302 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 6, 



and the section, fig. 5, p. 284, gives some idea of the relations of its 

 brim to tlie central trachyte. 



By reference to the section (fig. o) it will be seen, that however the 

 summit of the Monte della Croce be nearly equidistant from any por- 

 tion of that part of the adjacent brim properly called the "Cortinella" 

 (as insisted on by ^I. Pilla), the centre of the trachyte is nearly double 

 the distance from the less elevated segments of the brim south and 

 east of the ^-illage of Rocca Monfina ; and therefore it is, that the 

 occurrence of some trachyte in situ which I observed in the inter- 

 vening valley is of importance in leading us to any sound theoretical 

 inference. At the same time, no clearer geographical proof can be 

 given that the chief mass of the trachytic mountain is not in the centre 

 of the crater, than that in proceeding from Sessa to the town of Rocca 

 Monfina, the traveller having ascended by the chief watercourse on 

 the west by which mills are tuj-ned, and havmg passed through a 

 gorge in the volcanic brim, thence traverses the crater directly from 

 S.S.^^. to N.N.E., without touching upon the steep southern face 

 of the ]Monte della Croce, which he leaves about half a mile on the 

 left hand. 



Nothing can be more in contrast than the central trachyte of the 

 Monte della Croce and the dejections which surround the crater and 

 dip away into the surrounding low countries. To a person unac- 

 customed to trace the transitions between submarine volcanic rocks 

 and those formed in the atmosphere, it would necessarily appear 

 difficult to distinguish some of those dejections which slope dovMi to 

 Toro di Sessa and Sessa (such as the scoriaceous masses with some 

 lapilli) from what are now forming in Vesuvius ; but on the other 

 hand, he -uill, on careful scrutiny, observe that these graduate into, 

 and alternate icith, hard leucitic rocks and peperino (quarried as 

 millstones) and amygdaloids, both of which are unlike modem pro- 

 ductions, and have a porphyritic cast*. The distinctions between 

 these external productions of Rocca ]\Ionfina and the modern ejec- 

 tions of Vesuvius are indeed strikingly exhibited in the crystals of 

 leucite, which in the former or more ancient rock range from half an 

 inch to 2^ and 3 inches in diameter, whilst m the showers of Vesu- 

 vius they do not exceed the size of peasf . Again, instead of true 

 pumice, he meets with vast mounds of wliite felspathic aqueously- 

 formed trass, from which the Fiume Bianca and adjacent rivulets 

 have derived so milky a colour and so unwholesome a character, that 

 to improve the health of the population, it was deemed necessary to 

 construct an aqueduct, which conveys the purest water to Sessa from 

 the central dome of hard subcrystalline trachyte in which the 

 springs issue. Independently then of the manifest distinction be- 



* In respect to the an-angeraent of the external volcanic dejections, I cannot 

 admit as a general rule that the leucitic lava caps the trass and tuff, as cited by Dr. 

 Daubeny ; for in the ravines above Toro di Sessa, I met with numerous alternations 

 of these rocks, including those passages from the one to the other to which I here 

 advert. 



t A copious shower of these small crystals of leucite issued from Vesuvius in 

 1847, and is described bv Professor Scacchi. 



