Pit-coal and common salt, and almost all the metals, likewise occur in veins. Some veins are filled with water-worn pebbles, as one observed by Werner at Joachimstahl *. Some are filled with loam t. Nay, they even sometimes contain petrifactions. Thus the Baron de Born describes a petrified popites which he saw in a compact cinnabar vein in Hungary; and Mr. de Schlottheim communicated an account of a still more remarkable appearance of the same kind to Werner. In a calcareous mountain in Thuringia, there occur veins of marl five or six inches thick, containing petrifactions differing altogether from those which are found in the lime-stone. The petrifactions found in the marl are, cornua ammonis, terebrates, and turbinites; while those that occur in the lime-stone rock are trochites. Beds of the marl occur in the neighbourhood; and these beds contain the same petrifactions that are found in the veins t. 3. It is very common to find veins crossing each other in the same rock. When that happens, one of the veins may be traced passing through the other without any interruption, and cutting in two, while the other always separates, and disappears at the point of crossing. 4. Such is a short sketch of the most remarkable phænomena respecting veins. Werner supposes that they were originally fissures formed in the rocks, and that they were all gradually filled by minerals depositsd slowly from above, while the rocks in which they occur were covered by water, and that they were filled at the same time that the different formations were deposited. This theory he has supported in his book on Veins, by a very complete enumeration of all the circumstances respecting their structure and appearances. He has shown that they resemble fissures very exactly in their shape and direction; and that as they contain petrifactions and minerals altered by the action of water, they must of necessity have been filled from above. Veins of course, according to this theory, are newer than the rocks in which they occur; and when two veins cross, that is obviously the newest which traverses the other without interruption, as the fissures constituting the second vein must have been formed after * Werner, Nouvelle Theorie, p. 81. + Ibid. p. 82. Ibid. p. 88. the first vein was filled up. When different veins contain the same minerals arranged in the same order, he conceives that they were filled at the same time, and says that such veins belong to the same formation. When they differ in these respects, they belong to different formations. From the position of the respective veins with respect to each other, he deduces their relative age; and from this draws inferences respecting the relative age of the different mineral substances that occur in veins similar to the inferences drawn respecting the age of the rocks which constitute the grand classes of formations. [Thomson's System of Chemistry, Vol. IV. occasionally altered.] CHAP. VIII. VOLCANOES, EARTHQUAKES, AND SUBTERRANEAN FIRES. THAT fires to an enormous extent, and produced by various causes, may exist at different depths beneath the surface of the earth, must, we think, be clear to every one who has attentively perused the preceding chapters of the present book: and we have much reason to believe, from a very curious series of experiments lately conducted by Sir James Hall, that, where the substances in which such fires occur lie profound, and are surmounted by a very deep and heavy super-incumbent pressure; and, more especially, where they, at the same time, contain large portions of elastic gasses; the effects of such fires will be prodigiously greater, and more diversified, than where these circumstances are absent. Earthquakes and volcanoes may be reckoned, for the most part, among the most powerful and extraordinary of these effects; and as resulting from those chemical changes which the agency of fire principally produces in the interior of the solid crust of the globe. They have, probably, little further connection with electricity than as causes that occasionally destroy the equilibrium; for although some authors have inferred, from the great velocity with which the shock of an earthquake is transmitted from place to place, that its nature must be electrical, yet others have, with greater probability, attributed the rapid succession of the effects to the operation of a single cause, acting like subterranean heat, at a great distance below the earth's surface. There are, however, some circumstances which indicate such a connexion between the state of the atmosphere and the approach of an earthquake, as cannot easily be explained by any hypothesis. The shocks of earthquakes, and the eruptions of volcanoes, are in all probability modifications of the effects of one common cause; the same countries are liable to both of them; and where the agitation produced by an earthquake extends further than there is any reason to suspect a subterraneous commotion, it is probably propagated through the earth nearly in the same manner as a noise is conveyed through the air. Volcanoes are found in almost all parts of the world, but most commonly in the neighbourhood of the sea; and especially in small islands; for instance, in Italy, Sicily, Iceland, Japan, the Caribbees, the Cape Verd islands, the Canaries, and the Azores: there are also numerous volcanoes in Mexico and Peru, especially Pichincha and Cotopaxi. The subterraneous fires, which are continually kept up in an open volcano, depend perhaps in general on sulphureous combinations and decompositions, like the heating of a heap of wet pyrites, or the union of sulphur and iron filings but in other cases they may perhaps approach more nearly to the nature of common fires. A mountain of coal has been burning in Siberia for almost a century, and must probably have undermined in some degree the neighbouring country. The immediate cause of an eruption appears to be very frequently an admission of water from the sea, or from subterraneous reservoirs; it has often happened that boiling water has been discharged in great quantities from a volcano; and the force of steam is perhaps more adequate to the production of violent explosions, than any other power in nature. The consequence of such an admission of water, into an immense collection of ignited materials, may in some measure be understood, from the accidents which occasionally happen in founderies: thus a whole furnace of melted iron was a few years ago dissipated into the air in Colebrook Dale, by the effect of a flood which suddenly overflowed it. The phænomena of earthquakes and volcanoes are amply illustrated by the particular accounts, transmitted to the Royal Society by Sir William Hamilton, of those which have happened at different times in Italy. The earthquake, which desolated Calabria, in 1783, was fatal to about 40,000 persons, continuing its ravages for more than three months; it destroyed the towns and villages occupying a circle of nearly 50 miles in diameter, lying between 38 and 39 degrees latitude, and extending almost from the western to the eastern coast of the southernmost point of Italy, besides doing considerable damage to places at much greater distances from its origin, which is supposed to have been either immediately under the town of Oppido, in the centre of this circle; or under some part of the sea, between the west of Italy, and the volcanic island of Stromboli. This island, as well as Mount Etna, had smoked less than usual before the earthquake, but they both exhibited appearances of an eruption during its continuance; Etna towards the beginning, and Stromboli at the end. Before each shock the clouds were usually motionless for a certain time, and it rained violently; frequently also lightning and sudden gusts of wind accompanied the rain. The principal shocks appeared to consist in a sudden elevation of the ground to a considerable height, which was propagated somewhat like a wave, from west to east: besides this, the ground had also a horizontal motion backwards and forwards, and in some measure in a circular direction. This motion was accompanied by a loud noise; it continued in one instance for ten seconds without intermission : and it shook the trees so violently that their heads nearly reached the ground. It affected the plains more strongly than the hills. In some places luminous exhalations, which Sir William Hamilton thinks rather electrical than igneous, were emitted by the earth; the sea boiled up near Messina, and was agitated as if by a copious discharge of vapours from its bottom; and in several places water, mixed with sand, was thrown up to a considerable height. The most general effect of these violent commotions was the destruction of buildings of all kinds, except the light barracks of wood or of reeds, into which the inhabitants retreated as soon as they were aware of their danger: the beds of rivers were often left dry, while the shock lasted, and the water on its return overflowed their banks; springs were sometimes dried up, and new ones broke out in other places. The hills which formed the sides of steep vallies were often divided by deep chasms parallel to the vallies; and in . many cases large portions of them were separated, and removed by the temporary deluge to places half a mile or a mile off; with the buildings and trees still standing on them; and in this manner hills were levelled, and vallies were filled up. But the most fatal accident of this kind happened at Scilla, where so large a portion of a cliff was thrown into the sea, that it raised an immense wave, which carried off more than 2000 inhabitants who were collected on the beach, and even extended its formidable effects to the opposite coast of Sicily, where several persons perished by it in a similar manner. The eruptions of volcanoes are usually attended by some shocks like those of earthquakes, although commonly less violent. Open volcanoes continually throw out in more or less abundance, smoke, ashes, and pumice stones, or light cinders; but their most formidable effects are produced by a torrent of ignited lava, which, like a vast deluge of liquid or semiliquid fire, lays waste the country over which it runs, and buries all the works of human art, In March, 1767, Vesuvius began to throw out a considerable quantity of ashes and stones, which raised its summit in the course of the year no less than 200 feet, forming first a little mountain of pumice stones within the crater, which by degrees became visible above its margin. The smoke, which was continually emitted, was rendered luminous at night, by the light derived from the fire burning below it. In August some lava had broken through this mountain, and in September it had filled the space left between it and the former crater. On the 13th and 14th of October there were heavy rains, which perhaps supplied the water concerned in the eruption that shortly followed. On the morning of the 19th, clouds of smoke were forced, in continual succession, out of the mouth of the volcano, forming a mass like a large pine tree, which was lengthened into an arch, and extended to the island of Caprea, 28 miles off; it was accompanied by much lightning, and by an appearance of meteors like shooting stars. A mouth then opened below the crater, and discharged a stream of lava, which Sir William Hamilton ventured to approach within a short distance, imagining that the violence of the confined materials must have been exhausted; but on a sudden the mountain opened with a great noise at much lower point, about a quarter of a mile from the place where he stood, and threw out a torrent of lava, which advanced straight towards |