Mr, PouLETT ScROPE on the Geology of the Ponza Isles. 229 



duced by this process indifferently from both a trachytic conglomerate^, and a 

 solid rock of the same mineral constitution. 



These localities, however, do not alone furnish observations of this kind, I 

 could instance various spots in the vicinity of Naples and the isles of Procida 

 and Lipari, in which strata of pitchstone, porphyritic and occasionally with the 

 structure of pearlstone, intervene between intruded beds or dykes of feldspa- 

 those lava-rocks and their accompanying conglomerates, graduating into both. 



Rocks differing in mineral constitution when submitted to the circumstances 

 under which this modification takes place, will of course produce different 

 species of pitchstone. Thus in Ponza and Palmarola this substance varies 

 with the varying character of the rocks into which it graduates ; perhaps even 

 more than might have been expected from the great similarity of mineral 

 composition in the unchanged rocks. All are obviously varieties of resinous 

 or semi-vitrified trachyte. 



The pitchstone of Hungary, according to M. Beudant, passes by repeated 

 and easy gradations both into his per lite (semi-vitreous trachyte), and into a 

 compact lithoidal felspar rock resembling the " porphyre trachytique " and 

 " porphyre molaire." 



The pitchstone of the Cantal is derived from a clinkstone (slaty trachyte). 

 That of Egg appears to be basaltic, graduating into that rock. The pitch- 

 stone of the valley of Tribitsch in Saxony repeatedly passes into a compact 

 porphyritic felspar, described by M. Beudant as mineralogically identical with 

 varieties of the Hungarian trachytes. At Grantola, which I visited in 1821, 

 this substance intervenes between a felspar porphyry and overlying strata of 

 old red sandstone, and blends imperceptibly with both these rocks. The 

 pitchstones of Scotland are said to occur almost invariably in veins. This 

 situation is easily reconciled to the idea of the fusion of the congenerous rock 

 beneath some overlying bed, which must be split by the expansion accom- 

 panying this process, and its fissures occupied by the liquid matter, under 

 such circumstances of pressure and friction as seem requisite for the produc- 

 tion of the peculiar texture of pitchstone. 



5. The siliceous trachyte of Ponza, La Gabbia, and Zannone, is of consi- 

 derable interest ; since, though the existence of quartz as a constituent mine- 

 ral in trachyte is no longer a fact of such rare occurrence as it was once sup- 

 posed, yet in no instance that I am acquainted with, has it been found to 

 enter so largely into the composition of the rock. Even the porphyre molaire 

 of Hungary is far less siliceous than the analogous rock in Ponza, There are 

 two causes, to either of which a hasty decision might refer this abundance of 

 quartz : viz, 1st. Infiltrations of water impregnated with siliceous matter ; and 



