Quartziferous Variety of Trachyte found in Iceland. 371 



the supposition of an infiltration of silica into what was ori- 

 ginally a mass of normal trachyte ; or, on the other hand, 

 upon the supposition of a partial elimination of the bases, to 

 arrive by calculation, at compositions nearly identical with 

 No. 2. For, if to 5 portions of normal trachyte we assume 

 the infiltration of 1 portion of silica, the composition 3 will 

 be the result; and we obtain the composition 4, if, of 8 por- 

 tions of undecomposed trachyte, 2 portions undergo altera- 

 tion in such a manner as that their contained bases are, by 

 the action of volcanic vapours (Fumarolenth'atigkeit), elimi- 

 nated in the form of sulphuric salts, — their silica at same 

 time remaining : — 





No. 3. 



No. 4. 



No. 5. 



Silica, 



80-56 



80-76 



80-22 



Alumina and protoxide of iron, 



11-86 



11-98 



12«27 



Lime, .... 



1-20 



1-21 



0-31 



Magnesia, 



0-23 



0-24 



0-14 



Alkalies, . 



6-15 



5-78 



7-06 



100-00 100-00 ,100-00 



Thus, it might seem as if the question were incapable of 

 solution. But the alternative is really decided by the obser- 

 vation, that in No. 2 there is contained not more than 3 — 4 

 per cent, of free silica. In the calculated composition 3, in 

 which 1 portion of quartz becomes added to 5 portions of nor- 

 mal trachyte, there would be required, not 3 or 4 per cent., 

 but no less than 1666 per cent, of free silicic acid. Where, 

 again, (in 4) 0'2 of the whole mass is conceived, to have been 

 decomposed in the above manner, there remains just 3 per 

 cent, of free silicic acid, which might very readily appear in 

 the form of quartz and rock-crystal. Farther, if No. 2 had 

 arisen under the influence of a process of infiltration, the re- 

 sult of its composition must, after subtracting the 3 per cent, 

 quartz, have been identical with that of normal trachyte. 

 Such a calculation, however, gives as its result, not the com- 

 position of normal trachyte, but one containing a larger 

 amount of silica (5). 



If, then, an elimination of bases have actually taken place 

 in 1 and 2, it may be anticipated that this process will not 

 have extended in a precisely similar degree to the whole con- 



