Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/392

 integrant molecules of these substances is alone sufficient to prevent our assenting to it; and it certainly cannot be that of the Abbé Haüy. But does a real transition take place in this case, according to the sense in which he uses the term Epigène?

As an example of what he means to express by this word, he quotes the transition of sulphuret of iron, or pyrites, to the state of hepatic iron, which is a reddish-brown oxide of that metal, more or less compact in some parts, and frequently cellular in others. He quotes also the transition of phosphate of lead in hexahedral prisms to sulphuret of lead, or galena.

In the first of these examples we can easily understand the nature of the operation which takes place, and which consists in the actual decomposition of the pyrites by the loss of the sulphur that was combined with the iron: a decomposition probably produced by the slow oxygenation of the pyrites, by which the sulphur is changed into sulphuric acid, and is disengaged, leaving the iron in the state of a brown oxide.

The second of these examples, which offers more difficulty, may however also be conceived, though without our being able clearly to trace what was the primary or direct cause of the change. It appears, with respect to this transition, that the decomposition, while it deprived the phosphate of lead of the phosphoric acid, probably by the intervention of sulphuric acid, occasioned at the same time the deoxydation of the lead as well as of the sulphuric acid, which then combined in the state of sulphur with the lead, and produced galena, which is so moulded into the space occupied by the phosphate of lead, that it has completely retained its external form.

In these two natural operations, so interesting and at the same time so remarkable, the change which occasions the decomposition and transformation of the substance begins at the surface, without in the