Mr, PouLETT ScROPE oTi the Geology of the Ponza Isles. 221 



Scoglio della Boite, 

 A peaked insular rock, which has acquired this name from the rude resem- 

 blance of its form to a boot, rises above the sea-level at about a third of the 

 distance from Ponza to Ventotiene. It is composed of graystone similar to 

 that of the Montagna della Guardia in the former island ; exceedingly compact, 

 hard, and tough, of a coarse crystalline or granitoidal texture ; the crystals 

 being almost solely of felspar with a deep purplish gray tinge. The visible 

 crystals of augite are few, small, and imperfect. The mass of the rock is 

 split into rude prismatic blocks. It is superficially coated by a thin pellicle of 

 a blackish colour like a dull and poor varnish, which I conceive to be analo- 

 gous to the black glazing observed by Humboldt to invest the surface of the 

 granitic rocks of the cataracts of the Orinoco, and apparently produced by 

 long exposure to sun and moisture. Beneath this the rock is remarkably 

 unaffected by decomposition, perhaps because the process of mechanical 

 abrasion by the violence of the waves which break upon it, advances more 

 rapidly than the chemical influence of the atmosphere. 



In calm weather the base of this rock may be seen to extend at no great 

 depth below the surface of the water to considerable distances in most direc- 

 tions, but particularly towards the west. There can be little doubt indeed 

 that it is the solitary remnant of a much more extensive mass, and is indebted 

 for preservation to its extreme solidity, without which it must have been long 

 ago swept away by the powerful forces of destruction by which in so exposed 

 a situation it is almost without intermission attacked. 



Ventotiene.* 



This island is removed considerably to the eastward of the group already 

 described, lying nearly at equal distances from Ponza and Ischia, upon a 

 straight line which may be imagined to connect them. Its area has nearly the 

 form of an acute rectangular triangle ; the base, which is about four miles in 



sembling Carrara marble in aspect. The inferior strata of gypsiferous sandstone and shell lime- 

 stone (which I conceive to be nexa red and muschelkalk) are likewise more or less altered where 

 they approach the granite. This last rock passes into an augitic porphyry (basalt), and on parts 

 into serpentine. The whole mass appears to have broken through these secondary strata from 

 below, elevating them to a great height in massive pyramidal mountains. On the borders of the 

 principal mass of basalt, other smaller dykes traverse the same secondary rocks. 

 » See Plate XXV. fig. 7. 



