Page:Popular Science Monthly Volume 17.djvu/552

536 have been gathered at Santorin in the interior of tubular cavities like fulgurites, which have evidently been traversed by vapors at a very high temperature. The deposited crystals are of anorthite, sphene, and pyroxene under the forms of fassaïte and augite.

Other crystallized silicates are met occasionally in the lavas. They arise from the transformation of the calcareous masses which are taken from the subsoil in the region and carried out in the lavas, and consist chiefly of anorthite, fassaïte, the melanite garnet, and Wollastonite. The quartz and mica-schist, which, like the limestone, appear as inclosed lumps in the lava of Santorin, do not seem to have suffered any action from the matter surrounding them, notwithstanding the high temperature it possessed.

The mineralogical study of the lavas of 1866 offered great difficulties, on account of the small size and strong adherence of the mineral integrants. It was not practicable to extract the crystals by picking them out, and M. Fouque was obliged to seek new processes. One, founded on the employment of a powerful electro-magnet to draw out the ferruginous minerals, permitted the isolation of the feldspar; and other minerals, harder to deal with, were separated by the use of concentrated hydrofluoric acid.

The crystallization of minerals in the recent lava took place in two stages. In the first stage were developed crystals which frequently attained the length of about a fiftieth of an inch, and of which the other dimensions exceeded a tenth of that size; and, in the other stage, crystals of notably small dimensions were produced. The latter crystals, or microliths, unite the others and follow their contour. Before the microscope was applied to the study of the rocks, only the larger crystals were recognized, and the matter which in volcanic rocks envelops crystals large enough to be perceived through the glass was supposed to be wholly uncrystalline. In this matter, however, the microliths abound in immense quantities. The discovery of them has been one of the most signal triumphs of microscopic micrography.

The minerals observed in the microliths of the general lava of 1866 are feldspar and titaniferous oxide of iron. The predominant feldspar in large crystals is the labrador, but anorthite, and some oligoclase and sanidine are also found in the same condition. The microlithic feldspar is albite with a considerable proportion of oligoclase, and the whole is cemented with a vitreous matter which represents the residue of the crystallization, the part of the rock to which the latter owed its fluidity, at the moment when the minerals which had made a portion of it were already crystallized. This amorphous matter is of a composition similar to that of albite, but a little richer in silica and potassa; and it is curious that a substance of such a composition should have constituted the part of the rock which remained longest melted.

The order of crystallization of the minerals was as follows: 1. Magnetic oxide of iron in large crystals; 2. Apatite; 3. Silicated