THE VOLCANIC ROCKS OF ICELAND. 6? 



tion of zeolitic amygdaloids, the origin of the zeoUtic minerals 

 imbedded in the substance of the trap beds and the more com- 

 pact basalt, where it is obviously necessary to assume the former 

 prevalence of very much more elevated temperatures than those 

 which even, according to the above experiment, are compatible 

 with the formation of zeolites from palagonite, must still appear 

 very enigmatical. But even this phaenomenon admits of being 

 accounted for by means of an experiment, which would appear 

 completely to clear up the mystery attaching to the origin of 

 hydrated siliceous minerals imbedded in plutonic rocks. When, 

 for instance, a finely powdered mixture of 0*2 part lime and 1*0 

 silica is introduced into 9 parts of caustic potash, melted in a 

 silver capsule, and the whole allowed to cool slowly, after having 

 been kept for some time at a strong red heat in a muffle, upon 

 treating the mass with water, a network of prismatic crystals, 

 frequently 4 to 5 lines in length, is found partly attached to 

 the sides of the capsules. These crystals are hydrated silicate 

 of lime mixed with some carbonate of lime, and represented by 



the formula 3CaO 2Si03 + Aq :— 



74 



Silica 27-215 



Lime 22*241 



Potash 0733 



Water separated at 228°-2 . . 36*915 



Water separated by ignition . 9*508 



Carbonate of lime .... 2*603 



99*215 



The artificial preparation of this beautifully crystallized hy- 

 drated silicate, and still more the altogether uncommon mode of 

 its formation, are of very great interest in a geological point of 

 view. We have here to consider, not merely the fact that a hy- 

 drated silicate is formed at a red heat, but the still more import- 

 ant fact, that it is not destroyed again under this condition, 

 although, after it has once been separated from the surrounding 

 mass, it loses four-fifths of its water at 228°*2, and the re- 

 mainder at a temperature below red heat. It most undoubtedly 

 follows from this, of its kind isolated, phaenomenon, the fur- 

 ther discussion of which, with all its experimental and theoreti- 



F2 



