THE VOLCANIC ROCKS OF ICELAND. 5 



113. 



Silica ...... 47*05 



Alumina 10-91 



Protoxide of iron . . 12*66 



Lime 11*79 



Magnesia .... 7 '73 



Soda 1-23 



Potash 0*75 



Carbonate of lime. . 1*01 



Pyrites 0*20 



Water 6*67 



Gypsum trace 



10000 

 On account of the unequal distribution of the pyrites in layers, 

 it is not possible to compare this product of decomposition with 

 the original rock from which it is derived. It perfectly resembles, 

 not only in its composition, but likewise in its natural appearance, 

 a rock decomposed by solfatara action, while it does not present 

 the slightest similarity to the decomposed mass resulting from 

 the mere action of water upon the fissures of pyroxenic rocks. 

 Further towards the centre of the dyke the rock is more difficult 

 of fracture, has a less decomposed appearance, and is impregnated 

 throughout with calcareous spar. The composition of this sub- 

 stance was found to be — 



114. 



Silica 50-82 



Alumina 11*10 



Protoxide of iron . . 12*97 



Lime 4-34 



Magnesia .... 3*90 



Soda 1-93 



Potash 0*31 



Carbonate of lime . . 8*16 



Pyrites 0*26 



Water 5*05 



Gypsum trace 



100-00 

 In this instance, likewise, we meet with the remarkable fact 

 that the metamorphic substance of the dyke has not lost any of 



