Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/235

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surrounded by the solid rock on all sides. It is more common for the boundary of the quartz in the immediate vicinity of the trap to be formed of various zones of coloured chalcedony. The quartz in this case assumes a peculiar well-known aspect, and is called in the Wernerian nomenclature, amethyst, although most commonly of a white or watery appearance. Different zones of chalcedony and quartz will even at times succeed each other in the same nodule.

But the imbedded mineral from which this place has acquired its greatest celebrity, is the agate, or coloured chalcedony, with which it abounds, but which it possesses only in common with many other places in Scotland. The nodules of this substance vary exceedingly both in size and colour, and their general aspect is much too well known to need any description; yet a few circumstances respecting them deserve to be considered, as they involve difficulties which it is incumbent on any general theory of the formation of these rocks to explain. Their external surfaces, I believe invariably, bear those marks of indentation by the surrounding rock which determines their posteriority of formation, or at least their posteriority of induration, to that of the rock in which they are imbedded. Their internal structure is also most commonly zoned, with irregularities corresponding to those of the external boundary; but in some cases they exhibit a complication of structure, which as it cannot be well described in words, I have ventured to represent in the accompanying sketches. In the first example, a stalactite may be observed occupying a portion of a hollow cavity, marking as in the case of the larger quartz cavities described above, the gradual deposition of siliceous matter by infiltration.[1] The change of disposition in the zones in the figures N° 2 & 3,[2] seems to be the result of a similar process, the horizontal parallel lines

  1. Pl. 10. N° 1.
  2. Pl. 10. N° 2 & 3.