Site of the Ancient City of the Aurunci, 235 



Jleport on Mineral Waters (British Association EeportSjVol. v.. 

 1836, p. 24), namely, why these and other thermal springs 

 which issue from granitic chains, hold carbonate of soda in 

 solution, but not carbonate of potass. 



Now, by considering the nature of its felspathic material, 

 it being one of those varieties of the mineral in which the 

 silica is in the proportion of three atoms to only one of base, 

 we may see the reason, why in granite a certain proportion 

 of quartz, or uncombined silica, is almost universally present. 



Its amount, in fact, represents the excess of silica existing 

 in the rock over and above that which could combine with 

 the alumina ; and hence it implies, that at the time, and at 

 the place where the granite was formed, there was not a suf- 

 ficient quantity present of the several bases to combine with 

 the whole of the silica. 



And if we examine the composition of the various rocks 

 which have been produced by the operation of volcanic forces 

 in ancient and in modern times, we shall be able to trace a 

 gradual scale of diminution in the proportion of silica, and a 

 corresponding increase in that of the bases present. 



The first great division of them is comprehended under the 

 name of trachyte, a general term for a class of rocks of igne- 

 ous formation, characterised mineralogically by their harsh 

 and gritty feel, together with the frequent presence of cry- 

 stals of glassy felspar ; and, chemically, as being trisilicates 

 with or without an excess of silica. 



They are divided by Abich, who follows in a great degree 

 the classification of Beudant, into, 



1. Trachytic porphyry, in which quartz is present, but 

 neither hornblende, augite, nor titaniferous iron appear. It 

 is found, not only in Hungary, but also in the Ponza, and in 

 some of the Lipari islands. 



2. Trachyte properly so called, in which no quartz occurs, 

 but which contains crystals of hornblende and even of augite, 

 together with mica. 



3. Andesite, the trachytic rock of the Andes, described by 

 Abich as being of various degrees of compactness and con- 



