348 Mr. PouLETT Scrope on the Volcanic District of Naples. 



from the sea. It is, however, by no means so regular in form or structure. 

 There are appearances of a crater on the side towards Naples, but the thick 

 woodsj with which its slopes are covered, render this indistinct. The opposite 

 side facing the north presents a very steep escarpment; at the foot of which, 

 near the village of Pianura, a massive bed of trachytic lava, covered with a 

 thick layer of loose conglomerate, crops out from under the prodigious thick- 

 ness of indurated tufa which composes all the upper part of the mountain. 

 This rock is quarried copiously, being in great request at Naples for steps, 

 flagging, jambs, and all purposes which require a harder material than the 

 tufa. It is known there by the name of Piperno*. The galleries pierced for 

 the extraction of this stone, run a considerable way under the hill. It is an 

 earthy and porous trachyte of a dark ash-gray colour, of which the base is 

 composed almost entirely of crystals of felspar, with a very few of augite, but 

 is interspersed in a remarkable manner with lenticular or flattish masses; of a 

 much darker and harder substance, in which augitic matter predominates, and 

 of which the texture is close-grained, and structure cellular. It would seem 

 that there has taken place throughout this lava a concretionary separation of 

 the augitic from the felspathose particles, apparently analogous to the process 

 by which flints are formed in chalky or lenticular nodules of marl in clay. The 

 difference of texture and structure between the parts is to be accounted for 

 by the difference in the size of their grain ; the augitic matter being fine, the 

 feldspathose coarse ; owing to which, the latter remain separated by pores or 

 rude interstices, while the former was sufficiently liquid to allow of the for- 

 mation of vesicles. The augitic concretions are lengthened, as was observed 

 by Brieslak, in the direction in which the lava appears to have flowed ; — a 

 fact which proves the concretionary process to have taken effect before the 

 lava had settled completely in its present situation. 



A trachytic lava with nearly the same characters, and aptly illustrating this 

 peculiar internal modification of the piperno, is met with amongst the products 

 of the neighbouring volcano of Rocca-Monfina near Sessa and St. Agata, and 

 likewise at Sorrento. In these places the feldspathose base of the rock is still 

 more earthy, porous, and loosely granular than at Pianura; so much so, indeed, 

 as to be with difficulty distinguishable from tufa ; and the augitic concretions 

 are also more fine-grained, compact, and vesicular, so as to resemble spongi- 

 form scoriae. It is obvious that, in proportion to this difference in bulk of the 



♦ This must not be confounded with the term Piperino, which is appropriated to the conglo- 

 merate of a leucitic rock, of volcanic origin, which occurs at Albano and other points in the 

 neighbourhood of Rome. 



