206 Mr. PouLETT Scrope on the Geology of the Ponza Isles. 



In the globular pitchstone this process has been accelerated by the quantity of 

 cubic pyrites disseminated throughout it. Sometimes^ in the immediate vici- 

 nity of the prismatic trachyte, the pitchstone incloses small nuclei of obsidian 

 of an olive green colour, translucent at the edges, and with the conchoidal frac- 

 ture of glass ; and the pitchstone becomes more fine-grained, compact, and 

 homogeneous the nearer it lies to the boundary of the prismatic trachyte. 



This boundary is universally an abrupt and decided separation ; but, on the 

 other side, the former rock graduates insensibly into the earthy and friable 

 conglomerate, passing through a modification of pitchstone of which the pre- 

 dominant colours are an orange-green and a waxy yellow. It contains the 

 same minerals, and has the same texture and lamellar structure as the dark 

 bottle-green pitchstone ; except that, as it recedes from the latter rock, it be- 

 comes less compact, and slightly porous. The conglomerate structure may be 

 tmced very clearly through this intermediate rock. The imbedded fragments 

 sometimes assume the resinous lustre and texture more completely than the 

 base : sometimes the reverse takes place ; probably according to the original 

 nature of the fragments. When these were of earthy unvitrified trachyte, 

 they have resisted more or less the agency which tended to reduce them to 

 pitchstone, and remain visibly enveloped in the more resinous base. This 

 often exhibits a wrinkled appearance incircling these refractory fragments, 

 from which it appears to have possessed a partial fluidity. This rock is fre- 

 quently penetrated by minute veins filled with a white earthy substance, of a 

 feldspathose character; they traverse both varieties of pitchstone, and ramify 

 into the conglomerate without changing their direction. In the Chiaja di 

 Frontone, two vertical dykes, averaging two feet in width, cut through the 

 green pitchstone, which appears here of unusual extent. They are filled 

 with a loose white trachytic conglomerate, extremely fine-grained. It was 

 probably washed into fissures of the rock by the water which held it in 

 suspension. 



The constant occurrence of these bands of green and yellow pitchstone at 

 the junction of the prismatic trachyte and the semi-vitreous conglomerate *, 

 and the easy gradations of colour, texture and structure, by which they inva- 

 riably pass into the latter rock, unquestionably prove them to proceed from its 

 alteration by contact with the prismatic trachyte. In fact, this rock, as has 

 been already noticed, bears on most points the appearance of having forced 

 its w^ay upwards by cutting through the incumbent conglomerate in various 

 directions, and occasionally of having spread laterally over it. Its regular co- 



* See Plates XXIV. and XXV. 



