268 AN ENQUIRY INTO THE NATURE OF [Ch. XIII. 



cooling, from a state of fusion : this shows that they may 

 have had the same origin, but at the same time it renders it 

 possible that the lavas have been derived from the fusion of 

 primary rocks at a great depth ; a conjecture not so very im- 

 probable, since fragments of the older rocks are sometimes 

 projected from the craters of volcanos. We know that many 

 mineral substances have been and can be produced by aqueous 

 as well as igneous agency : but if we only knew of one mode 

 of formation, it might be surmised that like substances were 

 similarly derived ; the inference, however, would not be in- 

 fallible. Most persons, for instance, if they found within a 

 rock a mass of solid native copper, would conclude that it 

 had been melted by fire, because they know that this metal 

 can be so formed by art, but are not, in general, aware of any 

 other mode of production. The conclusion, however, might 

 not be correct ; for by the precipitation of a solution of sul- 

 phate of copper, either by the direct application of the poles 

 of a galvanic battery, or by the electrical action resulting 

 from the immersion of iron, sheets, nodules, or even pieces 

 of copper of no inconsiderable size may be obtained ; and 

 sometimes so perfectly solid, when precipitated by iron, that 

 they resemble the products of the furnace. 



Besides, admitting that our planet was originally in a state 

 of fusion, and that it is still undergoing refrigeration, only 

 the external part or crust being as yet consolidated, it does 

 not follow, although the lavas issue from the same central 

 igneous mass as that from which the granitic rocks have been 

 also derived, that the latter have, therefore, been protruded 

 in like manner, because they may have resulted from the 

 original consolidation of the earth. 



It is necessary, however, in order to ascertain whether 

 this be the case or not, to enquire into the respective posi- 

 tions of the granitic and volcanic rocks : and, proceeding from 

 the known to the unknown, we must, in the first place, exa- 

 mine the phenomena of existing volcanos. It will not be 

 necessary to enter into minute details on this subject ; it will 



