THE VOLCANIC ROCKS OF ICELAND. 91 



guished either mineralogically or chemically from those products 

 of decomposition which are observed in the rocks of the solfa- 

 taras, and in mere appearance they are totally different from those 

 decomposition crusts presented by fissures in the interior of the 

 same rock, and communicating with spring strata, whether the 

 decomposition may be less advanced, or still more so than in 

 these dykes. A basaltic dyke, which has penetrated through 

 the adjoining older trap on the north-eastern coast of Vidhey in 

 the harbour of Reykjavik, presents a fine example of this kind. 

 The trap rock contains a considerable per-centage of water, and 

 possesses exactly the composition of the normal pyroxenic mass. 

 Where it is in contact with the basalt dyke, a crust resembling 

 trachyte is observable, which has a pitchy-black appearance, 

 conchoidal fracture, and glassy lustre. Dried in the air at the 

 ordinary temperature, it contains only 0*84 per cent, of water, 

 and presents entirely the appearance of a fresh obsidian-like 

 glass. The following analysis shows that this crust is actually 

 nothing more than melted normal pyroxenic substance, having 

 the same composition as other trap, from the lateral fusion and 

 rapid cooling of which it has originated : — 



107. 



Silica 47-58 



Protoxide of iron .. 17*51 



Alumina .... 13-78 



Lime 11-36 



Magnesia . , . . 6*48 



Soda 2-90 



Potash 0-60 



Water 0-84 



101-05 



The blackish gray substance of the basalt dyke itself has a 

 dull earthy appearance, contains pyrites, together with calcareous 

 spar disseminated through its compact mass, and resembles in 

 the most deceptive manner the rocky nuclei which are sometimes 

 found in imbedded pyroxenic masses which have been already 

 converted by lying in the solfataras into pyritic clay on their 

 surface, while in the centre the decomposition is only in its 

 initial stage. The composition of this altered basaltic mass gives 



