Composition, and Analogies of Rocks. 33 



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 whicli, as far as we know, is merely that of mixture, and 

 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 circumstances which we are but im- 

 perfectly able to appreciate, there are formed numerous rocks, 

 either apparently simple, or compounded of the different mine- 

 rals that have been formed by the contending affinities of the 

 materials. These processes are imitable 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 man- 

 ner. Thus, by the slow cooling of the most compounded ma- 

 terials of the glass-house furnace, 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 the Vivarais and the Euga- 

 nean hills, but at the very seats of living volcanoes. If, therefore, 

 out of a common mass of rock, or among many different ones 

 evidently formed under the same circumstances, there are parts 

 which bear all the marks of an origin similar to that of volcanic 

 rocks, it is evident that the whole must be referred to the same 

 source, with certain exceptions arising from collateral circum- 

 stances that it is not within the limits of this paper to notice. 

 Vol. XIX. D 



