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daily see in the manufacture of the metals, glass, and earthenware. Such, we repeat, are the more general and likelycauses of rock-metamorphism; and as it is possible that several of these may be operating within the same locality at the same time, the reader will perceive that no hypothesis that limits itself to any one agent can be accepted as sufficient and satisfactory.
It has been already stated, that while every stratum or portion of a stratum, every formation or portion of a formation, may undergo metamorphism, it was to the older and deeper-seated slates and schists that the term Metamorphic was more especially applied — it being chiefly among these that mineral transformations were to be witnessed in their greatest intensity. It is true that in many secondary mountains, such as the Alps and Apennines, the stratified rocks are often highly metamorphosed; but this transformation is for the most part partial, some localities remaining little affected, and having all their fossils distinct and legible. Near one centre of vulcanic energy the shales may be converted into dark glistening schists and the limestones into crystalline marbles, while in another centre, and only a few miles distant, the shales and limestones may retain their original sedimentary aspect. Among the primary formations, on the other hand, the metamorphism is general, and whole mountain-ranges and vast tracts of country, like the Scottish Highlands and Scandinavia, are entirely composed of crystalline slates and schists—the clay-slates, mica-schists, chlorite-schists, gneisses, quartzites, marbles, and serpentines of the mineralogist. Among these rocks stratification is indistinct, fossils are obliterated, and the whole succession is massed into one enormous thickness of unknown origin and antiquity. Under this view these rocks have been successively regarded as "Primary," " Metamorphic," "Azoic," and " Hypozoic,"* or in other words, as marking the earliest stages of worldhistory, and before life had begun to make its appearance on our planet.
This view, however, like many others of the earlier geologists, has had to give way to more extended research and newer discovery. Even during the time of the German geologist Werner, a large portion of these primary strata was found to be partially fossiliferous, and separated under the term " Transition," as indicating the passage of the world from an uninhabitable to a habitable state. In our own time these Transition rocks (as will be more fully explained in a subsequent sketch) have been subdivided into " Silurian" and "Cambrian" systems, both of which have yielded abundant forms of life; and still more recently, in the schists and serpentines of the St Lawrence— the equivalents of the old gnarled gneiss-rocks of Scotland and Norway—traces of lowly life have been discovered, this giving rise to the "Laurentian" system — the oldest or earliest strata in which fossils have yet been detected. Or, tabulating the progress, we have first the Primary of the earlier geologists resolved into Transition and Metamorphic; secondly, the Transition resolved into Silurian and Cambrian; and, lastly, the Metamorphic resolved into Laurentian and Older Crystalline Schists; thus—
/•_ ( Silurian System.
| Transition. \ cambrian
) MwTAMnnpmn. J
Older Crystalline Schists.
Here, then, it is obvious that what has hitherto been especially regarded as the metamorphic system is merely a
* Primary, first or earliest formed; Metamorphic, changed or transformed in texture; Azoic, without life, or destitute of fossil remains; Hypozoic, under life, or lying beneath the fos3iliferous formations.
vast succession of stratified rocks of true sedimentary origin like any of the later systems, but so altered by mineral transformation that it is only in localities where the metamorphism has been partial and less intense that evidence of their aqueous and fossiliferous nature can possibly be discovered. As the extensive and mountainous tracts in which these schists occur have been but slenderly investigated, subsequent research may yet discover other fossils, and resolve the whole into definite and satisfactory lifesystems. And should such hopes be fulfilled, how inconceivably exalted will our notions of the world's antiquity become—new aeons of life and physical activity extending away into a past as vast perhaps in duration as all the later ages that geology has revealed! We can never hope to read aright these earlier pages of world-history, but passages here and there may be recovered, and from these scattered hints we may obtain enough to convince that then, as now, the physical agencies of nature were ever active and subject to the same laws, and that Life too was present as their accompaniment, though then merely coming into view under the operation of a higher and more complicated law of development.
How then, it may be asked, are we to deal, in a chronological point of view, with the metamorphic rocks in which no fossils have been detected? Shall we describe them as altered Silurians, Cambrians, or Laurentians? or shall we continue to regard them merely as metamorphic strata of unknown age, till some organism has been discovered that may lead to their identification t Where the stratigraphical succession is evident, it may often be convenient to mark them as Silurian, or Cambrian, or Laurentian; but where the succession is doubtful, and no trace of organism has been discovered, it will be much safer still to retain them as mere metamorphic strata. Much error, both in theory and practice, may arise from adopting an opposite course, while no inconvenience can result from the use of the term " metamorphic," which merely implies that the rocks under review have suffered intense mineral change, but advances no opinion as to their age or chronological co-ordination. We might colour on our geological maps the schists of the Scottish Highlands and Scandinavia, as Silurian, Cambrian, or Laurentian, and support our views by many plausible arguments, but nothing would be gained by such a course which is not already secured by the term "metamorphic," while the subsequent discovery of their real character would only be embarrassed by these hypothetical distinctions. Let us continue, then, to treat these old rocks simply as "the metamorphic," labouring to reveal their true nature by the discovery of unobliterated fossils, and encouraged by the success which, within the last thirty years, has resolved so much of them into Silurian, Cambrian, and Laurentian life-systems.
Such is the nature of metamorphism, or that internal mineral transformation to which all rock-matter in the earth's crust is incessantly subjected. Pressure, heat, chemical action, and the other agents above alluded to, are continually solidifying, hardening, and crystallising; and no sooner is a sediment laid down, or an igneous mass ejected, than it begins to be operated upon by one or other of these forces. Of course the latest laid down will have suffered less change than the older and deeper-seated; hence it is chiefly among the latter that metamorphism is to be seen in its greatest intensity. It may happen in certain areas, such as centres of vulcanic activity, that secondary strata may be as much metamorphosed, or even more so, than any of the primary—but still such instances are exceptional; and it may be safely asserted, as a general rule, that the older rocks have undergone the greatest amount of metamorphism or internal mineral transformation. So great, in many instances, has this change been, that stratification is rendered indistinct, fossils obliterated, new minerals evolved in the mass, and the rocks rendered so crystalline and homogeneous that it is difficult to determine whether they are really of sedimentary or of igneous origin. Normally speaking, between the most recent and most ancient rocks there will be every gradation of metamorphism, and it is to this process that the geologist must learn mainly to ascribe the different aspects and textures that prevail among the rockformations of the globe. The atmospheric, aqueous, and igneous forces may destroy the rocks in one region and re-form them in another, but metamorphism is the sure and silent agency by which they are compressed, solidified, crystallised, and converted into other and other aspects. No rock—be it shale, sandstone, limestone, coal, ironstone, lava, or greenstone—remains for ever the same. It is incessantly, however slowly, passing on to other and newer aspects, and metamorphism is the great wizard-power by which the transformations are effected. A sandstone, how. ever soft and granular now, will in time, and under the fitting conditions, be converted into a crystalline quartzite, an earthy limestone into a saccharoid marble, a tender and bituminous coal into a hard and nameless anthracite, a porous lava into a close-grained greenstone or basalt, and a greenstone itself, by farther change, into a large-grained crystalline granite. When once the operations of metamorphism are fully understood, the geologist has a key to much that is puzzling and perplexing in his science; and not till the efficacy of these is everywhere admitted can he be said to have adopted the right methods for the solution of his problems.