Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/523

Rh so decided, that no mode of crystallization or inorganic arrangement can be conceived capable of imitating it.

While on this subject it may not be useless to notice the various modifications under which chlorite when mixed with chalcedony presents itself. It is now I believe perfectly well understood (as I have already assisted in showing, in some observations on the island of Rum) that the colouring matter of heliotrope is chlorite diffused uniformly through the mass. From some specimens in my possession, I think it probable that plasma is indebted for its colour to the same substance; the different nature of the siliceous base alone constituting the difference of aspect. In these specimens (brought from Egypt) distinct grains of green matter may be observed independent of the general green stain which pervades the stone; and this probability is rendered still stronger from the colour and appearance which is occasionally assumed by the more transparent flinty matter associated with the chalcedony, the latter acquiring from the chlorite the aspect of heliotrope, while the more transparent parts put on the semblance of plasma. But of a substance so rare it is necessary to speak with hesitation. In quartz the chlorite is well known often to assume a contorted appearance resembling the intestines of animals, but in which the regular crystallization and superposition of the plates can be easily traced. Similarly contorted fibres occurring in chalcedony do not exhibit the crystallized structure, but appear to consist of very minute or of invisible particles, or even of a mere colour diffused in that particular form. At times the coloured fibre contains grains of chlorite attached to its sides or interspersed through its course, thus emulating the imbedded seeds or jointed appearance of real confervæ, and I have reason to think often mistaken for them. I am uncertain whether I should rank under the head of chlorite certain contorted fibres

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