Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/449

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439
various parts of Scotland.

forces have acted, independently of the neighbouring ones. It is equally evident, that a mere state of softness in the stratum of schist is insufficient to account for this partial and complicated effect, but that it must have taken place when the rock was in a state of tenacious fluidity. The effect produced by mechanical disturbance on fluid slags which consist of differently coloured laminæ, will illustrate my meaning, or, to leave out of the question any illustration which may appear to involve a cause, similar appearances may be produced by the disturbance of tenacious compounds of clay and water, or of other semifluid mixtures. It is not perhaps essential to the igneous theory, that this fact should be explained by an igneous softening; but it is necessary that the aqueous theory should admit of a disturbing force capable of producing a mechanical effect of this nature, since nothing short of mechanical force acting on yielding matter is capable of explaining it.

I may here add a fact somewhat illustrative of these partial contortions in schist, which is to be observed occurring in the basalt that forms the hill of Dun Can, in Raasa. Innumerable instances prove to us that the decomposition of basaltic as well as of granitic and other rocks, detects in them that structure which we should in vain seek in the fresh and entire mass. The weathered basalt to which I allude discovers appearances of contortion similar to those now under review, its surface exhibiting parallel prominences, separated by deep lines, which are waved in very complicated curves, and give reason to suspect a mechanical disturbing force, acting in this case also, upon a semifluid mass. As analogous appearances of curvature are also of frequent occurrence both in veins of quartz, and in veins of granite which traverse mica slate and gneiss, I have given figures of a few among the most remarkable of the innumerable specimens to be seen all over Scotland (Pl. 31*, excepting fig. 3.). I have

Plate 31*