AND ANALOGIES, OF ROCKS. 199 



the Vivarais and the Euganean 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 circumstances which will find a better 

 place in treating of this family hereafter. Thus ana- 

 logy, resemblance, and experiment, confirm that 

 opinion respecting the trap rocks which would be in- 

 ferred from the peculiarities of their chemical con- 

 stitution ; and thus also they confirm the conclu- 

 sions elsewhere drawn from their peculiar disposi- 

 tion, and from the nature of their connexion with the 

 various conterminous rocks among which they are 

 found. 



It is but a step from the trap rocks to granite; and 

 if the identity of specimens is not always so perfect, 

 or the resemblance so general and extensive between 

 these and the volcanic rocks, the analogical reasoning 

 is quite as unexceptionable. I have shown in another 

 place, (Chap, x.) that many rocks, forming integrant 

 portions of a granite mass, are undistinguishable from 

 many of the traps, and that among these, there are 

 many that resemble the productions of volcanoes. 

 Here then is an identity, even between granites and 

 volcanic rocks ; and, here also, what is true respecting 

 the origin of one part of the mass must be true re- 

 specting the whole. If that inference appears to be 

 drawn closer than the circumstances seem to warrant, 

 we may carry it through the intermediate stage of 

 trap; and having thus proved the identity of this rock 

 with the volcanic products on the one hand and with 



