stances, gradually evaporating, leaves the earthy matter behind it; this substance constitutes volcanic tuff. SECT. III-Of Veins. Veins are mineral repositories which cut throngh the strata or beds of which a mountain is composed, and which are filled with substances more or less different from the rocks through which they pass. We shall have a very distinct notion of veins, if we suppose that the mountains in which they occur were split by some means or other, and that the rifts thus formed were filled up by the matter which constitutes veins. They are distinguished from beds by their direction, which is either perpendicular to the stratifications, or at least forms an angle with it. Sometimes the strata through which veins pass are merely separ rated from each other; so that if we cut through the vein we find the same strata of the rock on both sides of it: but sometimes also the corresponding strata on one side are lower than on the other, as if the portion of the rock on one side of the vein had sunk a little, while the portion on the other side kept its original position. In such cases, the side of the rock against which the vein leans, or the floor of the vein, has always its strata highest up; whilst the strata of the portion of rock which leans over the vein, or the roof of the vein, are always lowest. So that this is the portion which appears to have sunk. Such a change of position in the strata is known in this country by the name of a shift. In considering veins, there are two circumstances which claim our attention: namely, 1. The shape of veins; and, 2. The substances with which they are filled. 1. All those mineralogists who have had the best opportunity of examining the shape of veins with correctness, agree in representing them as widest above, and as gradually diminishing in size as they deepen, till at last they terminate in a point, exactly as if they had been originally fissures. This is the account of Oppel, of Werner, and indeed of all those writers who have been professionally engaged in superintending mines. Sometimes, indeed, veins widen in different parts of their course, and afterwards contract again to their former size; but more commonly they continue diminishing gradu ally to their extremity. 2. Sometimes these veins are either partially or entirely empty. In that case they are denominated fissures; but most commonly they are filled with a matter more or less different from the rock through which they pass. Sometimes the vein is filled up with one species of mineral. Thus we have veins of calcareous spar, of quartz, &c.; but when it is of any size, we frequently find a variety of substances: these are disposed in regular layers always parallel to the sides of the vein, and they follow in their position a very regular order. One species of mineral constitutes the centre of the vein: on each side of this central bed the very same layers occur in the same order from the centre to the side of the vein. To give an example; the vein Gregorius, at Freyberg, is composed of nine layers or beds. The middle of the vein consists of a layer of calcareous spar; on each side of this is a layer consisting of various ores of silver mixed together; on each side of this a layer of brown spar; on each side of this a layer of galena; on each side of this again, and contiguous to the side of the vein, is a layer of quartz. The following sketch will give the reader some notion of the relative position of these layers: Gneiss rock. Gneiss Sometimes the number of layers of which a vein is composed greatly exceeds this. Werner describes one in the district of Freyberg, in which the middle layer is calcareous spar, having on each side of it no less than thirteen layers arranged in the very same order⚫. Almost every mineral substance which occurs in the mass of rocks has been found in veins. We sometimes find them filled with different well-known stony bodies. Thus veins of granite, porphyry, lime-stone, basalt, wacke, green-stone, &c. are not uncommon †; veins of quartz, clay, felspar, &c. are equally common. • Nouvelle Theorie de la Formation des Filons, p. 100. 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 aminonis, 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 veius 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. 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 |