Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/273

Rh EFFECTS or nE.A.'r.] in the meantime of only very partial solution. Leaving this question for the present, let us examine the effects which are clearly referable to the inﬂuence of heat. It is evident that the conditions under which these effects are produced must vary almost inﬁnitely. Ve have to consider (1) the temperature, from the lowest at which any change is possible up to that of complete fusion ; the nature of the rock operated upon, some materials being much more susceptible of change from heat than others; (3) the pressure under which the heat acts, the potency of this agency being much increased with increase of pressure ; (4) the presence of water, whereby chemical changes take place which would 11ot be possible in dry heat. In an interesting series of experiments the illustrious De Saussure (1779) fused some of the rocks of Switzerland and France, and inferred from them, contrary to the opinion pre- viously expressed by Dcsmarest-,1 that basalt and lava have not been produced from granite, but from hornstone (pierre de corne), varieties of “ schorl,” calcareous clays, marls, and niicaceous earths, and the cellular varieties from different kinds of slate.‘-’ He observed, however, that the artiﬁcial products obtained by fusion were glassy and enan1el—like, and did not always recall volcanic rocks, though some exactly resembled porous lavas. Sir James Hall, about the year 1790, began an important investigation, in which he succeeded in reducing various ancient and modern volcanic rocks to the condition of glass, and in restoring them, by slow cooling, to a stony state. Since that time many other researches of a more complicated kind have been undertaken, especially by Delesse, Daubrée, Deville, Bunsen, Bischof, H. and W. Rogers. By these observations it has been abundantly proved that all rocks undergo molecular changes when exposed to high tempera- ture, that when the heat is sufficiently raised they becon1e ﬂuid, that if the glass thus obtained is rapidly cooled it remains vitreous, and that, if allowed to cool slowly, a 1nore or less distinct crystallization sets in, the glass is devitrified, and a lithoid product is the result. Illustrations of the inﬂuence of different degrees of heat upon rocks of different kinds may often be very instruct- ively observed at lime—kilns, especially those roughly—built kilns or pits which may still be met with in outlying dis- tricts. Some of the stones lining such cavities will be found with no sensible change, others show a somewhat cellular texture, others have acquired a rudely prismatic structure, while some have had their surfaces fused into a rough glaze or enamel. The bricks or stones used for lining furnaces present similar illustrations, the columnar or prismatic structure being occasionally very perfectly developed in these materials. Mere contact with a highly-heated surface, and subsequent gradual cooling, have often produced this prismatic arrangement in rocks of the most diverse kinds. Thus, in Scotland, beds of sandstone and seams of coal have acquired" the most perfect prismatic structure from the intrusion of basalt dykes or sheets through them. To these and other effects of intruded igneous rocks reference will be made in the sequel. In these cases the alteration is merely local, and has obviously been produced by contact with a highly heated surface. But, besides such minor effects due merely to contact, others of a more general kind affect large masses of rock or whole districts of country. When rocks are exposed to temperatures as high as their melting-points, they fuse into glass which, i11 the great majority of cases, is of a bottle-green or black colour, the depth of the tint depending mainly on the proportion of iron. In this respect they resemble the natural glasses— pitchstones and obsidians. They almost always contain 1 Mam-. Acacl. Sc£en., 177], p. 273. '3 Dc Saussure, Voyages dams lcs Alps, edit. l803, tome i. p. 178. GEOLOGY 259 minute cells or bubbles, arising probably from the disen- gagement of water or of oxygen during the fusion. But after the most thorough fusion which has been found possible, minute granules usually appear in the solidiﬁed glass. Sometimes these consist of specks of quartz, which is especially apt to remain unmelted when the rock is very siliceous, or of other minerals of the original rock, But a microscopic investigation of fused rocks shows that, even in what seems to be a tolerably homogeneous glass, there are abundant minute hair-like, feathered, needle-shaped, or irregularly-aggregated bodies diffused through the glassy paste. These cr_2/stall-ites are in some cases colourless silicates, in others they are opaque metallic oxides, particu- larly oxides of iron. They precisely resemble the crystal- lites which are observed in many volcanic rocks, such as obsidian, pitchstone, and basalt. They may be obtained even from the fusion of a granitic or granitoid rock, as in the well—known case of the Mount Sorrel syenite near Leicester, which, being fused and slowly cooled, yielded abundant crystallites, including exquisitely grouped octo- hedra of magnetite. According to the observations of Delesse, volcanic rocks, when reduced to a molten condition, attack briskly the sides of the Hessian crucibles in which they are contained, and even eat them through. This is an interesting fact, for it helps to explain how some intrusive igneous rocks have come to occupy positions previously ﬁlled by sedi- mentary strata, and why, under such circumstances, the composition of the same mass of rock should be found to vary considerably from place to place!‘ It would appear that, i11 passing from a crystalline into a vitreous condition, rocks undergo a diminution of density, which, on the whole, is greater the more silica and alkali are present, and is less as the proportion of iro11, lime, and alumina increases. According to Delesse, granites, quartzi- ferous porphyries, and such highly silicated rocks lose from b‘ to 11 per cent. of their density when they are reduced to the condition of glass, basalts lose from 3 to 5 per cent., and lavas, including the vitreous varieties, fron1 0 to 4 per cent. This diminution of density (and consequent increase of volume) may account for minor oscillations of the ground so often observed in volcanic regions. If we suppose a sub- terranean mass of lava 500 feet thick to pass from the ﬂuid to the crystalline condition, this might cause a subsidence of the ground above to the extent of about 20 or 25 feet. The transition of a similar mass of 1‘ock from the solid to the ﬂuid condition would cause an uprise to a like amount. So far as experiments have yet been conducted, it has been found impossible to obtain fron1 a piece of fused rock a crystalline substance exactly resembling the original mass. Externally it may appear quite stony, but its internal structure, as revealed by the microscope, shows it to be essentially a slag or glass, and not a truly crystalline rock. There is another fundamental difference between the natural and artiﬁcial products. When a compound containing sub- stances of different fusibilities is artiﬁcially melted, and allowed thereafter to cool in such a way that the various in- gredients may separate from each other, they appear in their order of fusibility, the most refractory coming first, and the most fusible being the last to take a solid form. But in rocks which have crystallized naturally from a ﬂuid condition, it is often to be observed that the component minerals have been far from obeying what might have been supposed to be their invariable law. Thus, in all parts of the world, granite presents the very striking fact that its quartz, which we call an infusible mineral, has actually solidiﬁed after the more fusible felspar. In the Vesuvian lavas the 3 Bull. Soc. Géol. France, 2d ser., iv. 1382.