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Show CORRELATION OF COMPOSITION AND TEXTUEE. 91 importance to the differences in this respect which may exist between the various groups. Still, we have the investigations of Daubeny, Deville, and Mallet, which are so far concordant that they indicate decisively the existence of a true relation. The acid rocks have decidedly higher melting temperatures than the basic rocks. Many blast-furnace slags approach the volcanic rocks in constitution, a,nd the great amount of experience gathered in iron-smelting amply confirms the same relation so far as the cases are fairly comparable. We may, with considerable confidence, state as an approximate truth that the melting temperatures of volcanic rocks have a direct ratio to their acidity. The textures of volcanic rocks are no doubt due in part to peculiarities of chemical constitution. The vitreous character of the rhyolites, the coarse, harsh texture of the trachytes, the compact, fine-grained texture and peculiar fracture of the andesites and basalts are surely in due a great measure to their constitution, but how or why we do not know. There is, however, another sense in which texture is ordinarily spoken of, and to which high importance is attached, and this sense takes account of the degree or extent to which the groundmass of a rock is crystallized. By far the most important difference between a volcanic and a non-eruptive plutonic rock, so far as pure petrographic considerations are concerned, consists in the fact that the plutonic non-eruptive rock is wholly crystalline, while the volcanic rock is only partially so. Otherwise the two kinds might be quite indistinguishableâ€"might consist of the same constituents. This distinction, depending upon the extent of crystallization, however, is of great importance, since it arises in all probability from causes associated with the genesis and geological evolution of the rocks themselves. The nature and properties of the silicates are such, that under the conditions ordinarily existing their crystallization is attended with difficulty and proceeds very slowly. An indispensable requisite for crystallization is mobility of molecules inter se, and for this mobility a liquid condition of the magma is essential. But' the silicates possess the following peculiarity : at a temperature sufficiently high to render them very liquid crystallization is impossible; at a temperature just low enough for crystallization, they are exceedingly viscous and the mobility very much impeded. The crystals, |