been quoted in Dr. Thomson's journal, noticing the same fact, and professing the removal of his former doubts. It will be a sufficient apology for not recalling this notice after having thus discovered that the inquiry had not the merit of novelty, that so many should have overlooked, and so many others refused their assent to a fact of no uncommon occurrence.
The metallic arborizations emulating the vegetable form which occur in the fissures of many stones, and the similar well-known figures in the chalcedonies distinguished by the name of mochas, with the lively though superficial resemblance they bear to plants, have led to the hasty conclusion that all these appearances were of a metallic nature, and have probably prevented that accurate investigation of them which they deserved. The want of botanical knowledge has perhaps also assisted in concealing from most mineralogists their true origin, but I may now hope that the possessors of such specimens will hereafter by a more attentive examination confirm the frequency of an occurrence so interesting.
Another circumstance has assisted in this case in deceiving mineralogists, and that is the obscurity in which the vegetable is often involved either by the accidental mixture of metallic oxides in the same stone, or by the actual investment of the whole plant with a thick crust of carbonat of iron. Many of the specimens, and most commonly those which contain confervæ, exhibit at first sight nothing but a confused and entangled fibrous mass of oxide or carbonat of iron. It requires a patient observation to detect in these the existence of a real vegetable structure contained in the stone, and modifying the deposition of metallic matter. It will be found in fact that the whole plant is incrusted with the metallic deposit, and that it exhibits only here and there its true nature. An accurate