260 ditlicultly fusible lcucite may be seen to have enclosed crystals already formed of the fusible augite. In many ancient crystallin-3 rocks the pyroxenic constituents, which offer a less resistmce to fusion, have assumed a crystalline form before the more refr-a':tor_v triclinic felspars. 1"rom these facts it is clear that., in the fusion of rocks aml in their subsequent C')[lS0lltl'.ltl0ll, there must have been con- ditions under which the normal order of appearance of the minerals was disturbed or reversed. Yet another fact may be mentioned to show further the diflerence between the kind of fusion which has frequently obtained in nature and that of the ordinary operations of a glass-work or iron- furnace. As far back as the year 1846 Scheerer showed that there exist in granite various minerals which could not have consolidated save at a comparatively low tem- perature. He instanced especially several gadlolinites, orthites, and allanites, which cannot endure a hig 1er tem- perature than a dull-red heat without altering their physical characters; and he concluded that granite, though it may have possessed a high temperature, cannot have solidified from simple igneous fusion. U E () L O G - , . . I
e may conclude, therefore, that the manner in which
rocks have been melted within the crust is not that mere simple fusion which we can accomplish artificially, but that it has involved conditions which have not been successfully imitated in any laboratory or furnace. Two obvious differences must occur to the reader between the natural and artificial operations. In the first place, rocks which hive undoubtedly once been in a fluid or at least pasty con- dition, and which have been injected as veins and dykes into previously consolidated masses, contain water imprisoned within their component crystals. This is not water which has been subsequently introduced. It is contained in minute cells, which it usually does not now completely fill, but which it no doubt did occupy completely at the time and temperature at which the rock was consolidated. “'0 have seen (rmte, p. 250) how abundant are the discharges of water- vapour from volcanic fires, how the molten lava-streams issue from their vents, saturated, as it were, with steam, and how the steam continues to rise fro1n them long after they have congealed and come to rest. In the solid crystals of lava which were erupted only recently, as well as in those of early geological periods, the presence of water in minute cavities may be readily detected. It is in the quartz of such rocks, and still more in that of granite, that the detection of Water-cavities is most easily made. The quartz of granite is usually full of them. “A thousand millions,” says .Ir J. Clifton Ward, “might easily be con- tained within a cubic inch of quartz, and sometimes the contained water must make up at least 5 per cent. of the whole volume of the containing quartz.” Thus microscopic investigation confirms the conclusion arrived at by Scheerer in the memoir already cited, that at the time of its eruption granite must have been a kind of pasty mass containing a considerable proportion of water. It is common now to speak of the “ aquo-igneous" origin of some eruptive rocks, and to treat their production as a part of what is termed the “ hydro-thermal ” operations of geology. We may conclude that, while some rocks, like obsidian and pitchstone, which so closely resemble artificial glasses, may have been derived from a simple igneous fusion such as can be imitated in a furnace (though even in these the presence and influence of water may be traced), the great majority of rocks have had a more complex origin, and in a great number of cases can be proved to have been mingled with more or less water while they were still fluid. In the second place, there can be no question that, in the great hypogene laboratory of nature, rocks have been softened and fused under enormous pressure. Besides the pressure due to their varying depth from the s rface, they Y must have been subject to the enormous expansion of the superheated water or vapour which filled all their cavities. Mr Sorby has ingeniously estimated the probable pressure under which granite consolidated by taking the ratio between the size of the liquid cavities in the quartz and that of the contained bubble or vacuity. Assiuning the temperature of consolidation to have been 680° Fahr. (300 Cent. ), or a dull-red heat, he inferred that in many cases the pressure under which the granite consolidates must have been equal to that of an overlying mass of rock 50,000 feet, or more than 9 miles, in thickness. It is not probable that any such thick overlying mass ever did cover -the granite; the pressure, even if it be allowed to have been so great, must have been due partly to other causes, such as the compression due to secular contraction. It would appear therefore that perfect anhydrous fusion, or the reduction of a rock to the state of a completely homogeneous glass, has been a comparatively rare process in nature, or at least that such glasses, if originally formed, have in the vast majority of cases undergone’ devitrification and crystallization, until the glassy base has been reduced to a smaller or larger fraction of the total mass of the rock, or has entirely passed into a stony condition. In many volcanic rocks the original vitreous base or ground—mass can be readily observed with the microscope between the (lefinitely—formed crystals. Crystallites, or arrested st iges in the crystallization of iron oxides and of silicates, can often be detected in these rocks, more especially where it is evident that they must have cooled with COlllpCtl'.1l.l'C rapidity, as where they have been thrust into narrow [u1. ])YI'.-.ll(‘.L. ' fissures to form dykes. But i11 rocks such as granite, where no glass exists, but where the mineral constituents are all crystalline, no trace of the crystallites occurs. Doubtless such differences point to original distinctions in the kind and degree of fusion of the rocks. It seems reasonable to suppose that those rocks which showa glassy grouml-inass, and the presence of crystallites, have been fused under conditions more nearly resembling those of the simple igneous fusion of experiment. It has long been known that many mineral substances can be obtained in a crystalline form from the condensation of vapours. This process, called sublimation, may be the result of the mere cooling and reappearance of bodies which have been vaporized by heat and solidify on cooling, or from the solution of these bodies in other vapours or gases, or from the reaction of different vapours upon each other. These operations are of common occurrence at volcanic I ‘vents, and in the crevices of recently erupted and still hot lava-streams. They have been successfully imitated by experiment. In the early researches of Sir James Hall on the effects of heat modified by compression, he obtained by s11blin1atio11 “transparent and well-defined crystals,” lining the unoccupied portion of a l1ermetically-sealed iron t11be, in which he had placed and exposed to a high temperature some fragments of limestone (Trans. 1t’o_z/. Soc. ]:'dz'n., vi. ll0). .'umerous experiments have been made by Messrs I)elcsse, I )aubree, and others, in the production of Illlllt-I‘[1l5 by sublimation. Thus many of the metallic sulphides found in mineral veins have been produced by e.'posing to a comparatively low temperature (between that of boiling water and a dull-red heat) tubes containing metallic chlorides aml sulphide of hydrogen. By varying the materials employed, eorundum, quartz, apatite, and other minerals have been obtained. It is not diflicult, therefore, to understand how, in the crevices of lava—streams and volcanic cones, as well as in some mineral veins, sulphides and oxides of iron and other minerals may have been formed by the ascent of heated vapours. Superheated steam is endowed with a remarkable power of dissolving that
intractable substance, silica ; artificially heated to the tem-
