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Updated: June 17, 2025
With respect to the origin of the folia of quartz, mica, feldspar, and other minerals composing the metamorphic schists, Professor Sedgwick, Mr. Lyell, and most authors believe, that the constituent parts of each layer were separately deposited as sediment, and then metamorphosed. This view, in the majority of cases, I believe to be quite untenable.
In a pile of volcanic rocks on the Island of Ascension, there are strata, differing quite as much in appearance as the ordinary varieties of the metamorphic schists, which undoubtedly have been produced, not by successive flowings of lava, but by internal molecular changes.
I did not remark that these cavities had the same direction, or that the porous rock lay on compact strata, as happens in the currents of lava of Etna and Vesuvius. The sun discolours their surface, as it whitens several schists, by reviving a hydro-carburetted principle, which appears to be combined with the earth.
Foliation may be used for those alternating layers or plates of different mineralogical nature of which gneiss and other metamorphic schists are composed. That the planes of foliation of the crystalline schists in Norway accord very generally with those of original stratification is a conclusion long since espoused by Keilhau. In Scotland, also, Mr.
Dark trappean rocks full of hornblende have in many places burst through these schists, and appear in nodules on the surface. The highest rock seen is a fine sandstone of closer grain than that at Tette, and quite metamorphosed where it comes into contact with the igneous rocks below it. It sometimes gives place to quartz and reddish clay schists, much baked by heat.
This molten silica continually accumulating, spreading, and pressing against the horizontal Cambro-Silurian beds during a long period at length forced its way through the superincumbent strata in all directions; and it is abundantly evident, under the conditions of this force and the resistance offered to its action, that the line it would and must choose would be along any continuous and slightly inclined diagonal, at times crossing the strata of the schists, though generally preferring to develop itself and egress between the cleavage planes and dividing seams of the different schistose beds."
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.
There are yet a few other important deductions to be made from the foregoing facts: the same coal basin may, at the same level, contain fragments of coal of very different ages; its contour may have been much modified owing to the ravines made by the water which transported the ancient parts into the lowest regions of the basin; and finally, since the most recent sandstones and schists of the same basin may contain coal which is more ancient, but which is formed from the same species of plants that we find at this more recent level, we must admit that the conversion of the vegetable tissues into coal was relatively rapid, and far from requiring an enormous length of time, as we are generally led to believe.
Surrounding it on all sides are micaceous schists, and in the neighbourhood about half a mile distant is a large granite hill about 800 feet high. In the lode and its walls are large quantities of pyro-phyllite, and in some parts of the mine there are deposits of pure white translucent mica, but in the ore itself it is a yellow or pale olive green, and is never absent from the pyrites.
The fourth and last great division of rocks are the crystalline strata and slates, or schists, called gneiss, mica-schist, clay-slate, chlorite-schist, marble, and the like, the origin of which is more doubtful than that of the other three classes.
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