CONCLUSIONS.

If now we review the facts and arguments contained in the foregoing dissertation, and take into account those given in the appendices, several important conclusions would appear to be suggested.

1. It is a vain and useless task to think of finding any one principle of metamorphism, since this is exhibited in Nature in various degrees and in various phases. In the higher and more complex phases of change all the four principles discussed in this work are exhibited as having come into play, not necessarily simultaneously, but in an order of succession determined by general laws of nature; in other words the laws of nature being (so far as we have any possibility of knowing) persistent, unchanged, and unchangeable, admit however of an indefinite variation in the proportions in which their operation is manifested in any particular field of action. The almost infinite variety of their collocations is not therefore so much a qualitative as a quantitative variation; and to these the variations in the phenomenal results must correspond. We have in fact all gradations of change, from a simple metataxic change in the cleavage of slate, or a simple metatropic change in the crystallization of a limestone into a marble after fusion under pressure, to the most complex changes observable in regions of contact-metamorphism.

2. When we come to consider what is commonly understood as regional metamorphism,' very large elements of doubt and uncertainty are introduced; and considerable non-provable assumptions have to be made in order to construct anything like a coherent theory. Some of the most important data upon which it has been attempted in some quarters to build up such a theory are found to admit of a different explanation from that which has been given to them by the advocates of the theory; and the relative values of these different explanations is not a matter of opinion, but a question of agreement or disagreement with known laws of

nature.

3. The important revelations which the spectroscope (as a supplementary instrument to the telescope) has made in the last two or three decades, taken together with a firm grasp of the principle of dissipation of energy* furnish data which cannot fail to throw light upon the history of the developement

* It is because this great principle is ignored, or at least but feebly grasped, that some geologists follow so blindly the rigid 'uniformitarianism' of which we hear so much. Until geologists of that school come to realize the vast importance of this one simple physical principle, we cannot admit that their creed is a 'rational' one.

of our Earth; the only assumptions being (1) that the same universal laws have operated in that developement as are seen in operation in other bodies which roll through space;* (2) that the mass of the earth including its atmosphere has remained the same from the beginning, its material having neither diminished nor (with the slight exception of added meteoric matter) increased. Deductions from the known principles of thermal chemistry and physics lead to the conclusion that in the history of its developement the Earth must have passed through a pre-oceanic stage, in which deposition of the more stable minerals occurred to form a non-consolidated crust in the presence at high temperature and pressure of that chemical body whose composition is represented by the formula H,O; and in the minerals which for the most part constitute the archæan and pre-archæan rocks we can recoguise just those which, from their known high degree of stability, we must regard as most likely to have been formed at such a stage of the Earth's developement. This leads to the further conclusion that the process by which the archæan gneisses and schists were formed (so far as their essential mineral-characters are concerned) was essentially diagenetic' rather than 'metamorphic.' If this be admitted, such phrases as "the highly-metamorphosed archæan gneisses and schists" must be relegated to an obsolete nomenclature of geologic science.

4. The archæan rocks themselves, wherever we have an opportunity of studying them in anything like their full developement, exhibit a general, though not uniform, progressive series of changes of mineral character, from the oldest mixtures for the most part of the most (thermally) stable mineral compounds, quartz and felspar (the granitoid fundamental rocks), through well-foliated gneiss (with its many varieties) into the schists and phyllites, in which the silicates themselves tell the tale of a gradual increase in the two main

* Pfaff (Geol. als ex. Wiss., p. 31) thus summed up the evidence of this, fifteen years ago:

1. "No physical or chemical law, and in particular no known phenomenon can be brought forward, which would oppose the view of an original glowing (molten) condition of our Earth.

2. As a necessary consequence the oblate form of the Earth and the increase inwards of temperature, which we in fact observe, came about.

3. That we can recognise in the Sun and in other of the larger heavenly bodies the same glowing condition, makes it the more probable to us that the Earth was once in the same condition. The latter moreover, as one of the very small masses as compared with the Sun, must have cooled, so that its crust solidified, so much sooner than this can happen to such an enormous mass as the Sun."

factors which have determined their differentiation, (a) water,* (b) heavier (but less chemically-active) basic oxides, the high temperature of what we may call the 'Ur-gneiss stage' having been upon the whole too high for the formation of the silicates of the latter on anything like a general scale, and causing their deposition mainly as oxides, sulphides, and fluorides scattered as accessory minerals through the Ur-gneiss (in its many modifications) or precipitated more copiously in great intercalated metalliferous zones (Fahlbänder') often miles in area. And the truth of this as a deduction is not affected, though the difficulty of reading the record of it is increased, by the vast disturbances (resulting in fracture, faulting, overthrusting, and overfolding) which have since interfered with the original order, and the enormous waste they have suffered in furnishing materials for sedimentary rocks, the proportion of SiO2 (either free or in combination) in the one series and in the other being much greater than that of all their bases taken together.†

5. We thus come to regard the archæan series of rocks as representing upon the whole the primordial (first-formed) earth's crust, from which the siliceous materials of the sedimentary rocks have been for the most part derived: the

* This water was still for the most part highly superheated, in a condition, that is to say, such as that in which it operates in the high-temperature stage of contact-metamorphism in converting a clay-slate into a crystalline schist in immediate contact with granite, &c. Lawson's description of the crystalline schists in his Essay (op. cit.) may be referred to in illustration of this.

Of all the chemical elements Carbon alone can be mentioned (after perhaps Hydrogen) as endowed with such marvellous and varied potential ties as Silicon. Whether we note their tetravalency as elements, their remarkable relative positions in the Periodic or Natural Arrangement of the Elements (as broached by Newlands, and since worked out to a greater developement by Mendeljeff, Reynolds, Carnelly, and others), or the part they severally play in the economy of nature, we can hardly escape recognising a sort of conjugate relationship between them. The chemical potentialities of Silicon being called out mainly at high temperatures, and those of Carbon at more moderate temperatures, they seem to stand, as it were, at the two opposite poles of matter, dividing the empire between them into what we commonly call the Organic and the Inorganic, but with very undefined boundary lines, along which dwell a series of restless and turbulent tribes, the individuals of which own no permanent allegiance to either, passing from the domain of each into the other in the most facile manner. Is it too much to hope that the day will come when the study of the chemical potentialities of the element Silicon shall become the focus from which new light shall irradiate the chaos of Mineral Chemistry, as the last decade or two have seen the confused accumulation of facts in the 'Chemistry of the Carbon Compounds' brought to a great degree into order and symmetry by the recognition of the simple principle, that, "in the chemical properties of Carbon alone lies the essential fact, which determines the peculiar properties the Carbon-Compounds possess, as compared with all others."? (Wislicenus, Organ. Chem., § 1.)

fixation of CO, and oxides of some other non-metallic elements* as also possibly of the halogens, to form carbonates, sulphates, and haloid-salts, having continued on possibly into early palæozic times. The archæan stage of the earth's history is thus seen to fall into a place in a natural order of developement, and one more chapter is added to the history of the operation of that great Law of Evolution which is written upon all created things.

6. As the mists and clouds thus disperse our intellectual vision begins to descry a boundary to geologic time, and the physical geologist begins to feel that over this question he can join hands with the astronomer and the natural philosopher.

* While much of the sulphur once existing in the elementary state in the primordial atmosphere may have combined directly with metallic vapours to form sulphides (the combustion of metals in sulphur-vapour being a fact well known in the laboratory), a large proportion of it no doubt underwent combustion into SO2 with the further formation of free sulphuric acid; and this free acid no doubt played a part subordinate to that played by CO2 in the decomposition of the earlier-formed silicates, the SiO2 thus set free contributing to the production of the grauwacke quartzites. The insoluble sulphates of the alkaline earths thus formed were deposited, while the soluble sulphates of other bases were removed in solution. In some cases the sulphates may have been reduced to sulphides, which, as ores of the heavier metals (iron, zinc, copper, lead, &c.,) are of such common occurrence

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98

Appendix i.

Notes of Laboratory-Work.

a. Sulphur.

Sulphur was obtained in the vitreous state' in two ways: (1) by pouring molten sulphur into cold water; (2) by just melting lumps of brimstone in an open evaporating-dish, allowing a crust to form in the first stage of cooling, perforating this and pouring out the portion still liquid. Some of the residual 'crystallites' (App. ii. Note F.) were examined microscopically. In some instances minute triangular thin plates of clear glass were found attached to the edges of the crystallites, giving them a very serrated appearance. These, when detached and examined microscopically, gave beautiful and interesting results.

At the ends of these crystallization sets in rapidly so as to point off the lath-shaped bands with devitrified wedge-shaped interstices. Gradually, but not uniformly, devitrification is seen setting in along the margins of contiguous bands of the glassy material, and here and there the crystallization cuts transversely into the glassy prisms, forming little nests of crystals, which, in a few minutes (in cases where they begin to form at opposite points of the prism) coalesce and completely intersect the glassy band.

As devitrification proceeds in its earlier stages a marked pleochroism is exhibited along the devitrified lines, while the bands retain in the interior their isotropic character as glass. In the course of half-an-hour with strong sun-light devitrification may be so far advanced as to render the sulphur opaque. In one very thin plate of sulphur-glass, upon which the full-powered sunbeam was allowed to fall directly as well as by reflection from the mirror of the microscope, I was able to watch the progress of the devitrification; and in that case the plate became opaque in five minutes. Sunlight is thus seen to aid in devitrification of sulphur as well as of phosphorus.

Latent heat of vitreous sulphur.

1.-Vitreous (plastic) sulphur obtained in the usual way by pouring sulphur at about 400°C into a deep jar of cold water.

2.-One portion of this on being hammered on a cold stone slab became in a minute or two so hot that it could not be comfortably held in the hand. The brown colour gave place to a pale yellow, partly stringy, mass very tough. 3.-The portion treated as in (2) was placed out of doors on a cold stone slab until it was cooled down so far below the temp. of the room as to show considerable cold (40° to 50° on the scale) when placed on the metallic face of the thermopile; yet after a few minutes exposure to the air of the room it gave a large reading for heat on the scale (100° to 200°) when again placed on the thermopile. For this purpose it was lightly pressed down on the face of the pile with a glass rod so as to get a large surface of contact. The mass hardened and stiffened rapidly.

4. A piece of the same hammered mass was examined after between one and two hours under the microscope in reflected light (it had then become quite opaque) and was found quite crystallized, with the exception of a few small granules of brown amorphous sulphur.

5. After about 18 hours the residue of the hammered mass had become quite hard, inflexible, and brittle, falling easily to a crystalline powder when broken.