OVERLYING AND TRAP ROCKS. Ill 



The general structure is granitic; varying from that 

 of the coarsest granite, through all possible varieties, 

 to a degree of minuteness such that the particles are 

 scarcely distinguishable by the eye ; and thus, in many 

 of the intermediate states, resembling common green- 

 stone, with which it had been confounded by minera- 

 logists, who had even walked over miles of it. Some- 

 times containing superadded distinct crystals of felspar, 

 it resembles the porphyritic granites, and it also pre- 

 sents one other general resemblance to granite, in occa- 

 sionally containing garnets ; while, lastly, it sometimes 

 resembles gneiss; being fissile,from the parallel position 

 of the crystals of hypersthene. It resists decomposition 

 powerfully; appearing to undergo no waste, gene- 

 rating no soil, and thus presenting an aspect of bar- 

 renness unequalled by any other rock; while the inde- 

 structible hypersthene continues to project on the sur- 

 face, so as to cause a singular roughness by which it 

 is easily recognised, by the touch, or even by the feet, 

 blindfolded. 



The term PORPHYRY is applied to a rock in which 

 crystals of common felspar are imbedded in a simple 

 or a compound base. Hence the numerous varieties 

 noticed in the work so often mentioned. But this 

 structure does not form a geological distinction, nor 

 accompany any steady difference of character among 

 these rocks: occurring, even as an accident, in the 

 most recent traps, as in the most antient, where the 

 prevailing character is simple, and also in the other 

 members of this family here enumerated. To some of 

 the recent porphyries, the term Trachyte has been 

 lately applied; being extended also to other associated 

 rocks, and even to Pumice. I am not aware of any 

 peculiar advantages in this contrivance; which, like 

 the rest of this Gallic neology, seems to serve no pur- 



