198 ON THE ORIGIN,, MATERIALS, COMPOSITION, 



pressive sources of those rocks which are now daily 

 forming on the surface of the globe. By the agency 

 of their fires, the earths are ejected in a state which, 

 as far as we know, is merely that of mixture, arid 

 united in the fluidity of fusion. By repose during a 

 process of slow cooling, various combinations take 

 place in these fluid masses ; and, according to cir- 

 cumstances which we ar$ but imperfectly able to ap- 

 preciate, there are formed numerous rocks, either 

 apparently simple, or compounded of the different 

 minerals that have been formed by the contending 

 affinities of the materials. These processes are imi- 

 table by art ; which, having first reduced the natural 

 compounds furnished in basalt or other rocks, to a 

 fluid and uniform glass in the laboratory fires, dis- 

 poses them so as to cool during long repose in a 

 gradual manner. Thus, by the slow cooling of the 

 most compounded materials of the glass-house fur- 

 nace, various imitations of rocks are formed ; and 

 thus, more precisely, the greenstones of the trap 

 family are destroyed and again regenerated. 



In examining, now, those rocks which have been 

 formed out of our sight, we find one family which 

 produces many counterparts to the volcanic rocks, 

 namely, the family of trap. So absolute indeed is 

 the identity between many members in each set, that 

 no eye nor any analysis can distinguish them. To 

 attempt to prove this by an enumeration of specimens 

 in each, would be only to give a list of names that 

 would carry no conviction. But no more convincing 

 proof is wanted than this ; that, to this moment, 

 geologists continue to dispute about what belongs to 

 the trap family and what is of volcanic origin ; not 

 only in countries remote from volcanoes, or no longer 

 containing the marks of former activity, not only in 



