probable reaction of free CO2 upon silicates containing CaO. As for the limeand magnesia-micas, it would be difficult to deny that they may have been formed from muscovite, derived mechanically from the neighbouring archæan rocks, by such reactions upon it of the salts contained in the sea-water, as are suggested on p. 29 of this work, the richness of these waters in salts of CaO and MgO being evidenced by the gypsum and the dolomitic rocks. Again, in the decomposition of the felspar, which must have formed a portion of the detritus of which these sediments are composed in order to furnish their argillaceous materials (Thon '), it is at least rational to suppose that some of the silicate of alumina may have acquired a more basic character, such as is indicated by the formula Al2SiO5, which, as the analyses show, represents the composition of the disthene here. On the other hand, the direct derivation of disthene from the rather unstable mineral zoisite involves a comparatively simple reaction in the wet way:

Ca4 Al6H 2SiO26 +4CO2=4CaCO3 + H2O + 3SiO2+3Al2SiO5.*

Zoisite.

Disthene.

That this is no vagary of the chemist is proved by Dr. Grubenmann's own observation (op. cit. p. 15) of the passage (in the weathered rock) of zoisite into CaCO3, SiO2, and (in some cases) disthene in the cracks and fissures of the zoisite itself.

Substitute in the above reaction KзCO, for free CO2, and we should get the reaction for the conversion of zoisite into white potash-mica, which is a matter of observation (Roth, op. cit., p. 352). If such changes, of a purely chemical nature, took place in the fine silt derived from the adjacent siliceous rocks antecedently to the subjection of the sedimentary mass to the pressure which has given to these rocks their present schistosity, it is clear that we should have no right to attribute them to the operation of that pressure; even though we admitted that this might operate upon materials of the right chemical composition (previously formed by ordinary chemical reactions) so as to impart to them a crystalline structure and form. (cf. pp. 54-55 of this work).

"It is easy [as Dr. Grubenmann sagaciously remarks] to be tempted to allow too bold a flight to hypothesis in the region of rock-genesis"; and our only safe rule is to give hypothesis no place until we have exhausted all the resources which the ever-active known principles of hydro-chemical action can furnish towards the explanation of the facts observed. In the present instance of extreme mechanical action within a very limited and confined space, we have probably the ultimate results of the action of pressure upon materials previously prepared for it; the whole observed phenomena being not the result of pressure merely, but rather the resultant of the combined factors of time, hydrochemical change, physiographic conditions,† and pressure, with its concomitant metataxis, and perhaps in some cases metatropy.

(4). On general grounds it may safely be asserted that conditions of high temperature are those favourable to the building-up of the more complex silicates; while at lower temperatures and in the presence of water the tendency is towards the resolution of these into simpler or proximate constituents. The one is synthetic, the other is analytic. The vast difference in the degrees of probability of these two processes is at once appreciated by the chemist; and the experience of the chemical laboratory is on this point in * See Naumann-Zirkel's 'Mineralogie.' Graphically we might represent the composition of disthene thus:

the radicle (SiO3) being the exact analogue of the (CO3) of the carbonates. Anyone who has taken careful note of the nature of the detritus of modern valleys in the gneiss and schist regions of the Alps will appreciate the importance of this consideration.

perfect accord with observations in the field and under the microscope, as the study of secondary paramorphism soon reveals to us. The tendency of zoisite to undergo decomposition (which is pointed out by Dr. Grubenmann), the much greater abundance of disthene than of zoisite in these altered shales, the decomposed state of the garnets* and of much of the zoisite which remains, the way these two minerals (as also some of the others) are fractured and water-worn, the frequent accompaniment of these garnets by such well-known decomposition-products of garnet, as iron oxides, and quartz, (which commonly separate out as the lime of a garnet is removed in solution and the iron gets oxidised from the protoxide into the peroxide), and a comparison of these facts with numerous instances which Justus Roth+ has collected of the formation of decomposition-products from garnets, the general occurrence of clay and carbonaceous matter in the most highly-altered of these shales, the fractured and worn condition of many of the quartz-grains, seem to furnish strong cumulative evidence of a very considerable period of exposure of the most characteristic minerals of these thin shales to conditions inducive of chemical change, before the enormous pressure, which has given their present structural character to these altered shales, quartzites, and limestones, was brought to bear upon them. The absence of any record of felspar in these sediments is significant, as pointing again to a vast period of hydro-chemical action upon the original detrital minerals; and the probability of this is heightened by the single reflection that-if the Swiss geologists are right in assigning these altered shales to the age of the Trais-we have in this Airolo district a very insignificant series of shales and quartzites, not approaching—it appears— anything like 50 metres in thickness, to be regarded as the equivalents in time of the Triassic Dolomitic Series, which in the Eastern Alps is measured by thousands of metres.

(5). The succession of the strata under consideration is by no means certain, as Dr. Grubenmann candidly admits; the field-evidence, owing partly to alluvial deposits, and partly to the appropriation by a dense vegetation of the rich soil furnished by the weathering of the rocks themselves, being altogether insufficient for determining this with certainty. But this uncertainty leaves the broad distinction between these localized rock-phenomena and the great archæan series of gneisses and schists as clear as ever, except perhaps to those who only know Alpine geology through the medium of books.

A glance at Studer and Escher's Geol. Map of Switzerland, is almost enough to show one that the principal major longitudinal line of flexure in the Central and Western Alps is that which commences with the valley of Chamounix, (containing the Vernayaz series referred to above‡), is continued along the valley of the Upper Rhone (all that part above Martigny), the Urseren Thal, and the valley of the V. Rhein down to Chur. On general grounds, and taking into account the distribution of the 'Verrucano,' and T. Anthracit' along this zone of the Alps, there would seem to be little room for doubt that the initiation of this line of flexure dates from Palæozoic time; nor am I aware of any facts in the geognostic structure of the Alps which militate seriously against such a supposition. The protrusion of the great granite-gneiss-schist massif of Pitz Rotondo and St. Gotthard, has caused a bifurcation of this great line of flexure as we trace it eastwards, and has given us, as a consequence, the line of flexure of the Val Bedretto and the Val Leventina, which is deflected still more to the south by the great crystalline massif of the mountains which furnish the head-waters of the H. Rhein §

* So much so that the garnet could not be separated from the accompanying minerals in sufficient purity for an analysis to be made.

+ Allgem. und Chem. Geol., pp. 352-363. See in particular Lemberg's experiments on Grossular (p. 363). See page 90.

I am of course aware that Dr. Heim considers the architectonic structure of the Alps to have played only a very insignificant rôle in determining lines of erosion and denudation, but in this I am unable to follow even so great a master.

As narrow zones of sedimentation they must date from a very early stage in the building of the Alpine system, and the nature of the sediments formed here would differ (as pointed out above) presumably from those formed in more extended basins, so as to afford special facilities for pressure to impart to them such a 'schistosity' as may well simulate true foliation. There is no question here-it appears-of the grey schists (schistes gris= Bündner Schiefer) which form a narrow zone all along the south flank of the Val Bedretto, but only of a very limited series, their exposure in the section worked out by Dr. Grubenmann, not amounting even when reduplicated by overfolding (in part) to more than a very small fraction of the whole section, in which impure limestones and quartzites predominate.

NOTE U.

On the Developement of the Archæan Crystallines. *

"The stratified sedimentary formations (palæozoic and neozoic) form a great series, whose origin and present developement we can for the most part explain with apparent certainty. But besides these there exist at the base of our formations vast series of strata of gneiss, mica-schists, phyllites, hornblendeschists, chlorite-schists, talc-schists, and granulites, interstratified with which occur granite-gneiss, marble, graphite, and iron-ores, as to the mode of developement and building-up of which the views of geologists are still not merely confused and indefinite, but to some extent widely diverse from one another.+ The originally sedimentary mode of developement of all this complexus of archaic gneisses and schists cannot be doubted: in their stratiform differentiation, in their massive structure, and in their parallelism, which dominates the whole series, we are presented with a stratification (Schichtung) as real as in the fossiliferous series of clay-slates, limestones, shales and sandstones: concordantly with their distinctiveness as stratified masses frequent interstratifications (often on a diminutive scale) of the most diverse kinds of rock repeat themselves; between gneisses and mica-schists regular layers of granular quartzite and of conglomerates are met with; with the greatest regularity gneisses and granulites of the most manifold varieties are interstratified with chlorite-, talc-, mica-, quartzite-, and hornblende-schists; between these may occur layers of granite-gneiss, beds of crystalline limestone, magnetic iron-ore, and graphite, as well as complexes of graphite-schists; finally the gneisses pass into mica-schists, these into phyllites, and these again, so far as the stratification-order indicates, into the lower palæozoic fossiliferous formations: the phenomena as a whole indicating some sedimentary mode of developement of the gneisses and crystalline schists. Their present rockcharacter is, however, according to the view of many geologists, not original, but rather a change, a metamorphism has taken place in the course of time in the originally clastic material, out of which have arisen the crystalline structure and the petrographical habit which the gneisses and the crystalline schists at present exhibit. This process of alteration has been termed 'general metamorphism' or 'regional metamorphism.' As to the nature and origin of it the views however differ widely. By some it is regarded as the result of the active agency of the high temperature of the glowing internal mass of the earth; by others, as the result of intense pressure; by others again, as the result of hydrochemical processes, that is to say, of chemical processes carried on through the agency of fresh water penetrating to depths. In other words it is regarded, on the one side as a reaction of vulcanicity, on the other as atmospheric in origin.

*Translation of § 13 of the 'Petrogenetische Geologie' of Credner's 'Elemente,' 6th edition, (Leipzig, 1887).

↑ J. Roth, 'Ueber die Lehre von Metamorphismus aud die Entstehung der krystallinischen Schiefer,' Berlin, 1874. G. W. Gümbel, 'Ostbayer. Grenzgeb.' Gotha, 1868.

Plutonic Regional-Metamorphism.

"According to the views of the Huttonian school, and especially more recently of Lyell and von Cotta, the metamorphism of the originally sedimentary material is the result of a long-continued heating through the internal heat of the Earth, whereby, under the influence of the simultaneously existing pressure of the overlying formations, a melting of the lowermost rock-masses concerned and an intimate recrystallization with transfer of material and reconstruction have come about. In this process an important part is assigned on the one hand to the waters which originally filled the interstices of the sedimentary rocks, owing to its increased efficiency as a vehicle of heat by convection and as an active agent in the solution and decomposition of mineral-matter in the super-heated state; on the other hand, gases and vapours developed in the glowing-liquid kernel of the Earth and driven under pressure through the rocks are considered important factors in metamorphism. In plutonic regional metamorphism the processes would thus be similar to those of contact-metamorphism in the neighbourhood of eruptive rocks, except that they would arise from an universally-active source of heat, the glowing interior of the Earth. Since now the metamorphic influence of the latter diminishes in intensity from the interior towards the periphery, the most deeply seated are by it affected and transformed in the highest degree. On account of this the gneiss occupies the lowest position, and above this follow in succession the mica-schists, the chlorite-, talc-, and hornblende-schists, and lastly the phyllites so closely resembling the shales (Schieferthon). There would follow however instead of a mere partial softening or recrystallization a complete fusion of the originally sedimentary rock-material, so that this would, like our volcanic lavas, be forced in the plastic condition into clefts and fissures to solidify as granite and syenite. According to this view the eruptive rocks had their origin in the upper zones of the earth's crust which once formed the bed of the ocean.

"This theory presumes that in a higher zone, that is to say, in that which included the sedimentary deposits of the primeval ocean, considerably high temperatures were produced from within the sphere, and explains this as the result of elevation of the surface of the Earth through the deposition of vast stratified systems, so that the isothermal planes of temperature were transferred upwards. In this way, by the filling-up of an ocean of over 10,000ft. in depth, through the deposition of a mighty complex of stratified rocks, the temperature of the originally superficial formations was raised to about 100°C.

Mechanical (tectonic) Regional-Metamorphism.

"In some places, always however in regions very limited in space, it has been established as a fact, that the clastic normally-constructed strata, where they have undergone specially intensified pressure, in the process of mountainbuilding, and thus exhibit complicated disturbances of their stratification, have taken on at the same time a crystalline habit. Observations of that sort have afforded the tempting opportunity to refer the crystalline schists generally and collectively to practically similar processes, and the developement of their crystalline character to the energetic thrust, folding, and pressure, which originally clastic strata have suffered, together with the heat generated by the accompanying friction and pressure. Indeed, so far has this idea gone, that the archaic gneisses and granulites extending through whole regions have been accounted for as eruptive rocks metamorphosed by pressure into schists along with the rocks which are subordinately interstratified with them. All these hypotheses as to such a widespread mechanical metamorphism prevailing similarly through vast areas and resulting from the pressure concerned in mountain-building, dispense however with convincing evidence for their data, and stand in part in direct contradiction to known facts. They are disproved by the simple fact, that many highly crystalline archaic regions exhibit in the highest degree a simple little disturbed architectonic structure, while the

neighbouring Silurian and Devonian areas, in spite of powerful folding, crumpling, overthrusting, crushing, and transversal cleavage have retained their original structure unchanged, as slates, (Thonschiefer), grauwacke, sandstone, and ordinary limestone.

Hydrochemical Regional Metamorphism.

"In contradistinction to these views as to the origin and process of metamorphism by plutonic action of heat or by pressure of whole systems of stratified rocks, the hydrochemical theory of general metamorphism, as it was especially taught by Bischof, recognizes in the saturation of the rocks with water through a vast period of time the cause of the metamorphism of rocks on a large scale, and ascribes to this a substantial alteration and recrystallization of metamorphosed strata. According to it the process consists in the transference to depths in the Earth of the materials taken up in the superficial strata of the Earth's crust, by the chemical activity of water in decomposing and dissolving mineral substances. This water, holding carbonic acid and oxygen, penetrates, after its precipitation from the atmosphere, through the rocks near the surface, where the oxygen it holds is taken up in processes of oxidation and the carbonic acid is removed from it by the decomposition of certain silicates, until both gases, after the water has penetrated to greater depths, are used up entirely and so these reactions must cease. When these waters laden with the soluble mineral-substances arrive at the more deeply seated strata, they serve as agents in the alteration of rocks. The alkali and lime-silicates transported below in this fashion combine with the alumina- and magnesia-silicates already present there to form compound silicates (e.g. felspar, mica,) which, since this process goes on exceedingly slowly, separate out in the crystalline form. Besides single silicates which combine with one another, silica is present in larger quantities than is required for the resultant double silicates, and so in this process there is a separation-out of quartz. Thus the hydro-chemical metamorphism of rocks consists in the introduction of mineral-solutions from the more superficial zones into the deeper-seated; further in effecting pari passu combinations and decompositions between those solutions and the rockmaterials which are saturated with them; and lastly, in the resultant formation of new minerals which, on account of the slowness of the process, crystallize. The result is the complete transformation of the chemical composition, of the petrographical constitution, and of the structural habit of the original rock. This process, even when aided by the pressure of the overlying strata and by the increase of temperature at depths, requires enormous time. The direct consequence of this theory is that (e.g.) from one and the same limestone, according to the nature of the circulating mineral-solutions and the chemical processes induced thereby, in some places a pyroxene- or amphibole-rock, in others a garnet- or epidote-bearing rock, in others again, a quartz- or felspar-rock may be developed.

"A capital objection to this theory of hydro-chemical metamorphism consists in the fact that it requires an enormous period of time, longer even than that which has elapsed from the beginning of the palæozoic ('Silur') to the present, for the complete morphological transformation of the rocks through saturation with water; since all the formations from the Silurian down to the most recent ore found, where their normal construction is displayed, not yet in the condition of metamorphosed rocks * On the contrary, all the palæozoic formations, and especially so the Cambrian and Silurian stratified systems, which follow immediately upon the crystalline schists, contain rolled fragments both of gneisses and of crystalline schists, which possess exactly the same structural character as their parent-rock (Muttergestein). The hypothetical long-continued process of hydrochemical metamorphism of the pre-Cambrian formations was therefore already completed at the entry of the Earth into the palæozoic stage of its developement (in die silurische Periode), and could not * Compare § ii. pp. 16-17 of this work.