prepared salts of metals by its very great stability. It only begins to undergo dissociation (under a pressure of one atmosphere) at temperatures above 300° C, and is only attacked very feebly at ordinary temperatures by dilute acids. Thus chemistry settles in a very definite manner the question as to the modus operandi of dolomitization; and its answer is in accord with observations in the field on loss of bulk of the rock-mass.* A further confirmation of this is found in the high percentage of carbonate of lime which springs issuing from dolomitic limestones contain, and the enormous quantity of calcareous travertine which they deposit. This latter fact can hardly fail to force itself upon the attention of an observer, who in the light of chemical ideas will examine the magnificent coast-sections of the magnesian limestone of Durham. It also explains the formation of dolomitic sandstones out of the residual dolomitic sand of the rock-mass of which the Mansfield freestone is a well-marked and well-known instance. It perhaps accounts, as I suggested in a discussion in Section C of the British Association four years ago, for the great quantity of carbonate of lime which is mingled with the fine siliceous Alpine detritus in the Löss of the Upper Rhine country the derivation of this mineral from the regions of the head-quarters of the Rhine and the dolomitization of the limestones of those regions being thus seen to be concomitant phenomena. The comparative readiness with which carbonate of lime undergoes dissociation enables us to see how heat may act as an important factor in dolomitization within certain limits: lime, one product of dissociation being readily taken up as a soluble hydrate by pure water, and carbonic acid (the other product of dissociation) contributing to the solvent activity of contiguous water within the neighbouring rock. These principles taken into consideration along with the great tendency of magnesium to form double salts seem to throw some light upon the obscure process by which the drusy cavities of so-called "potato-stones are lined with a deposit of pure dolomite or bitter-spar, in the dolomite rocks of the magnesian limestone of England and in the Rauchwacke of Thüringen and other regions of the continent. Hyperphoric + change as illustrated by dolomitization includes then all those simple processes in which the character of the rock is changed by the removal of one or more mineral constituents from, or the introduction of one or more constituents * Dwellers on the Magnesian Limestone districts of Durham are from time to time unpleasantly reminded of this by the Erdfälle which occur as a consequence of it, cavernous portions of the rock collapsing into brecciated masses. † ὑπερφερω. into, the rock-mass. The active agencies concerned (partly chemical, partly physical) vary in different cases. But it will be seen that hyperphoric change is the essential principle of such minor processes of metamorphism' as (1) The conversion in some instances of a vesicular dolerite into an amygdaloid, especially where the dolerite is intrusive among limestones. (2) The gradual formation of gigantic masses of nearly pure rock-salt (NaCl) as in the Eastern Alps and in the Stassfurt district. In the latter district the massive saline deposits are known to reach a thickness of 400 metres, of which the lowest 200 metres and more are said to consist of pure rock salt. How was this formed? Apparently not by original deposition from concentrated sea-water or mineral-springs, since in all these other salts are invariably present. The chemistry of solubilities seems to help us here. We have only to recollect that the coefficient of solubility of Na Cl in water is only very slightly affected by temperature (increasing only from 34 to 36 % for a rise of temperature from 0° to 30°C) while the coefficients of solubility of the other salts contained in sea-water and in the upper layers of the saline deposits of Stassfurt and other districts, increase rapidly (that of sylvine for example from 30 to 38% and that of Glauber salt from 5 to nearly 40 %) for an equal elevation of the temperature of the water. The application of these facts as involving hyperphoric change is obvious enough, on the assumption that the temperature of the water in the lower beds is higher than that of the water in the beds above. (3) One of the most interesting and best understood instances of hyperphoric change is the derivation of the selenite found in clays from pyrites, involving a transfer of the element sulphur. We have in this case (i.) introduction of free oxygen in solution in atmos pheric water, with a consequent oxidation of the sulphide of iron into a soluble hydrated sulphate of that metal, and a transfer of the mineral thus formed by water; (ii.) oxidation of the base into a hydrated peroxide, which is precipitated, while the acid radicle is carried away as free sulphuric acid; (iii.) a direct chemical reaction, CaCO,+H,SO, CaSO, + H2CO, as soon as the free sulphuric acid comes into contact with carbonate of lime, the sulphate of lime thus formed crystallizing as selenite. I have observed very fine examples of this in the clays exposed in railway-cuttings near Grantham, and in other places. As pointed out in the introductory section of this work, changes such as we have here named 'hyperphoric' do not (acting alone) constitute true metamorphism; yet it must be admitted that in many cases, and especially in contact-metamorphism,' they play an important part conjointly with other agencies. The allotment of a short space to their consideration seems therefore to be justified. This part of our subject has been so thoroughly studied as to need only a brief consideration here. The phenomena presented by it exhibit the operation of all the four principles discussed in the foregoing sections of this work: a. Paramorphic change (§ ii); d. Hyperphoric change (§ v). There is one consideration however involved in the study of contact-metamorphism, which-so far as I know the literature of the subject-does not appear to have had the attention bestowed upon it which it deserves; that is the factor of time, upon which Lyell laid so much stress. We have no right to assume that all the results, which we can recognise to-day in any region in which the rocks have undergone the metamorphism which can be traced to the contiguity or proximity of intrusive masses of igneous rock, are traceable directly to its action. We have to consider also the possibility of further changes occurring after those forces, which the intrusion of the igneous mass called into action, ceased to operate. This will be seen as we proceed. If we try to work out the probable sequence of changes (in outline) it is not difficult to conceive of them as falling into several distinct stages. First stage. Direct effects of heat and pressure. From the principle of dissipation of energy it follows that the glowing intrusive mass must continue to impart heat to the neighbouring rocks until equalization of temperature is reached. The glowing mass having to make room for itself must displace the neighbouring rocks; and, as results of this, fracture, crushing, contortion, and shearing must follow. Also by transmission of *The terms 'exogenous' and 'endogenous' have been used to denote respectively the appearance of new minerals in the adjacent rocks and in the intrusive mass. (See Kalkowsky, Lithologie, p. 33). the pressure cleavage may be induced in rocks of the outer zone of the region affected. The proportion of these will vary in different instances according to the nature and condition of the rocks acted upon; the more rigid rocks suffering more crushing and fracture, those in a more pasty condition suffering more shearing. There will also be some variation in these results in different parts, according to the direction in which the pressure is applied to them. In every case mechanical force is transformed into heat, and this is added to that received by conduction from the glowing intrusive mass, together with that produced by the friction of the intrusive mass against the rocks into which it is intruded. The direct effects of all this would be mainly metataxic and metatropic in their nature. As metatropic results we may note such as vitrification, fritting and baking, both of the adjacent rocks and the fragments caught up and enclosed in the molten mass, the results being often most marked in the latter. In many cases they are melted directly into a glass, in others crushed and recemented by a glassy cement. Some are molten wholly or on the exterior into slags, of which latter I have a good example which I obtained some years ago from the ash-cone of the Pulvermaar in the Eifel; others are rendered porcellanic in their hardness, or acquire in some instances a columnar structure (as I have known lumps of fire-clay do in the burning pit-heaps of the coal-country); others again are coloured or bleached (according to the nature of their accessory materials) by baking. According to Lehmann cases occur in which the enclosed masses are (wholly or in part) so completely liquefied as to allow diffusion of their material through the surrounding magma, reacting upon it to change locally its actual composition. In the Vorder-Eifel and other parts of the Continent fragments of clay-slate and grauwacke have been observed baked on the exterior; fragments of mica-schist, quartz, and gneiss are found covered over with a vitreous crust; fragments of sericite-schist (in the particular locality Naurod near Wiesbaden) have only their layers of sericite and chlorite vitrified. Even the fragments caught up from older igneous masses, through which (or upon which) later flows find their way, are not exempt from this kind of metatropic change; as in the case recorded in the Isle of Ischia, where fragments of an older trachyte-flow are found included in a later flow and converted into slag. Fragments * * A very striking instance of this has been described by Macfarlane in the basic intrusive rocks of the Huronian Series of Lake Superior. In some places the included granite fragments have been so completely dissolved by the basic magma as to convert the rock locally by shearing into a 'siliceous slate rock.' (Canadian Naturalist, May, 1867.) of limestone are converted into marble, but not always wholly so; the larger fragments retaining in some cases an unmetamorphosed core, which gives a direct clue to their parent-rock. As remarkable examples of this we may cite the fragments of Silurian limestone at Escabar in the Pyrenees described some years ago by Zirkel, the fragments of Trias limestone in the augite-porphyry of the Grödnerthal described by Richthofen. Similar in their nature are the changes produced upon the neighbouring rocks, so far as the action of dry heat is concerned. Sandstones are decolorised and often fritted to a glistening enamel-like or porcellanic mass; in other cases where the cement is of a calcareo-argillaceous nature this is melted into a glass; clay and marl are converted into porcellanite or brick, with marked change of colour in many cases; tuffs and phonolites are so far vitrified as to acquire a character resembling that of obsidian; brown-coal is altered into seamcoal or anthracite, and these in other cases into a substance more resembling graphite, while in others (probably under less. pressure) the coal is converted into coke,* the various shades of metatropic change of brown-coal into anthracite, carbonglance, bituminous coal, and black-coal being observed in some cases in the same section through several metres of the mass; a prismatic structure is developed, not only in clays and marls, but even in sandstones, in brown-coal, in seam-coal, in dolomite; limestones are altered into crystalline marble, often with complete effacement of their stratification and even of all traces of their fossils; the finer varieties of grauwacke and its associated shales are converted into hornstone, as in the classical region of the Brocken. Of these metatropic changes by the action of dry heat under pressure, that of the formation of marble has been experimentally verified years ago by G. Rose, and more recently by Richthofen and others. The amount of metataxic change which the neighbouring rocks may undergo will vary greatly according to the nature of the rock upon which the force is brought to bear; and it would be difficult to deny that along with cleavage as a direct result of pressure in some of the rocks, especially in the more argillaceous, we may have a crude sort of foliation developed in some cases by shearing, pari passu with fluxion-structure and a certain degree even of foliation in the intrusive mass * Marked instances of this are known in the Meissen district; and several years ago I noticed a fine example in the Museum of the Reichsanstalt at Vienna of the local conversion of seam-coal into coke by a basalt dyke which traverses it. These are of course instances of 'destructive distillation.' In Ayrshire, graphite is said to result from the action of intrusive masses of dolerite upon the seams of coal; probably a dissociation-product by contactaction of the hydro-carbons distilled over from the coal. |