Mr. PouLETT ScROPE ou the Geology of the Ponza Isles. 217 



after a partial separation had taken place of the pure feldspathose part from 

 the mixed siliceous base; the local obstacles^ both internal and external, that 

 more or less impeded this motion, were obviously the cause of the irregulari- 

 ties of the laminae. This species of trachyte composes the <^reater j)art of 

 the island and all the insulated rocks around, apparently detached from it 

 by the destruction of the intervening conglomerate, which yields with 

 greater facility to the erosive action of the waves. The columnar division 

 prevails, but the columns are generally larger than those of Ponza. On 

 the north side they present vertical ranges of great magnificence and regu- 

 larity, running up to a height of from one to two hundred feet. Many of 

 the columns are articulated. The exposure of these mural ranges to the 

 breach of the sea at the north point, has produced several caves of no great 

 depth, but with a striking architectural effect ; resembling that of Fingal in 

 Stafla, but having in general a much more lofty entrance. Some detached 

 rocks have by the same process been perforated with arches. 



The zones of this rock are unequally affected by the wasting influence of 

 the atmosphere, the loose feldspathose layers decomposing more readily than 

 the others ; and this circumstance gives the false appearance of a schistose or 

 laminar structure to the exposed surfaces of the rock. The direction of the 

 zones, which is continuous through a great extent, and seems usually to coin- 

 cide with that of the bed or dyke itself, cuts the axes of the columns indiffe- 

 rently at every angle. The surface of the rocks, as at Ponza, is of a rusty or 

 bluish brown. ?*- ;-.Jt,j v;. 



The huge rocky boss that forms the northern extremity of Palmarola, en- 

 tirely consists of prismatic trachyte. It may be observed to abut at a very 

 high angle upon rocks of a different nature on either side of the island. To 

 the east the supporting rock is the ordinary white conglomerate with frag- 

 ments of glassy trachyte, which is converted, as usual, into dark green resi- 

 nous trachyte (pitchstone), the passage taking place, as elsewhere, by imper- 

 ceptible gradations through a band of reddish yellow semi-resinous trachyte. 

 On the opposite side of the hill fronting the west, the northern rock of pris- 

 matic trachyte is separated from another mass of the same nature, by an inter- 

 vening bed, nearly vertical, of a dark coloured pearlstone, from 23 to 30 feet 

 in thickness, which graduates into the latter rock, and appears to be the re- 

 sult of an alteration produced by the contact of the neighbouring trachytic 

 rock, similar and contemporaneous to that effected by the same mass in the 

 conglomerate on the other side of the island. 



The prismatic trachyte subjected to this change has lost its columnar divi- 

 sion, but preserved its zoned structure ; the pearlstone presenting the same 



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