Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/524

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Dr. Mac Culloch on Vegetable remains

which occur in carnelian, and (though much more rarely) in the agate nodules of trap. These are of a colour sometimes ochry yellow, sometimes white, and sometimes brown. Their forms, so similar to the chlorite fibres, would induce us to class them together, and it is probable that they actually consist of this substance, having undergone some decomposition by which the green colour has become brown, from affections of the metallic colouring matter. It is at present impossible to account for this very singular disposition of the chlorite. In some cases it evidently forms the centre of a fine stalactite, of which the minute ramifications, afterwards preserved in further additions of chalcedony, put on the appearance well known in the green stained chalcedonies of Faroe. But that it is not necessarily stalactitical is certain, from its assuming the very same disposition when existing in quartz crystals. It is probably the result of a species of arborization, that obscure circumstance in crystallization which appears to depend on a sort of polarity between distinct crystals, or throughout the interrupted parts of the same one. Thus water on freezing, and various metallic substances on crystallizing, ramify in certain directions. Thus it will often be found that fibres of mesotype will hold their straight and radiating course across stilbite or other associated minerals, the continuity of direction in the portions of any crystal remaining unchanged, however the crystal itself may be interrupted by obstacles occurring in its course. The same appearance may frequently be observed in quartz and other crystallized substances, and it affords among the various phenomena of the obscure process of crystallization, not the least curious subject of inquiry.

It is in the transparent chalcedonies that the fibrous structures are most visible, but they are also of common occurrence in the opaque ones or agates, as they are usually called, although from the