into quartzite I have myself observed a local instance in the metatropic alteration of the basement-beds of the Keuper sandstones of Grinshill, Salop, where they are faulted (as seen in the lane leading from that place to Clive) at a high angle against the Bunter Sandstone. The Bunter Sandstone is altered against the fault into a sort of brick, and the alteration of the Keuper Sandstone into a quartzite extends from twelve to fourteen feet into the Keuper beds, beyond which these assume their normal character. There is a developement of quite large macroscopic crystals of a flesh-coloured silicate along certain zones of the rock near the fault. The phenomena presented in this faulted section seem to me to afford a very good example of the effects of the intensity of heat when suddenly developed by pressure and sliding friction sufficiently concentrated, as in some of the late Mr. Mallet's experiments. It seems reasonable to regard such occurrences as concomitant subterranean phenomena of great earthquakes, and as being no more related to such so-called 'regional metamorphism as is seen in the 'crystalline schists' of the archæan rocks than a sprat is to an elephant or a whale. (See App. ii.) In nature no limit can be recognized in the operation of those changes to which we have assigned the name 'Secondary Paramorphism,' in effecting which water is the chief agency. They are seen in operation not only in rocks commonly called 'metamorphic,' but in igneous (plutonic and volcanic) and sedimentary rocks also. Secondary quartz crystals (as was shown by the late J. A. Phillips, and cases of which I have myself recorded*) are developed on sand-grains; quartz crystals and intergrowths of crystals of quarts and felspar are found developed on the surfaces of blocks of felspathic rocks in some of the older conglomerates, as in those in Euba near Chemnitz quoted by Crednert; crystalline granular aggregates of quartz, orthoclase, oligoclase, mica, and tourmaline are found deposited by mineral waters in fissures; veins are formed in granitoid rocks by the infilling of fissures with secondary minerals dissolved out of the rock-mass, as in the granulitic rocks of Saxony, in the Riesengebirge, in the Isle of Elba, in the gneiss of North America (Credner, op. cit. and the authors there cited.) Deposition of minerals in the drusy cavities of rocks, from which those minerals have been derived, are further examples of this, one of the most universal of nature's operations. * Geol. Mag., dec. ii, vol. X., p. 412. + 'Elemente der Geologie,' 3rd ed., p. 203. To these we may add the cases of secondary change recently described by Bonney (Q.J.G.S., February, 1888,) (i) in the matrix of the Obermittweida conglomerate, (ii) in the matrix of the conglomerate of Sudbury, Canada. "The To sum up, in the words of the writer just quoted,tendency of water universally is either to dissolve the mineralconstituents of a rock directly, or, after decomposition of insoluble compounds, to remove at least some portion of them in solution."** Of the changes considered here under the head of Secondary Paramorphism' it may be said generally, that they are local, partial, and accidental they result in giving varietal differences to different portions of great rock masses, but they are in no way essentially connected with those characters which give to any great rock-mass a definite individuality, as performing a determined function in the developement of the earth's lithosphere. As Judd cogently remarks, (Q.J.G.S., vol. xli, p. 362), "much confusion has been introduced into petrographical literature in consequence of all the characters presented by minerals being treated as if they had precisely the same [degree of] significance. While some of the characters of the rock-forming minerals are original and essential, others are, as certainly, secondary and accidental. The minerals, since their first crystallization, may have undergone several series of changes totally dissimilar in kind, and resulting from causes altogether different." The italics in this quotation are mine. In using this term, we must give ourselves rather more latitude than is allowed in the use of the sister-word 'allotropy' in chemistry. Strictly, allotropy implies the assumption under different physical conditions of a different set of physical properties by one and the same chemical body; and its use is generally confined to the elementary bodies. Here we shall have to preserve the main idea, and, while extending the term metatropy to chemical compounds, and even to mixtures of them in some instances, include under it only those instances in which essential alteration takes place in the physical character (poros) of the rock (hardness, crystalline form, cleavage, fracture, optical properties, conductivity, specific gravity, and so on), while no essential change occurs in its chemical composition. Such changes, it will be seen, may be considered without reference to the original genesis of the rock. The conversion of seam-coal into anthracite (as in the Culm of the Alps and Germany), and that in some cases into graphite, are changes in the physical character of rock-masses which may be designated 'metatropic,' concomitant with other changes in the rocks among which they are interstratified. In this case we have a slight change in chemical composition, with a corresponding increase in the percentage of the elementary carbon, but the chemical composition remains essentially the same. *Roth has discussed the whole thing in his usually masterly way in his Allgemeine und Chemische Geologie. To his laborious collection of facts as chemical data for the study of such phenomena, references are made in the sequel of this work. Ꭰ The formation of palagonite and hydrotachylite out of the glass of basalts by hydration, the change of anhydrite into gypsum by taking up water of crystallization, may perhaps be considered cases of metatropy, as also the conversion of arragonite into calcite by dry heat, and the opposite change of calcite into arragonite by solution and reprecipitation at a higher temperature. Such instances of contact metamorphism' as the conversion by heat of the finer grauwackes and shales into hornstone or porcellanite, the conversion of coal into coke, the conversion of sandstone into quartzite, and the conversion of limestone into marble, are all instances of metatropy. Occasionally there would appear to be a slight and subordinate metataric change also, resulting in the appearance of small inclusions of earthy or marly matter in crystalline marble, unequal distribution of the oxides of the heavy metals, a linear arrangement of authigenous mica in cipollino (as is well seen in the weathered columns of the Roman Forum) the cleavage of some Alpine marbles. In some of these changes heat has been the sole or principal agent; but in the case of marble pressure also was absolutely essential to prevent the destruction of the carbonate of lime by dissociation into calcium oxide and carbonic acid gas. Polymorphism, a phenomenon exhibited by many minerals, often to the extent of rock-masses of considerable magnitude, may be included under the head of metatropy. Here again the study of the mineral constituents of rocks on the chemical side seems to help us, as the facts next to be cited plainly show. (1) Temperature may affect the crystalline form of the mineral, as illustrated in the dimorphism of carbonate of lime, to which reference has been made before. (2) The crystalline form of the same essential compound may vary with the amount of molecular water which its molecule holds in combination, as in the case of carbonate of magnesia, which crystallizes out of an aqueous solution of the salt in carbonated water, when the solution is left to stand in the air, in the following forms: (a) in monoclinic plates with the composition Mg CO2+ 5 H2O in the cold of winter; (b) in a mixture of monoclinic prisms with the com- * Wislicenus, Anorg. Chem., § 600, With the case of the variation of molecular structure along with differences in the proportions of molecular water presented by magnesium carbonate, as observed in the chemical laboratory, may be mentioned the case of the native hydrates of alumina: Hydrargillite, Al2O3+3 H2O [H6 Al2 06] chalcedonic. Bauxite, Al2 O3 + 2 H2O [H4 Al2O5], earthy. Diaspore, Al2O3 + H2 0 [H2 Al2 04], highly crystalline (isomorphous with chrysoberyll (Be Al1⁄2 04). The most crystalline form here, it will be noted, has least water, while in the Mg CO3 series it has the highest proportion of water. In the silicates of copper again we have Dioptase, Cu Si O3 + H2 O, crystalline: Kieselmalachit, CuSi O3+2H20, non-crystalline. (3) The presence of an accessory mineral may influence crystalline form. In the crystallization of carbonate of lime the presence of small quantities of the carbonates of barium and strontium (for example) increases greatly the proportion of arragonite, with which those minerals are isomorphous; a fact which would suggest that the normal crystalline form of the carbonates of the alkaline earths is rather that of the rhombic prisms of arragonite than the hexagonal system, to which the many crystalline forms of calcite may be referred. Recent researches seem to suggest an allotropic relation between certain augites and certain hornblendes. Within the wide range of variation which these minerals severally display in their chemical composition, it is not at all unlikely that for certain percentage-compositions, the same mixture may crystallize under certain physical conditions as augite, and under other physical conditions as hornblende. But have we any right to generalise to the extent of asserting that any augite may undergo a metatropic change into hornblende? A negative answer to this question is suggested by the following average percentages: Augite Hornblende from analyses given by Rammelsberg. Rammelsberg (op. cit. pp. 411, 414) regards both minerals as "isomorphous mixtures of silicates" with the sequi-oxides Fe2O3 and Al2O3, though his earlier notion that the Al2 O3 was present in hornblende as aluminate, may not be altogether untrue; and, if so, this would possibly help to determine the crystalline form of hornblende. More importance perhaps is to be attached to the frequent occurrence in the aluminous hornblendes of such accessory components as TiO2, CrO3, and F, as influencing crystalline form. (Ibid, pp. 416-418.). Again, Epidote and Zoisite are almost allotropic forms. In this connection may be mentioned those curious globular concretionary structures with a radiating prismatic subcrystalline texture-suggestive to a chemist's mind of crystal lization out of supersaturated solutions-which abound in the Magnesian Limestones of Durham, occurring sometimes nearly a foot in diameter, while smaller ones often make up the entire rock-mass of whole beds of limestone, containing 98 per cent. of carbonate of lime.* Of all the changes which we should call metatropic those of vitrification and devitrification of rock (as, e.g., obsidian and tachylite) are by far the most complicated. It is pretty generally recognized that the former of these is the result of solidification by rapid cooling. Much obscurity however still hangs over the latter phenomenon; but perhaps the consideration of a few known chemical facts may go a little way towards penetrating it, and will lead us to recognise a change of molecular structure as the essential part of the process of devitrification; though the physicist and the chemist pronounce with one voice our ignorance of the actual constitution of the molecules of solid bodies. Two of the best-known instances of allotropy are those of the elements sulphur and phosphorus.† Both these bodies may undergo such a metatropic change as to assume an allotropic modification, which we are justified in calling the vitreous condition.' I have for some years used this term with reference to them, and observe that this use of it is gradually finding its way into text-books of chemistry. In both cases the vitreous condition is induced by rapid cooling; common or vitreous phosphorus, by the arrangement employed for casting it into sticks; sulphur, when poured in a molten state at temperatures not far below its boiling point into cold water. The translucent flexible needle-shaped prisms which are formed when sulphur solidifies in the dry way at the higher temperature of its fusing point (115°C) must be regarded as a vitreous form, so far as their internal texture goes, since they are quite isotropic. That in the vitreous condition of both these bodies the molecular structure is not the most permanent or stable appears from several considerations. Vitreous sulphur (both in the plastic and in the prismatic state) is known to assume the crystalline state in forms of the ortho-rhombic system in a few days, the translucent mass becoming opaque (devitrified) as the result of crystallization. * They must not be confounded with concretionary dolomitic 'mudstones' found in other parts of the same rocks, on the weathered surface of which a laminated structure can be easily traced. In the summer of 1886, to the surprise of Mr. Howse, of the Newcastle Museum, I split some of these with the hammer, and disclosed casts of Axinus within them. + Carbon, another allotropic element, is omitted here, as it is not known in the glassy or isotropic modification; but we do know that in its crystalline state (Diamond, Sp. Gr. 3.5) its density is greater than in the amorphous state (Charcoal, Sp. Gr. 1·82) (cf. Note L, App. ii.). They are in fact crystallites. |