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.
FORMATION OF MINERAL-VEINS.
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.
CHAPTER VI.
THE VARIOUS STRUCTURES BUILT UP AROUND VOLCANIC VENTS
From what has been said in the preceding chapters it will be seen that while some of the materials ejected from volcanic vents are, by the movements of the air and ocean, distributed over every part of the face of the globe, another, and by far the larger, part of the matter so ejected, accumulates in the immediate vicinity of the vent itself. By this accumulation of erupted materials, various structures are built up around the orifices from which the ejections take place, and the size and character of these structures vary greatly in different cases, according to the quantity and nature of the ejected materials, and the intensity of the eruptive forces by which they were thrown from the orifice. We shall proceed in the present chapter to notice the chief varieties in the forms and characters of the heaps of materials accumulated round volcanic vents.
These heaps of materials vary in size from masses no bigger than a mole-heap up to mountains like Etna, Teneriffe, and Chimborazo. The size of volcanic mountains is principally determined by the conditions of the eruptive action at the vent around which they are formed. If this action exhausts itself in a single effort, very considerable volcanic cones, like the Monte Nuovo with many similar hills in its vicinity, and the Puys of Auvergne, may be formed; but if repeated eruptions take place at longer or shorter intervals from the same vent, there appears to be scarcely any limit to the size of the structures which may, under such conditions, be formed. It is by this repeated action from the same volcanic vent going on for thousands or even millions of years, that the grandest volcanic mountains of the globe have been built up. Such volcanoes have sometimes a diameter at their base of from 30 to 100 miles, and an elevation of from 10,000 to 25,000 feet.
The form of volcanic mountains is determined in part by the nature of the materials ejected, and in part by the character of the eruptive action.