Mr. PouLETT ScROPE on the Geologi/ of the Ponza Isles. 203 



half an inch to four or five in diameter, are closely grouped together, in vast 

 numbers, into clusters ; ami by the sharpness of their angles, seen in the per- 

 pendicular face of the cliffs, exhibit an appearance, which was compared both 

 by Dolomieu and Hamilton to some kinds of Roman brick-work. Those co- 

 lumns which are piled horizontally are universally the least and shortest, the 

 vertical ones much longer and of greater diameter. 



The superior hardness of this rock, aided probably by its prismatic structure 

 (which under a certain arrangement of the columns adds to the durability of 

 the mass as much as a different disposition often facilitates its destruction), has 

 enabled it on many points to resist (he breach of the sea more effectually than 

 the other varieties of trachyte, and the conglomerates. Hence it constitutes 

 most of the insulated rocks that surround the island ; its columnar structure 

 adding much to the picturesque effect of their outline. The prismatic tra- 

 chyte also forms a large part of the main island, particularly of its southern 

 moiety. It abounds throughout the range of vertical cliffs of the whole east- 

 ern shore, as well as on the opposite side of the island from the Punta del 

 Fielo to that de' Faraglioni, 



In these natural sections it is constantly accompanied by a rock which I am 

 about to describe, with which it alternates in irregular massive beds, assuming 

 every possible direction, except perhaps the horizontal. 



This other rock may be designated as a 



Semi-vitreous Trachytic Conglomerate. 



In its ordinary character it consists of an earthy and very friable substance 

 of a pure white or grayish white colour, inclosing numerous fragmentary 

 blocks of an ash gray or grayish green semi-vitreous trachyte, which have in 

 general much of the fibrous structure and elongated cells of pumice, are ex- 

 tremely brittle and light, and occasionally pass into common glassy pumice. 



On a close examination the basis of the rock appears to consist of pulveru- 

 lent particles of the same substance. It crumbles easily, and seldom possesses 

 sufficient solidity to allow a specimen to be preserved entire. The inclosed frag- 

 ments reach the size of a foot or more in diameter. They contain imbedded 

 glassy felspars and bronzed mica. These crystals occur also, but less fre- 

 quently, in a loose state in the earthy base. Some few of the fragments consist 

 of trachyte without any glassy lustre, and resembling the prismatic trachyte 

 described above, or other varieties not met with in situ in the island. One large 

 block which I took out of this conglomerate was identical with the loose cry- 

 stalline trachytic lava of the Monte Tabor, in Ischia, of a dead leaf colour, 

 with numerous large and distinct crystals of glassy felspar. None of the frag- 



