as have been previously noticed, could scarcely but produce vibrations, supposing the needful tension and cohesion of parts. It is by no means required that these fissures should always rise to the surface of the ground; indeed, in many volcanic accumulations, the rents formed, and subsequently filled with molten rock, are observed to terminate before they reach it. From the absence of the proper cohesion of parts amid great masses of ashes and cinders, these may so yield, that though a fissure might be suddenly produced in more solid matter beneath them, they could adjust themselves above in a very general manner over its upward termination.

It would be anticipated that, all other things being equal, vibrations of the ground around volcanos would be more intense after a vent had long been closed and dormant, so that time for the consolidation of tuff beds had elapsed, the whole well braced together by lava streams of various dimensions, than when the vent was

still open, the volcano active, and the ashes and cinders incoherent. It may also be inferred that a certain thickness of trachyte, dolerite, or basalt, if not too much divided by columnar, or other joints, would offer greater resistance to any given volcanic force employed than tuff beds, unless these were so changed and consolidated as to assume the character of palagonite, or others of that class. Again, different effects would be expected from the resistance of intermingled sheets of tuff and rocks which had been in fusion, such as those described as occurring in the Val del Bove, Etna, and where similar substances are mixed, as narrow lava streams and irregular piles of matter, in both cases prior fissures, more or less filled by dykes of lava, considerably modifying the effects produced.

A connexion has often been inferred to exist between volcanic eruptions and vibrations of the ground at distances far beyond the immediate vicinity of the former, as if the volcanos were great safety-valves, through which, under ordinary circumstances, a certain amount of force escaped, mere local disturbances being thereby produced; while at others, from the overloading of the valves, or a greater exertion of power, larger portions of the earth's crust were shaken. Without including dormant or extinct volcanos, active vents are so widely dispersed over different parts of the world, that considerable areas may readily be disturbed by vibrations more or less depending upon general conditions, of which the discharge of molten rock, vapours, and gases, at certain points, is only one of the effects thereby produced. Hence, as respects this mode of viewing the subject, volcanic eruptions and earthquakes may be intimately connected, volcanic eruptions being equally regarded in the same general manner, and other adjustments of the earth's surface included, by which great fissures have been formed, and huge masses of rocks squeezed, broken, and thrust up into great ridges and mounds of varied forms and magnitude.

Many instances are given of the inferred connexion between earthquakes and volcanic eruptions, as, for example, the sudden disappearance of smoke in the volcano of Pasto, when the province of Quito, 192 miles distant, was so violently shaken by the great earthquake of Riobamba, on the 4th of February, 1797, and the sudden tranquillity of Stromboli from its otherwise constant activity, during the great earthquake in Calabria, in 1783. As we are quite assured that in minor areas there is often much vibration of the ground prior to such eruptions, and that subsequently to them

tranquillity is restored, at least for a time, an observer would be led to inquire how far such apparent causes and effects may be extended. Herein caution is much needed, so that, from a preconceived opinion, accidental circumstances may not have an undue value assigned them, some of the inferences drawn respecting the immediate connexion between given earthquakes and the eruptions from certain volcanos being scarcely borne out by the facts adduced.

It would be anticipated that in regions of volcanos, such as those of South America, great vibrations of the ground should be experienced, these vibrations extending to variable distances, not only according to their intensity, but also to the kinds of rocks through which they are transmitted. In certain regions earthquakes are sometimes of such frequent occurrence, that except when of particular intensity they are so little regarded, that these, and similarly circumstanced portions of the earth's surface, may be considered in a more unstable state than others. The great earthquake of Chili, in 1835, was merely one of a more intense kind in a district often shaken by such vibrations. It is described as having been felt from Copiapo to Chiloe in one direction, and from Mendoza to Juan Fernandez in another; and the volcanos of that part of the Andes are noticed as having been in an unusual state of activity immediately prior to, during, and subsequent to it. In a previous earthquake (1822) the same region of South America was shaken through a distance, from north to south, of about 1,200 miles.

With respect to the areas actually disturbed by earthquakes, as waves are necessarily raised by them in the sea adjoining the lands shaken, or by the vibration of the rocks beneath it, attention has to be directed as to the amount of dry land moved, and the extent to which any adjoining portion of the sea-bottom may have been simultaneously shaken. For instance, this has to be done with the great earthquake of Lisbon, the area disturbed being represented as spread over a large portion of the Northern Atlantic, and comprising a part of North America, with some of the West India Islands (Antigua, Barbadoes, and Martinique) on the one side, and a part of Northern Africa and a large portion of Western Europe on the other. In such a case the extent to which the sea-wave produced by earthquakes may have been propagated, has to be well considered.* The known amount of dry land shaken

Sir Charles Lyell (Principles of Geology, 7th edit., p. 344), calls attention to the great Lisbon shock, as having come in from the ocean, remarking that "a line drawn

in Europe was alone very large, comprising Portugal, Spain, France, the British Islands, the southern portions of Norway and Sweden, Denmark, Germany, Switzerland, and the north of Italy.

As respects earthquakes, the transmission of the vibrations has to be regarded with especial reference to the kind of substances through which an earthquake-wave may have to pass, so that, even, for illustration, assuming the impulses given to be equal, the extent of the vibrations and their amount might be very materially modified.* Mr. Mallet infers that an earthquake "is the transit of a wave of elastic impression in any direction, from vertically upwards to horizontally in an azimuth, through the crust of the earth, from any centre of impulse, or from more than one, and which may be attended with tidal and sound waves, dependent upon the impulse, and upon the circumstances of position as to sea and land." At the same time, he admits that the truth of this view has not yet been fully and experimentally demonstrated.

The movement of the great earth-wavet is commonly classed as undulatory or vertical, as the ground may be observed to roll onward in a given direction, or simply rise and fall in a nearly perpendicular manner. We have descriptions, in the one case, of the surface of the ground moving in a wave-like manner, and in the other, of a mere sudden rise and fall, as far as regards a particular locality. Of the latter the great earthquake experienced at Riobamba, in 1797, would appear an excellent example, many bodies of the inhabitants having, according to Humboldt, been hurled to a height of several hundred feet on the hill of La Cullca, beyond the small river of Lican. We may readily infer that these two classes of earthquake movements are only modifications of the

through the Grecian Archipelago, the volcanic region of Southern Italy, Sicily, Southern Spain, and Portugal, will, if prolonged westward through the ocean, strike the volcanic group of the Azores;" hence inferring, as probable, their submarine connection with the European line.

* Mr. Mallet (Naval Manual of Scientific Enquiry, Art. Earthquakes, p. 197), in order to illustrate the transmission of waves through different materials, supposes a person to stand upon a line of railway, near the rail, and that a heavy blow be struck upon the latter a few hundred feet distant. "He will," Mr. Mallet remarks, "almost instantly hear the wave through the iron rail; directly after he will feel another wave through the ground on which he stands; and, lastly, he will hear another wave through the air; and if there were a deep side-drain to the railway, a person immersed in the water would hear a wave of sound through it, the rate of transit of which would be different from any of the others--all these starting from the same point at the same time."

+ (Admiralty Manual of Scientific Enquiry, Art. Earthquakes).- Mr. Mallet defines the "great earth-wave" as the "true shock, a real roll or undulation of the surface travelling with immense velocity outwards in every direction from the centre of impulse."

"Kosmos," Art. Earthquakes.