THE VOLCANIC ROCKS OF ICELAND. 



53 



Without here entering into any special examination of the 

 various members of the tulF rocks, it may be as well to mention 

 that the palagonitic substance appears to occur everywhere as a 

 characteristic constituent of these formations when the pyro- 

 xenic rocks of the volcanic period are especially well developed. 

 Besides Iceland, where it occurs in extensive masses, it is met 

 with in the more considerable basaltic elevations of Germany 

 and France, in the Euganean islands, on Etna, the Azores, 

 Canaries, Cape Verde Islands and the Galapagos Islands, pro- 

 bably also upon the volcanic islands of the South Sea. The 

 following analyses will give an idea of the degree of uniformity 

 in the chemical composition of this very generally occurring 

 cementing substance of the volcanic tuff: — 



Iceland^ Seljadalr. 



Silica . , . . 

 Alumina . . . 

 Protoxide of iron 

 Lime . . 

 Magnesia . 

 Potash . . 

 Soda . . 

 Water . . 

 Foreign residue 



Oxygen found. 

 19-43 



9-47 



5-12 



Oxygen calculated. 

 19-44 



9-72 



4-86 



100-00 



