Mr. PouLETT ScROPE 071 the Geology of the Ponza Isles. 213 



of the Montagna della Guardia^ and forms a partial coverings to the slopes of 

 the hills to the north and west of the harbour. 



This stratified conglomerate is in turn surmounted by the huge bed of rock 

 which caps the Montagna della Guardia^ and occasions its superior elevation 

 over all the other heights of the island, where no subsequent formation covers 

 the primary trachytes. 



This rock varies in colour from a dark iron gray to a reddish or purplish 

 gray and lead blue. It is uniformly dense, heavy, and extremely hard, strik- 

 ing fire readily with steel, and so tough that specimens can with difficulty be 

 obtained from it by a hammer of the largest size. Very few porous parts are 

 observable. It appears to be composed principally of crystalline felspar, vary- 

 ing in the size of the grain from the compactness of some clinkstones, which 

 it then resembles, to a coarse granitoidal aggregation of large and brilliant 

 but imperfect crystals of gray or reddish white felspar, with a few small cry- 

 stals and numerous minute grains of augite, and some hexagonal prisms of 

 mica. I met with some felspar crystals nearly three inches in diameter, and 

 these are accompanied by augite, bearing the same proportion to them in size 

 as the ordinary minute grains to the common-sized felspars. In some parts 

 the basis alone is gray, and incloses a vast number of distinct rhomboidal 

 felspars of a reddish colour, which project from the exposed surfaces of the 

 rock by the destruction of the intermediate matter. In general the colour 

 appears to be owing to the dissemination of minute augite ; and even in the 

 large felspar crystals which are decidedly tinged with the predominant hue of 

 the rock, there is probably a certain proportion of augitic matter introduced 

 sufficient to produce this effect. 



This rock is every where more or less columnar. The columns of the upper 

 part of the bed which caps the hill are rudely formed, and of an enormous 

 size. Lower down they become more regular and smaller. The detached 

 rock forming the Punta della Guardia*, the extreme southern point of the 

 island, is an example of very perfect columns collected into a fascicular group 

 and with a double curvature. Many columns, by exposure to meteoric action, 

 and particularly to the sea-spray, become carious, wasting exactly in the man- 

 ner, if I may be allowed the comparison, of a decaying tooth. Their coating 

 is of a more durable nature than the interior, so that when once decomposition 

 has worn through this crust on any single point, it advances rapidly in all 

 directions through the inner substance, leaving the shell comparatively whole. 



The rock described above is one example of a particular species of felspar- 



* See Plate XXIV. fig. 3. 



