impossibility of transmitting a ray of light through these specimens, our sight of them is limited to that portion which happens to run along the fractured or polished surface. Numerous red, white and yellow agates are every where to be found among collectors and dealers, in which the appearance of fibres is such as to render it very probable that they are portions of vegetables incrusted by oxides of iron, although the obscurity of the specimens must necessarily render this uncertain until they are submitted to chemical trials. It these are vegetable fibres there is no difficulty in accounting for this concealment of the vegetable by the metallic deposit, since it is easy to understand how from a compound solution of flint and iron in water, the iron might first be precipitated on the vegetable, in the same way as we daily see it deposited in chalybeate springs, and how the subsequent deposition of flinty matter might involve and penetrate the whole. It is besides known to botanists, that many plants possess the property of attracting from their state of combination the earthy base of some of the salts which are dissolved in natural waters, a property which may be subservient to some purpose in the economy of the plant, unknown to us. Such is the case with Chara vulgaris, which is always found incrusted with a coating of chalk or calcareous subcarbonate, and such appears also to be the case with Byssus nivea.
It is worthy of remark that in almost all the specimens of chalcedony which appear to contain aquatic confervæ, not only the vegetable structure is perfectly preserved, but the plant, however light and yielding its texture, is disposed in as free a manner as if still living and floating in the water which was its native element. Together with these circumstances, the natural colour is often equally well preserved, and the various specimens of the confervæ in particular, which are the plants of most common occurrence,
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