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OVERLYIN'6 AND TRAP ROCKS. 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 parallelposition 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-