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



show a minute spherolitic structure*. The globules are of felspar, generally 

 hollow, and internally ragged and angular. The base is compact, fine-grained^ 

 somewhat siliceou=, and of a dark gray or a reddish brown colour. Complete 

 and gradual transitions from this to the ordinary zoned or marbled variety may 

 be observed within the compass of the same block ; the spherolitic concretions 

 increasing in number, and appearing drawn out into a long ellipsoidal or len- 

 ticular form, until they form together a whitish earthy zone or vein ; while 

 the intervening compact matter is equally stretched out into stripes which re- 

 tain their colour and texture. These passages lead me to be of opinion that 

 even where no traces of spherolitic structure are visible, the ribboned arrange- 

 ment of the rock is owing to the mass of matter having been drawn out in the 

 direction of the zones while still liquid, after a previous separation had taken 

 place of the feldspathose spherolites, (which are analogous to the vitro-lithoid 

 globules of glass furnaces,) and the impure semi-siliceous base. A close exa- 

 mination of these zones, makes it obvious that the finer and darker parts pos- 

 sessed a higher fluidity than the earthy and more feldspathose ones, and that 

 the different degrees of mobility of these alternate layers, was the chief cause 

 of their irregular wavings and contortions. Almost every distinct crystal of 

 felspar or mica may likewise be observed to have produced a similar irregu- 

 larity in the neighbouring zones, proportioned to its size and form, by the 

 resistance it offered to their freedom of motion. The elongated crystals of 

 both minerals have generally taken the direction of the movement, their longer 

 axes lying more or less in the plane of the zones ; and frequently a single 

 crystal of felspar will appear to have been broken up and carried away piece- 

 meal, by the friction of the neighbouring laminae, leaving a train of half-fused 

 fragments with empty spaces between them. 



The columns into which this rock is divided assume every direction, though 

 with a general tendency to the vertical. They are sometimes united into fan- 

 shaped clusters. At others, the rock appears separated into irregular vertical 

 beds, composed of columns arranged in bundles, like faggots, and disposed 

 horizontally, or nearly so ; their axes being always perpendicular to the sides 

 of the bed or vertical seams. The width of the bed is always proportioned to 

 the diameter of the columns. They are sometimes bent in one or more di- 

 rections, preserving however their general parallelism, and recovering by de- 

 grees the straight direction on either side. They are usually small, and the 

 smallest are the most regular. Very perfect five- or six-sided prisms, from 



* Similar to that of some of the Hungarian trachytes, the porphyres trachytiques of M. 

 Beudant. 



