THE VOLCANIC ROCKS OF ICELAND. 3? 



11 is a massive non-columnar basaltic rock from Skardhsf jail, 

 near Hecla, which, like the hills of tuff in the same locality, is 

 thrown up parallel to the cone of Hecla, and which presents in 

 some cavities a scoriaceous surface, showing signs of previous 

 liquidity. 12 is a hand specimen of an extremely homogeneous 

 ancient lava from Hecla. 13 is a light gray lava, filled with 

 cavities, forming the precipice of Almannagja, and whose prin- 

 cipal mass consists of an intimate and uniform mixture of about 

 equal parts of a felspathic silicate free from iron, and a dark- 

 coloured ferruginous siUcate in granular crystalline particles. 

 Small augitic concretions may be distinguished in the latter 

 mineral, possessing the most deceptive resemblance to olivine : — 



8. 9. 10. 11. 12. 13. 



100-00 100-00 100-00 100-00 100-00 100-00 



As the proportion of the oxygen of the silicic acid to that of 

 the bases is here, with slight variations, as 3 : 1-998, all these 

 rocks may be regarded as a constant mixture of bibasic silicates, 

 when we consider only their entire mass, wdthout reference to 

 the fact of their constituents being grouped into minerals of 

 definite composition. 



The quantity of silica almost always bears a constant propor- 

 tion to the lime and magnesia, while the relation between the 

 quantities of alumina and protoxide of iron is subject to con- 

 siderable variations. The reason of this is not difficult to dis- 

 cover, for there are abundance of well-known phaenomena which 

 can throw light upon it. The silver dissolved in lead separates 

 in that portion of the metal which solidifies first in a different 

 proportion from that in which it separates afterwards ; and a re- 

 melted bar of auriferous silver never gives the same per-centage 

 of gold throughout its entire mass. The same inequality occurs 

 in the solidifying silicates. The more easily solidifying consti- 



