"Nur die überzeugendsten Gründe, also sowohl der Nachweis der Ursache des Umwandlungsvorganges, als die Beobachtung inniger, durch allmählige Uebergänge erwiesener Verknüpfung des umgewandelten Gesteines mit dem ursprünglichen Muttergestein geben die Berechtigung, ein Gestein als metamorphisch zu bezeichnen." PROF. HERMANN CREDNER; Elemente der Geologie ROCK-METAMORPHISM CONSIDERED FROM THE CHEMICAL AND PHYSICAL SIDE. § i. INTRODUCTION. IN Physical Geology there is no subject more complicated than Metamorphism, nor one which presents to the student of Geology so many vexed and complicated questions. True progress in this direction can only be made by a threefold attack upon the problems which it presents to us. They must be studied— 1. in the field; 2. with the microscope; 3. in the light of known physical laws and chemical principles. In the first of these three lines of work our own country has undoubtedly taken the lead, but it has been rapidly overtaken by the geologists of Germany, Austria, France, and America. In the application of the microscope and microscopic methods to petrology it would be admitted, I think, on all sides that we have to thank Germany* mainly for the elaboration of this more exact mode of research; but thanks to the indefatigable energy of several workers-e.g., Bonney, Sorby, Allport, Judd -such rapid strides have been made in this country in the last decade or so that English petrology may be said now to take a place in this respect second to that of no country in the world. With respect to the chemico-physical side however it would be rash to assert as much. There is some advance here in the latest text books; the physical and chemical sides of many geological phenomena are handled much more freely than they used to be; but there is room yet for much improvement. It is not the "" text-book taster in chemistry or physics who will rise to that masterly handling of such problems which we see in some of the best geological works on the Continent, but the investigator who can bring such living ideas to the consideration of them as can only be acquired through bonâ fide laboratory-work. It is not too much however to hope that, with the new impetus which has been given of late years to laboratory * Zirkel's earlier work on the 'Basalt Rocks' (Bonn, 1870), is dedicated to Henry Clifton Sorby, F.R.S., who initiated microscopic petrographical methods. B work at the universities and the increased facilities which are now afforded by our provincial colleges, the next few years will be marked by rapid strides in this direction; and that the day will soon pass away in which we shall find Bischof quoted as a final authority on such questions; or hear a professor of geology excuse himself for not having made himself acquainted with memoirs on interesting and important questions in physical geology on the ground that they are 'too chemical'; or find a distinguished author of a text-book stating, after writing about dolomites, that “we are still quite in the dark as to the exact nature of the reactions by which they have been produced "; or find another eminent, but now deceased, professor invoking the notion of the "spheroidal state" of water to account for paroxysmal explosions at the mouth of a volcano in activity; or the reducing action of heated protoxides of the heavy metals upon steam overlooked—as it generally is when the suggestion of Davy,* as to one possible mode of production of free hydrogen at the mouth of a volcano, is considered; or the discussion at the Geological Society of the chemical evidence which may bear upon a stratigraphical question a practical impossibility; or a learned President of that same Society attempting to extend our ideas of those differentiated forms of energy which are concerned in the vital activity of living organisms to the building up of a crystalline mineral.t General and Preliminary. The principle of Conservation of Energy is a recognition of the fact, that-so far as the material universe is concernedthe sum-total of its energy is a fixed quantity. This energywhen manifested in operation-is differentiated in the various ways or modes to which we apply the term force, so that the forces of nature may be defined as differentiated energy. Without entering into the psychical side of the question we may regard vitality in a living organism, so far as its physical side is concerned, as a summation of forces. When a selective differentiation of the forces essential to vitality takes place in connection with special organs (i.e., differentiated structures), powers or capacities are developed ; and a summation of powers constitutes that indefinable thing which we call the individual. To give a full account of the individual in any grade of existence we must trace back each intermediate power which is an essential constituent of the individual to those simple elementary principles which we can recognise as laws uni * See Appendix ii. Note A. + Appendix ii. Note B. versal. A scientific imagination may be useful in projecting an idea; but to call in that faculty in support of an idea, when direct evidence from nature fails us, is-whatever it may be— certainly not scientific. No true advance can be made by such a use of it. No piling-up of opinion upon opinion can establish a truth of nature, unless such opinions rest ultimately upon evidence furnished by Nature herself. In the conflict of opinions and views which has been waged for years over those phenomena connected with rockstructure which are generally understood to be included under the term "metamorphism 'metamorphism" in its more restricted sense, it is to be feared that many "theories which have been put forward from time to time are simply suspended in the air; instead of hanging on to a series of rigid inductions from facts they have often little more than the imaginations of their authors to rest upon.* A really solid basis for theory can only be laid in a careful and laborious observation by impartial minds of the facts presented to us in nature, or arrived at through the experimental work of the laboratory. We must get our ideas our way of looking at things-our "theories"-by inductions from the hard facts of nature; and this some of our best workers are beginning to realise. But we must not imagine that an inference drawn from a set of facts of one kind is of itself sufficient to give a full account of the manifold and complex phenomena presented to us in the more highly (so-called) metamorphic rocks, such as the crystalline schists. The term "Metamorphism." The root idea of μopon is no doubt shape or form. But in scientific nomenclature the idea is limited to internal structure. In Botany, for example, there is no difficulty in distinguishing between morphology and external conformation. The former -the structure and growth of the cell, the mode of elaboration of tissues and organs-is determined by definite laws and the operation of definite forces (not in every case clearly defined, it may be); that is to say, differentiated forms of energy applied in Nature's laboratory. The gardener trains a fruit-tree to a wall, and thus alters its external conformation; but no one would be so foolish as to say that the morphology of the plant was changed thereby. It is equally unscientific to attempt to extend the word metamorphism in petrology to such accidental changes in the conformation of a rock-mass (large or small), as may occur, for example, in the indentation of pebbles by * See Appendix ii. Note E. the pressure of a harder pebble upon a softer one, as is often seen to be the case in the Nagelfluh conglomerate and elsewhere. How necessary it is to fix such limitations to the meaning of the term is seen from the fact that a learned professor in this country * only five years ago brought forward a similar case from the Old Red Sandstone Conglomerates of Scotland, as furnishing what he called "an example of an early stage of metamorphism." 6 In a letter dated Zürich, 1849, Von Cotta tells us that Escher v. d. Linth showed him a great case full of such in the Museum at Zürich. Many of the larger indented calcareous pebbles were split and their cracks filled with calcspar, just as the cracks in the septaria of the London Clay are filled up. No one has yet ventured to bring forward the latter as instances of metamorphism,' nor on the other hand can it be shown, I think, that any continental writer has indulged in such an abuse of the term, although some of them at least as is shown above-have long been familiar with the phenomena. As well might we call the striations and grooves of glaciated rock, or the work of a mason's chisel upon a block of stone, 'metamorphism,' as to apply that term here. In this thesis metamorphism' will be used to mean only changes in the internal structure of rock-masses (i.e. in their morphology); everything connected with external conformation, which is purely accidental, is excluded. In an attempt to deal with the vast subject of metamorphism from the chemical and physical side, as thus outlined, it is not possible within the limits of a thesis to do more than touch upon its more salient points. One thing however must be premised; we are not dealing merely with the socalled metamorphic rocks' of the systematist, but rather with principles. For this reason it will be best to rise above the text-book level of looking at the facts and ignore the restrictions and limitations which may be convenient and even necessary in classifying rocks. Here as in so many cases we have to recognise the fact that Nature knows no sharp lines of demarcation, though for the conveniences of study and description they are admissible. Divisions of the Subject. A year or two ago (see Brit. Assoc. Report, Birmingham, 1886, p. 658) I proposed to exclude from metamorphism in the stricter sense of the word, as defined in this work, all such changes as could not be included under the two terms * Brit. Assc. Report, Southampton Meeting (1882), p. 536, |