For 1. 



For 2. 



LOO-00 



100-00 



3-56 



2-51 



0-53 



1-18 



54-40 



57'56 



372 Quartziferous Variety of Trachyte found in Iceland. 



stituents. Now this inequality will actually appear, if we 

 calculate the composition of normal trachyte and that of the 

 varieties 1 and 2, without reference to the silica, for like 

 quantities of one of the bases, e. g., the alumina : — 



For Normal 

 Trachyte. 



Alumina and protoxide of iron, 100-00 



Lime, . . . . 10-11 



Magnesia, . . . . 1*81 



Alkalies, .... 52-88 



For these reasons, as well as from the external appear- 

 ance of the rock, which of itself indicates the operation of 

 volcanic gases (Fumarolenwirkung), I think we are entitled 

 to conclude that the quartz, which we so unexpectedly find in 

 this trachyte, is derived, not from the infiltration, but from 

 the secretion of silica. Almost everywhere in Iceland, where 

 trachytes occur, I have observed amongst them compact and 

 earthy varieties, resembling the fumerolle clay, frequently 

 along with spicula of iron pyrites, — as near the Baula, at the 

 Indridastadir, near Skorradalsvatn, at the Illvidrishnukr, in 

 the northern district of the island, not far from where variety 

 2 is found. The analysis of the trachyte of Langarfjall, 

 at the Geisir, which continues to be decomposed by the influ- 

 ence of fumerolles, exhibits a similar elimination of bases, 

 especially a striking diminution of the alkalies. It is not im- 

 probable that, whilst the silica secreted in 2, in the course of 

 the partial decomposition, remained in the rock ; of the liber- 

 ated bases, the alkalies, and about half the alumina, were 

 eliminated in the form of alum, the other half of the alumina, 

 together with the whole lime and magnesia, being eliminated 

 in the form of sulphuric salts.* 



* Liebig's Annalen der Chemie, &c, vol. lxxxv., part 3. 



