Mr. PouLETT ScROPE ow the Geology of the Ponza Isles. J 97 



that the examination of them is facilitated by the constant degradation of their 

 steep and naked chlTs, occasioned by their exposed situation in a boisterous 

 sea of great depths and the friable nature of many of their component rocks. 



The few trachytic districts which have been as yet examined with atten- 

 tion, and according to the improved method of modern science, occur inland, 

 and great difficulty has uniformly been experienced in every endeavour to 

 ascertain the relative positions of the different varieties that compose them; 

 a difficulty perhaps exaggerated by the prepossession for some favourite theory, 

 but cliiefly owing to the facility with which the rocks of this family yield to 

 the wasting influence of the weather; in consequence of which their debris 

 usually reach to so great a height against their escarpments, that few natural 

 sections of any extensive surface are to be obtained. In traversing such 

 inland districts the observations of the acutest geologist are almost necessarily 

 limited to geographical data ; the intervals between the knolls or protruding 

 masses of different varieties being concealed by detritus. This is a source of 

 peculiar obscurity among the trachytic formations, which, being unstratified, 

 give no indications, by the dip of their beds, of the relative positions of their 

 discordant members. Vertical sections in this case are invaluable, and without 

 them the geological observer is liable to be led into unavoidable and perni- 

 cious errors. So general indeed and fatal is this difficulty, that there still 

 exist doubts in the minds of geologists of great experience, and who have had 

 opportunities of examining more than one trachytic district, (but under the 

 disadvantages to which I allude,) as to whether this rock ever occurs in abso- 

 lute superposition to any other. 



For these reasons the Ponza Isles, whose internal structure is completely laid 

 open in the surfaces of their worn and skeleton forms, become objects of con- 

 siderable interest, notwithstanding the comparative smallness of their extent. 



Such were the considerations which induced me to visit them in the begin- 

 ning of May, 1822. I was on the whole very fortunate in weather, and was 

 enabled by the calmness of the sea to explore almost every part of them at 

 leisure, making the tour of each in a boat, and landing wherever it was judged 

 necessary to examine more closely the composition of the cliffs by which they 

 are invested. 



Ponza*. 



This, the largest island of the group, has a very remarkable figure ; of ex- 

 treme length in comparison with its width. The former may be estimated at 

 little more than four miles ; its width is exceedingly unequal. The shores are 

 eaten into numerous deep concavities, seemingly produced by the undermin- 



* See the Map, Pi. XXIII. 



