248 Notice of Active and Extinct Volcanos. 



tude of little globules analogous to those in the pearlstone, which 

 seem to be of a felspathx nature, and when broken, are found 

 to contain in their centre a little crystal of quartz, or a speck of 

 some siliceous substance. 



" The<e globules in some cases compose the whole substance 

 of the paste, in others they are held together by a sort of har- 

 dened clay, which here and there resembles porcelain-jasper. 

 Notwithstanding these distinctions, there is a greater degree of 

 uniformity in the characters of this, than in those of the other 

 species of trachyte, and the most obvious differences between 

 the several parts of this formation relate to the size and direc- 

 tion of the cells, which are sometimes so small and narrow, as to 

 give to the rock a fibrous character, sometimes of considerable 

 size, in which case they are in general coated internally with 

 crystals of quartz. 



" The fifth and last species comprehended by Beudant under 

 the generic term of Trachyte, consists of those heaps of pum- 

 ice, and other loose materials, that occur agglutinated together 

 on the slopes and at the base of the rocks belonging to the four 

 preceding classes. Although the prevailing constituent is pum- 

 ice, every variety of rock found in the neighboring hills is met 

 with amongst the fragments. The latter vary extremely in size, 

 as well as in the mode of their aggregation ; the cement which 

 unites them is often of a porphyritic character, hardly distin- 

 guishable from the fragments themselves. Like them it often 

 contains crystals of felspar, mica and hornblende, and sometimes 

 grains of titaniferous iron are diffused through it, or it is colored 

 red by the peroxide of that metal. 



u The fragments of pumice are united together either imme- 

 diately, or by the intervention of a paste of a vitreous character 

 resembling obsidian, into which the pumice passes insensibly. 

 Here and there the rock itself has become decomposed, and its 

 destruction has given rise to beds either of a cellular nature ari- 

 sing from minute portions of pumice, .which still preserve their 

 fibrous texture, or (where all traces of this have been oblitera- 

 ted) to masses of an earthy character similar to the trass of the 

 Rhine volcanos or the " tripolV of those in Auvergne. r 



The trachytic conglomerate has evidently undergone de- 

 composition in various ways. Sometimes it contains marine 

 shells, similar to those found in the calcaire grossier of the 

 Paris basin ; sometimes the infiltration of siliceous matter has 

 mineralized in it the stems of vegetables of a cylindrical 

 form, often hollow, and crystals of felspar, mica, quartz, and 

 garnet, are distributed through the substance of the mass. 



It is also decomposed into alum, which is profitably ex- 



