But this wearing away of the structure of a volcanic cone by the denuding forces may proceed even one stage farther, and we may then have revealed for our inspection and study the mass of originally fluid materials, from which one or more volcanoes have been fed, cooled and consolidated in their original reservoir. There are many examples of masses of granitic or highly crystalline rocks, having precisely the same composition as the different varieties of lavas, which are found lying in the midst of the sedimentary rocks, and sending off into these rocks veins and dykes of the same composition with themselves. No one who has carefully studied the appearances presented by volcanic mountains in different stages of dissection, by the action of denuding forces, can avoid recognising these great granitic masses as the cooled reservoirs from which volcanoes have in all probability been supplied during earlier periods of the earth's history.

The eruption of these great masses of incandescent rock, impregnated with water and acid gases, through strata of limestone, sandstone, clay, coal, &c., may be expected to produce striking and wonderful chemical changes in the latter. Nor are we disappointed in these anticipations. Whenever we examine the sedimentary materials around volcanic vents, we find that, in contact with the once-fused materials, they everywhere exhibit remarkable evidences of the chemical action to which they have been subjected. The limestones are converted into statuary marble, the sandstones pass into quartzite, the days assume the hardness and lustre of porcelain, while the coals have lost their volatile ingredients and assumed a form like coke or graphite. And these changes are found to extend in many cases to the distance of many hundreds of yards from the planes of junction between the igneous and the sedimentary materials.

Among the most interesting effects resulting from the extrusion of masses of incandescent rock, charged with water and various gases, through beds of limestone, clay, sandstone, &c., we may mention the production of those beautiful crystalline minerals which adorn our museums and are so highly prized as gems. By far the larger part of these beautiful minerals have been formed, directly or indirectly, by volcanic agencies.

These gems and beautiful minerals are, for the most part, substances of every-day occurrence, which entirely owe their beauty to the crystalline forms they have assumed. The diamond is crystallised carbon, the ruby and sapphire are crystallised alumina, the amethyst and a host of other gems are crystallised silica; and in almost all cases the materials of gems are common and widely diffused, it is only in their finely crystalline condition that they are rare and therefore valuable.

Crystals are formed during the slow deposition of a substance, either by the evaporation of a liquid in which it is dissolved, by its volatilisation, or its cooling from a state of fusion. In many cases it can be shown that the formation of large and regular crystals is aided if the work goes on with extreme slowness and under great pressure. By sealing up various substances in tubes containing water which can be kept at a high temperature, minute crystals of many well-known minerals have been artificially formed by chemists. Part of the water converted into steam has formed a powerful spring, which, reacting upon the remainder of the liquid in the tube, has subjected it to enormous pressure, and under these conditions of extreme pressure and temperature, chemical actions take place of which we have no experience under ordinary circumstances. The experiments of Mr. Hannay seem to prove that when carbon is separated from certain organic substances at a high temperature and under great pressure, it may crystallise in the form of the diamond. And the recent discovery of diamonds in the midst of materials filling old volcanic vents in South Africa seems to show that this was in many cases the mode in which the gem was originated. Even under the conditions which prevail at the earth's surface, however, minute and unnoticed chemical actions taking place during long periods of time, produce most remarkable results. This has been well illustrated by M. Daubrée, who has shown that in the midst of masses of concrete which the Romans built up around the hot springs of Plombières and other localities, many crystalline minerals have been formed, in the course of 2,000 years, by the action of the waters upon the ingredients of the concrete.

But most of the crystals of minerals which have been thus artificially formed are of minute, indeed often of microscopic, dimensions. In the underground reservoirs beneath volcanoes, however, we have all the necessary conditions for the formation of crystals of minerals on a far grander scale. High temperatures, pressures far greater than any we can command at the earth's surface, the action of superheated steam and many acid gases on the various constituents of both igneous and sedimentary rocks, and, above all, time of almost unlimited duration; these constitute such a set of conditions as may fairly be expected to result in the formation of crystals, similar to those artificially produced but of far greater size and beauty.

If we visit those parts of the earth's surface where great masses of fused volcanic rock have slowly cooled down in contact with sedimentary materials, we shall not be disappointed in our expectations. Diamonds, rubies, sapphires, emeralds, topazes, garnets, and a host of equally beautiful, if less highly prized, crystalline substances, are found in such situations, lying in the subterranean chemical laboratories in which they have been formed, but now, by the action of denuding forces, revealed to our view.

In some cases it is not necessary to penetrate to these subterranean laboratories in order to find these beautiful gems and other crystallised minerals; for the steam jets which issue from volcanic fissures carry up fragments of rock torn from the side of the vent, and in the cavities and fissures of such ejected masses beautiful crystallised products are often found. Such rock-fragments containing minerals finely crystallised are found abundantly on the flanks of Vesuvius and other active volcanoes, and among the materials of the Laacher See and other extinct volcanoes.

But it is not only the finely crystallised minerals and gems which we owe to volcanic action. The various metallic minerals have nearly all been brought from deep-seated portions of the earth's crust and deposited upon the sides of rock-fissures by the agency of the same volcanic forces. It is these forces which have, in the first instance, opened the cracks through the solid rock masses; and, in the second place, have brought the metallic sulphides, oxides, and salts—either in fusion, in solution, or in a vaporised condition—from the deep-seated masses within the earth, causing them to crystallise upon the sides of the fissures, and thus form those metallic lodes and veins which are within reach of our mining operations.

There is still one other important class of minerals which owe the existence, though indirectly, to volcanic agencies. The cavities of igneous rocks, especially the vesicles formed by the escape of steam, constitute, when filled with water, laboratories in which complicated chemical reactions take place. The materials of the lava are gradually dissolved and re-crystallised in new combinations. By this means the most beautiful examples of such minerals as the agates, the onyxes, the rock-crystals, the Iceland-spars, and the numerous beautiful crystals classed together as 'Zeolites' have been formed. No one can visit a large collection of crystalline minerals without being struck with the large number of beautiful substances which have thus been formed as secondary products from volcanic materials.