Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/522

512 examination will always detect in these the vegetable disposition, and the sections of the stone will, by cutting through the branches in various oblique directions, lay bare the true structure and distinguish the plant from its adventitious and metallic covering.

A third, and the most common source of deception and obscurity, will be found in the whimsical and fibrous disposition occasionally assumed by chlorite, its colour often imitating the natural hue of a plant as perfectly as its fibrous and ramified appearance does the disposition and form of one. It is by this substance particularly that appearances strongly resembling confervæ are produced, and nothing but a very accurate investigation, with considerable experience in the various forms which this substance puts on, as well as with the aspect and characters of the plants imitated, is sufficient to enable us to distinguish them. In many cases even the most scrupulous attention will fail, a fact which need excite no surprize when it is considered with what difficulty the examination must often be conducted, from a deficiency of light when transmitted through these stones, and from the impossibility of bringing into view any considerable portion of the imbedded substance. Here we must have recourse to the chemical means which I shall presently describe.

It is I believe from the deception to which those specimens have exposed mineralogists, that so much incredulity on the subject exists, every green fibre having been supposed to be chlorite, because it was apparent that chlorite sometimes assumed the form of green fibres. However difficult the distinction, it will be found real, and with sufficient care, generally practicable. The subjoined drawings contain representations of some specimens which are undoubtedly chlorite, and of others which present a very suspicious aspect. Among them however will be found some exhibiting an organization