Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/393

 least affecting its figure or dimensions. It then proceeds gradually, till the decomposition and regeneration are complete. Specimens of pyrites are found, in which only the surface to a very slight depth is in the state of hepatic iron, as well as prisms of phosphate of lead, which are precisely in the same circumstances. We find also crystals of these two substances, in which, though the centre has participated in the same decomposition and regeneration, particles of greater or less bulk, that are nowise altered, remain interspersed here and there in the regenerated substance. In phosphate of lead, which has passed, into the state of galena, we frequently observe one or more laminæ, of different degrees of thickness, parallel to the planes that form the exterior surface of the hexahedral prism, which have still retained their primitive form. In the interior of these prisms the galena is in a state of confused crystallization with small laminæ, frequently lying in different directions, so that the fracture, which is irregular and granular, and has no resemblance to what we should expect in sulphate of lead or phosphate of lead, exhibits nothing but shining laminæ of galena without any determinate direction. Frequently too we observe that in the two transitions of which I have been speaking, when they are completed, there are several small cavities, in which the decomposed substance has not been replaced.

In the two examples quoted, though a perfectly exact explanation. of the means employed by nature is very difficult, yet we can conceive