Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/415

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various parts of Scotland.

edges of the plates which have formed them. But the most common of the spherical bodies are, when fresh broken, of a white colour, becoming at first yellow by exposure to air, and subsequently of a dark brown. Their internal texture, as far as it can he determined by high magnifying powers, appears platy, and although it is difficult to speak with certainty of bodies so minute, and so much out of the reach of examination, they seem greatly to resemble brown spar; a conclusion rendered more probable, by the changes which they undergo on exposure to the air. The whole of these chalcedonies are occasionally penetrated by laminæ of calcareous spar, which are often so numerous as to form nearly one half of the stone, and which from its yielding in their direction, rather than in that of the chalcedony, do often seem on a superficial view to form the whole of it. In the rifts of these chalcedonies, we may occasionally find detached masses of green earth, which by its combination with the stone itself appears beyond all doubt to be the colouring ingredient. Its unequal diffusion through the stone, is equally demonstrative of the same supposition.

We have now to compare the green chalcedony of Rum, with the stone known by the name of heliotrope, and commonly called oriental, since that name has been applied to it, and as it will appear, with perfect propriety. The fracture and translucency of the true oriental heliotrope, show, as Brongniart has well remarked, that it has been improperly ranked among the jaspers; and the description and natural analysis, if I may so call it, which I have now given of the stone of Rum, confirm the propriety of his remark, and refer it to its true place among the varieties of chalcedony. It is true, that the stone of Rum very rarely exhibits the red spots which so often occur in the oriental kinds, but these