214 Mr. PouLETT ScROPE on the Geology of the Ponza Isles. 



rock, which occurs very frequently among the volcanic districts of Italy. It is 

 always closely related to trachyte, and occasionally assumes, in some of its 

 parts, the mineralogical features of that rock. It is generally, however, distin- 

 guished from it by a greater closeness of grain and weight, by its uniformly 

 darker colours, and the compactness and brilliancy of its felspar crystals. It 

 passes on the other hand, by acquiring a scaly texture, great compactness, and 

 a laminar structure, into phonolite(cUnkstone) ; and when the latter character 

 is wanting, into compact felspar. In mineral constitution it is identical with 

 many rocks which have been classed by geologists under one or other of these 

 terms, being composed principally of felspar with a small proportion (never 

 perhaps above 10 per cent) of augite, hornblende, or mica. Its specific gra- 

 vity ranges from 265 to 2-90. To avoid circumlocution, and at the same 

 time distinguish it from those other rocks of the same family, I shall designate 

 it for the present by the name of gray stone*. Its colour is in fact the most 

 constant as well as obvious character of the rock ; and, wherever I have ob- 

 served it, is uniformly of some shade of gray, generally leaden, purplish, or 

 iron gray ; even the disseminated felspar crystals usually share more or less in 

 the predominant tint of their basis. 



The same rock appears in the Scoglio della Botte, Ventotiene, and San 

 Stefano, amongst the Ponza Isles. It is of very frequent occurrence throughout 

 Ischia. The insulated rock on which the castle is built is a well-known example. 

 It is found in the north-west angle of Procida, in the hill of Cumae, at Marmo- 

 rata, in the Monte Olibano, and many other points of the Phlegraean fields, in 

 almost all the Lipari Isles, and among some of the older currents of iEtna. In 

 most of these spots its disposition is obviously that of a lava bed or current, 

 and in some the crater whence it issued can still be distinguished. 



The massive covering of graystone from which the Montagna della Guar- 

 dia derives its boss-shaped form, is upwards of 300 feet in thickness. Towards 

 the east it rises in a bluff cliff from a rapidly sloping talus, strewn with enor- 

 mous blocks that have fallen from above. Where the vertical face of the rock 

 takes off from this inclined plane, a stratum of bright red friable tufa, like coarse 

 brick-dust, crops out from beneath the graystone, and reposes on the white 

 semi-vitreous conglomerate. Here, therefore, is another instance of partial 

 alteration in a conglomerate by contact with a later mass of lava-rock, though 

 in this case the alteration has not proceeded to any degree of fusion or vitri- 

 fication, as in the instance of the pitchstone beds. 



* It is the rock frequently described by Dolomieu and Spallanzani under the name of " lave 

 a base de roche de come." See "A Classification of Volcanic Rocks." Journal of the Royal 

 Institution, June 1826. 



