ITALY. LETTER XXVI. 325 



that the fpots of feld-fparfi have got by the action 

 of the air an opaque milk-colour ; and that thofe 

 of the porphyry hills, now under confideration, 

 confift of a flem-coloured feld-fpath, or of a vi- 

 treous tranfparent Iherl, fimilar to that which in 

 the Vefuvian lavas formed garnet-like cryftals, 

 but here appears in undetermined grains. The 

 black porphyry in thefe hills is fpotted with the 

 fame vitreous fherl, either in oblong or undeter- 

 mined forms. The fimilarity of this porphyry 

 with fome Vefuvian lavas is fo very great, that 

 even the ikilfulleft Mineralogift might confound 

 them ; and I am convinced that thefe Newmark 

 porphyries are true lavas, though I would not 

 venture any general afiertion. Another circum- 

 ftance, which furprized me, countenances this 

 opinion. All thefe porphyry hills confifted either 

 of feparated or Hill coherent rhomboidal columns-, 

 or they had, upon the whole, a tendency to fplit 

 into thefe forms, which is a quality of feveral forts 

 of bafaltes. Thefe high porphyry hills continued 

 almoft to Brandfol y and were generally cracked 

 and fplit, moftly in fquare columns, with a flat 

 oblique broken top , their form fo regular, that 

 it cannot be confidered as accidental, but as that 

 of regular cryftallized columns. The angles of 

 the oblique mTures were commonly oblique, that 



