Mr. PouLETT ScROPE OH the Geology of the Ponza Isles. 219 



Zannone* 



I have already adverted ft) the island of Zannone as lying in the ideal pro- 

 longation of Ponza, with which it is connected by a chain of sunk rocks 

 detected by soundings, besides the more obvious ones which rise above the 

 water-level in the same line. 



The area of this island is scarcely a mile in its longest dimension, which 

 runs North and South, and half a mile in width. It consists of a ragged 

 longitudinal ridge rising gradually from the southern extremity, and after 

 suffering one abrupt depression from which it recovers immediately, termina- 

 ting towards the N. at a considerable height in a steep and rocky promontory 

 called Capo Negro. The slopes on either side are covered by low brushwood, 

 and afford occasional pasture to a flock of goats and some cattle belonging to 

 proprietors resident in Ponza. 



The geological constitution of this island is singular, since above two 

 thirds of its mass is composed of trachyte, which rests at the North point upon 

 limestone. 



The former rock is extremely siliceous, and identical with that of La Gabbia, 

 and the northern portion of Ponza, exhibiting the same intimate mixture of 

 concretionary ramifications having a flinty texture and a more or less porphy- 

 ritic structure, with white earthy patches. It is associated in the same manner 

 with marbled and brecciated masses assuming a great diversity of textures 

 and colours in the same block. The white porphyritic trachyte with crystals 

 of quartz is, however, the predominant rock ; and composes in particular the 

 mass which immediately overlies the limestone of the Capo Negro, The mica 

 which is here abundant, in some parts of the rock has decomposed to a certain 

 depth from the weathered surfaces into a reddish ochre. The felspar is bril- 

 liant, and the base extremely siliceous, and of a pure white. This rock is 

 occasionally divided into tangled groups of very regular columns varying in 

 size, but sometimes as small as those of the earthy prismatic trachyte in Ponza. 

 In its general character, however, it presents a singularly rude, shapeless, and 

 craggy outline, and all the external features of burr-stone ; the decomposition 

 of the mica and corrosion of the earthy parts, giving a cellular and carious 

 aspect to its superficial parts. 



It is possible that some of the columnar groups which I observed on the 

 eastern side of the island may consist of earthy trachyte ; but, as the shores 

 of the trachytic portion present few vertical sections of any considerable sur- 

 face, and are rendered otherwise difficult of access by heaps of enormous 



* See Plate XXV. fig. 5 and 6. 



