On Mineral Metamorphism. 365 



change proceeds. In many other cases the metamorphic 

 pi'ocess itself remains a mystery, and from the nature of the 

 products alone do we conclude that such a metamorphic pro- 

 cess has actually taken place ; as, for instance, when we find 

 neptunian rocks gradually passing into others, which, to judge 

 from their present condition, could not have been formed in 

 water. Geological science is in the same position in refer- 

 ence to metamorphic rocks, as mineralogy is with reference 

 to the pseudomorphic crystals : it acknowledges the change to 

 be a fact established by accurate observation, but by no means 

 depending on the probability or possibility of an explanation. 



Metamorphosis of Rocks by Heat and Cementation. 



The influence exercised by quick or slow cooling, the 

 pressure which keeps gases in their combinations, and the 

 effect which is thus exercised upon the aggregation and struc- 

 ture of the solidifying fused masses, has already been men- 

 tioned.* Also, regarding the change of stratified neptunian 

 rocks by heat, a multitude of facts have, in later times, been 

 collected, partly by direct observation of the effect of the 

 furnace-fires on the bricks of the furnace, or of burning beds 

 of coal on the adjacent strata ; and partly indirectly, by con- 

 cluding that this effect exists from the changes which such 

 neptunian rocks shew in the neighbourhood of others, of which, 

 it is supposed, they were at one time in a state of fusion, or 

 had, at least, been strongly heated, that is to say (i.e) we 

 reason from the relations of contact. Metallurgical processes 

 have shewn that the chemical condition of solid substances, 

 when exposed to high temperatures, may undergo alterations 

 without previous fusion. In the process of cementation, ii'on, 

 when nealed for some time with pounded charcoal, unites 

 with the charcoal and forms steel ; so also copper, when nealed 

 with zinc, is changed into brass. In Agordo and Roraas, 

 pyrites, containing only 2 per cent, of copper, when roasted in 

 pieces the size of a fist, become changed in the centre into 

 copper pyrites, yielding 7 per cent, pure copper, and this cen- 

 tral mass detaches itself with a smooth variegated surface, 



* Vide vol. i. p. i36 of Studer's Lehrbuch. 



