222 Dr Charles Daubeny on the 



pacted aggregates of volcanic sand, and of stones promis- 

 cuously heaped one upon the other. 



In this tuff are often imbedded, not only its usual concomi- 

 tants, sand and rapilli, but also large blocks of a kind of por- 

 phyry, peculiar, as it would seem, to the products of the an- 

 cient volcanoes, of Monte Somma and of Rocca Monfina in the 

 Neapolitan territory, and of Acquapendente and Viterbo in 

 the Roman, characterised by crystals of leucite, which, at the 

 spot now under consideration, are often of extraordinary di- 

 mensions, sometimes two inches and a half in diameter, and 

 are accompanied by minute crystals of augite, both imbed- 

 ded in a felspathic basis. As the felspathic portion was more 

 readily decomposed than the imbedded crystals, these latter 

 might often be detached from their matrix in a state of great 

 integrity. 



Near the little village of Tuoro de Sessa, which is situate 

 very little below the external margin of the ancient crater, we 

 observe a continuous bed of this leucitic porphyry, resting upon 

 the tuff, and extending for some distance along a ravine which 

 runs obliquely down the sides of the mountain. This was the 

 only instance that occurred to me, in which the above rock 

 appeared in any other form than that of detached blocks. 



The most remarkable feature, however, in the physiognomy 

 of this mountain, and that which distinguishes it from every 

 other volcano I had seen, is the protrusion, from the interior 

 of the crater, of a conical mass of rock resembling trachyte,* 

 large enough to fill up two-thirds of the area comprehended 

 within the walls of the crater, and so lofty as to rise consi- 

 derably above the most elevated point in its margin ; consti- 

 tuting, indeed, when observed from a distance, the most con- 

 spicuous object embraced within the compass of the moun- 

 tain. 



This trachytic rock is much more abrupt than the tuff 



* See in Plate I. the section and ground plan of Rocca Monfina, the former re- 

 duced from a sketch taken by the artist who accompanied me on my visit to the 

 mountain ; the latter borrowed from a memoir by Professor Pilla, who, how- 

 ever, has represented the crater as though it were entirely effaced on the east, 

 whereas it appeared to me there distinctly traceable, although, undoubtedly, 

 much lower than it was on the west. 



