Quartziferous Variety of Trachyte found in Iceland. 369 



posed in beds, was nowhere to be observed ; even the thickness 

 of the vein itself, which seemed to run from 6 to 14 feet, could 

 scarcely be determined. A dark green vitreous rock of junc- 

 tion (Contactgestein), which is found at the Baula as well as 

 almost everywhere else in Iceland, where the trachyte sepa- 

 rates from an augitic or tuff rock, was not observed here, but 

 may very possibly have been concealed under the loose rubbish, 

 which the unfavourable state of the weather unfortunately 

 put it out of my power to subject to a very close examination. 



The rock No. 1, is composed of a greenish mass in which 

 are imbedded small white globular particles, very distinctly 

 separated from the matrix. These globules are either entirely 

 filled with a hard white quartzose mass, or else they form a 

 globular crust, of which the internal cavity is occupied with 

 beautifully transparent points of rock crystals. The rock 

 No. 2, appears to be more decomposed than the former ; the 

 mass, which is of a light yellow reddish colour, is almost 

 friable, and the quartz crystals are found distributed in more 

 irregular cavities. On comparing the two varieties, they 

 seem to exhibit different phases of the same process of decom- 

 position ; the globules disappearing and giving place to cavities, 

 as the matrix itself becomes more friable. The reddish 

 powder of No. 2, after being digested for a short time in con- 

 centrated muriatic acid, totally loses its colour, and becomes 

 snow-white. 



The composition of these two varieties, as appearing on 

 analysis, calculated on the anhydrous substance, is as fol- 

 lows : — No. 1 losing 1-847 per cent., and No. 2 losing 1*656 

 per cent., by being brought to a red heat. 







No. 1. 



No. 2. 



Silica, .... 





78-149 



81-364 



Alumina, 





11-522 



10-241 



Oxide of iron, 





1-655 



1-931 



Lime, .... 





0-465 



0301 



Magnesia, 



Protoxide of manganese, 



} 



0-067 



0-058 

 0-076 



Potassa, 





2-898 



4-878 



Soda, .... 





4-195 



2-030 



98-951 100-879 



