THE VOLCANIC ROCKS OF ICELAND. 87 



this process, whose special modes of formation I have already 

 fully discussed in another place*, are crystals of iron pyrites, silica 

 in the form of hyalite, hydrated oxide of iron and anhydrous oxide 

 of iron, formed from it by long- continued boiling with water, as 

 well as in some instances carbonate of lime and gypsum. 



The springs of the Great Geyser have formed their siliceous 

 incrustations over a surface of palagonitic tuff, which, as I have 

 already mentioned, is penetrated by solfatara gases. From this 

 palagonitic bed, covered with siliceous tuff and boulders, rises 

 the small trachytic chain of mountains called Laugarfjall, which 

 extends along by the side of the springs in a north-eastern direc- 

 tion. The traces of a not inconsiderable geyser action may be 

 followed on the declivities of these hills as far as their trachytic 

 ridge of rocks, and which, with the exception of a few isolated 

 and unimportant springs of vapour, is almost entirely extinct. 

 Below these is one spring, issuing from the trachytic rock itself, 

 which it has converted into a white, earthy, friable mass, of a 

 dull appearance, or even into a plastic clay. The following 

 analyses of the original and the decomposed trachyte, show it 

 was especially the alkalies which were extracted from the rock, 

 while water was taken up : — 



102. 103. 



Undecomposed trachyte. Decomposed trachyte. 



Silica 75-48 75*84 



Alumina .... 12-97 13-71 



Protoxide of iron . 2*61 Peroxide of iron . 3*21 



Lime 1-01 0*70 



Magnesia. . . . 0-03 0-14 



Potash .... 5-43 1*24 



Soda 2-72 1-94 



Water 0-32 2-18 



100-57 98-96 



The earthy mass passes finally under the increased action of 

 the fumarole gases into a fat, pliant pipe-clay, in which the iron 

 of the original rock is found in the form of small crystals of iron 

 pyrites, the formation of which, as I have already shown, de- 

 pends upon a very simple process of decomposition. The com- 



* Liebig's Annalen, vol. Ixi. p. 1. 



