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Those who believed the hypogene rocks to have originated antecedently to the creation of organic beings, imputed the absence of lime, so remarkable in metamorphic strata, to the non-existence of those mollusca and zoophytes by which shells and corals are secreted; but when we ascribe the crystalline formations to Plutonic action, it is natural to inquire whether this action itself may not tend to expel carbonic acid and lime from the materials which it reduces to fusion or semi-fusion.
These metamorphic phenomena, though important, are obscure, and their elucidation demands some knowledge of petrographic science, that branch of geology which considers the principles of rock formation. They will therefore not be further considered in this work.
There is no igneous, no metamorphic rock here; nothing but limestone of the Jurassic formation. The convexities on one side of the fissure correspond with marked regularity to the concavities on the other. For awhile I walked on the lush grass by the brimming river, where in the little creeks and bays the water-ranunculus floated its small white flowers that were to continue the race.
Such metamorphic rocks occur in formations of all ages; and, in various cases, there are very good grounds for the belief that they have contained organic remains, and that those remains have been absolutely obliterated.
The gold-bearing quartz veins of California traverse the metamorphic slates of the Sierra Nevada Mountains. But to the depth of about fifty feet from the surface the pyrite of the vein has been dissolved, leaving a rusty, cellular quartz with grains of the insoluble gold scattered through it.
Thus where a region is underlain by sedimentary rocks, as is the drift-covered area from the Hudson River to the Missouri, the drift contains not only fragments of limestone, sandstone, and shale of local derivation, but also pebbles of many igneous and metamorphic rocks, such as granites, gneisses, schists, dike rocks, quartzites, and the quartz of mineral veins, whose nearest source is the Archean area of Canada and the states of our northern border.
In the deep gorges of the Dordogne and its tributaries, the snow rarely lies more than a few days upon the ground, whereas upon the wind-swept plateau above the scanty population have to contend with the rigours of that French Siberia which may be said to commence here on the west, and to extend eastward over the whole mass of metamorphic and igneous rocks, which is termed the great central plateau of France, although it lies far south of the true centre of the country.
Each layer of quartz was separated from its fellows, by one of mica scales; and was broken up into cubical fragments, whose surfaces are no doubt cleavage and jointing places. I had previously seen, but not understood, such flexures produced by metamorphic action on masses of quartz when in a pasty state, in the Falkland Islands, where they have been perfectly well described by Mr.
Of course, this view cannot be extended to THICK masses included in the metamorphic series, which are of totally different composition from the adjoining schists, and which are far extended, as is sometimes the case with quartz and marble; these must generally be of the nature of true strata.
Astronomical observations give great weight to this interpretation, especially so far as the planetary bodies of the solar system are concerned. It is also supported by such facts as the small mean density of the earth, the increasing temperature at increasing depths, the phenomena of volcanoes and injected veins, and those of igneous and metamorphic rocks.
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