ON THE TOLCANIC PnENOMENA OF VESCVIUS. 293 



represented by a fibrous, silvery grey pumice. The upper part is com- 

 posed of 2m. of coarse gritty sanidinic sand, with minute frngments of 

 the different rocks beneath, and represents the diminution of the erup- 

 tive activity. Finally this passes up into 0"7m. of reddish gritty earth, 

 representing the last ejections of fine ash, eventually converted into vege- 

 table soil : at least 12m. 



Grey pipernoid tuff, quite similar to that of Nocera and other locali- 

 ties, containing fragments of marialite-bearing piperno — the lowest, 

 0'30m., reddened; about 4"30m. 



Black dust band, O'lOm. 



Bed of coarse masses of light coloured pumice, the upper third 

 reddened, O'SOm. 



White pumice interbedded with thin dust bands, some buff and others 

 red, conformably under last, 400m. 



Rather uniform bed of nearly pure small white pumice, O'OOm. 

 lying conformably on. 



Brown ashy tuff, with bands of white pumice, 400m. 



Grey pipernoid Uiffs of the Campania. — The remarkable relationship of 

 this section, together with that of Piauura-Soccavo to the grey tuffs that 

 crop out all round the Campanian plain has led me to examine these 

 further. Although that examination is not completed it is advisable to 

 give the results, as already the number of facts I have collected is sufii- 

 cient to afford some explanation of this i-emarkable deposit. 



Professor A. Scacchi, in a series of valuable memoirs on the peculiar 

 metamorphism that the enclosed fragments of limestone exhibit, has 

 touched upon the origin of this tuff. On account of its slight variation 

 in colour, the number of enclosed scoria fragments and the different 

 degrees of alteration of the enclosed limestone blocks, he supposes 

 that innumerable eruptive mouths have broken out around the bases of 

 the limestone mountains, from which volcanic mud has been poured forth 

 and blocks of limestone ejected, which had already been to some extent 

 acted upon by hydroflaosilicic acid. Now, when we examine this deposit 

 in the field, we find that it chokes all the valleys, forming an important 

 deposit 20 to 30 kilometres around the actual limits of the Campanian 

 plain. To explain such deposits it would be necessary to admit the ex- 

 istence of hundreds of eruptive mouths, or to suppose that these great 

 streams of mud had actually flowed up instead of down the valleys In 

 the next place, I have met with important deposits of this tuff, as much 

 as 500 meters or more above sea level. There are dozens of such localities 

 known to me, where a slight interruption on the steep slope of a mountain 

 has permitted this material to resist the denuding agencies. In some of 

 these localities the limestone rock above the deposic is bare right up to 

 the summit, and there is no eruptive mouth visible. We must therefore 

 conclude that this tuff was not mud, since there is nowhere from which it 

 could have flowed. It is evident that it was an ash that fell, and, swept 

 down the mountain by rain, was converted into mud ; this is quite 

 different from the supposed eruption of mud, still a questionable pheno- 

 menon from real volcanoes. Neither is there evidence of a single eruptive 

 orifice ; and all the variations can be explained by wind action, which 

 would carry scoria fragments more in one direction than in another, and, 

 by changing during the eruption, cause irregularity of distribution in any 



