Mr. Poui.ETT ScROPE on the Geology of the Ponza Isles. 199 



which connect Zannone and La Gabbia, indicate its former extension in a 

 longitudinal direction. Besides those which appear above the water-level, there 

 are many sunk rocks between and around them. At a certain distance how- 

 ever from the island, the water suddenly deepens and becomes unfathomable. 



The southern extremity swells into a bulky and rounded eminence called 

 La Montagna della Guardia, a name common to the principal height of almost 

 every island in the Italian part of the Mediterranean, and derived from the 

 look-out which till within a very few years was necessarily kept there day and 

 night, to give notice of the approach of the Barbaresque corsairs, which were 

 in the habit of paying frequent spoliatory visits to these straggling and unpro- 

 tected islands. This mountain, if it may be called so, rises to about 800 feet 

 above the sea, while no other point of the island exceeds 500. It has a gradual 

 and concave slope on the north-east towards the circular cove which forms the 

 port, and the neck of land which connects it with the remainder of the 

 island. On every other side it is bounded by cliffs, with or without a talus at 

 their base. 



Throughout the remaining circumference of the island, in consequence of 

 the depth of the sea by which it is continually corroded, and, probably, by 

 reason, also, of the existence of some marine current which sweeps away the 

 detritus of its shores, no accumulation of fragments appears to take place at the 

 foot of their escarpments. These, therefore,, present the most complete natural 

 sections, in which the constitution of the island may be studied with the greatest 

 facility. 



The brilliant whiteness of many rocks, the strongly contrasting colours of 

 others, their remarkable forms, and the irregular manner in which they are 

 mutually disposed, give an extraordinary and striking aspect to the island, 

 particularly when approached from the east. The range of cliffs on this side, 

 seen from a short distance, presents the characters of an ideal geological 

 section coloured with conventional tints to distinguish the different rocks 

 which compose it. The colours made use of for this purpose in the accom- 

 panying sketches are the same as in Nature, and scarcely deepened in tint.* 



The substance of the island, with the exception perhaps of the massive bed 

 which forms the capping and the southern front of the Montagna della Guar- 

 dia, is trachyte f . This rock, however, varies considerably, and frequently 



' * See Plates XXIV. and XXV. 



t Trachyte occurs so rarely, if at all, in this country, and appears in consequence to be so little 

 known amongst English geologists, that I may be perhaps pardoned for introducing here a brief 

 description of its general characters, and principal specific varieties. 



Trachyte, then, is a rock composed essentially of felspar at least in the proportion of 90 per 

 VOL. II. SECOND SERIES. 2 D 



