THE VOLCANIC ROCKS OF ICELAND. 63 



equal to the melting-point of platinum, such a sublimation of car- 

 bonated alkali, together with cyanide of potassium, takes place, 

 that these products are in some instances collected by hundred 

 weights. Unless this separation and sublimation of alkalies be 

 attributed exclusively to the reducing influence of the coal, it 

 would not be inadmissible to assume that similar processes take 

 place in the immediate neighbourhood of those volcanic foci 

 which are surrounded by the substance of pyroxenic rocks in a 

 state of igneous liquidity. Such conditions therefore may pos- 

 sibly have contributed largely to the formation of palagonite. 

 However, the enormous extent of the Icelandic tuff- rocks renders 

 it very improbable that we have here to do with a mode of origin 

 vvhich would in any case bear the character of a merely local 

 one. It is therefore certainly more in accordance with the strict 

 principles of scientific inquiry to dispense with any merely hy- 

 pothetical explanation of this question, and to rest satisfied with 

 the assumption, justified both by experiment and observation, 

 that in the volcanic periods there was, besides the trachytic ftnd 

 pyroxenic foci, a third focus in a state of activity, which has 

 now become extinct, and whose contents consisted of siHcates 

 rich in alkalies and sufficiently superbasicto break up under the 

 influence of water into palagonitic substance and soluble com- 

 pounds, which were washed away. The occurrence of the pala- 

 gonitic cementing substance, scarcely ever absent in Iceland from 

 among the conglomerates and masses of fragmentary rock desti- 

 tute of fossils and accompanying eruptive rocks, is readily ac- 

 counted for by means of this assumption. It is indeed a direct 

 and necessary consequence of the pouring out of such masses of 

 rock over the surface ; and the palagonitic tuff's containing fossils 

 are nothing more than the products of submarine accumulations 

 of sediment, which received the material for their palagonitic 

 cement from the above-mentioned eruptive silicates, rich in alka- 

 lies, while in a state of metamorphism. 



2. Zeolitic Formations. 



The zeolitic amygdaloid rocks are most intimately connected 

 with those belonging to the palagonitic and pyroxenic classes. 

 They constitute, indeed, the intermediate metamorphic members 

 which connect together these two classes. As the relative mean 



