52 BUNSEN OiN THE FORMATION OF 



formations, in a manner similar to that which I have adopted 

 with regard to the volcanic rocks. It would be necessary to 

 select for this purpose only the more extensive and largely deve- 

 loped rock systems, in order to avoid those disturbing influences 

 which may have resulted from the contact of chemically meta- 

 morphosed exogenous rocks with those of plutonic origin. 

 These disturbing influences manifest themselves even where 

 volcanic rocks have penetrated in isolated elevations, through 

 calcareous or siliceous sedimentary beds. 



It still remains, after the above consideration of this subject, 

 to point out the connexion which exists between the mode of 

 formation of Icelandic and Caucasian rocks just described, and 

 their mineralogical composition. This investigation affords the 

 most interesting results, but it would not be compatible with the 

 limits of this abstract to trace these various relations. Their 

 consideration must therefore be deferred for the present. 



II. Genetic Relations of the Metamorphic Rocks, 

 1. Palagonitic Rocks. 



The metamorphic rocks, which constitute a by no means in- 

 considerable part of Iceland, are far more interesting than the 

 unaltered rocks. The most remarkable member of this group 

 is certainly the palagonitic tuff, which consists of a mixture of 

 anhydrous and hydrated silicates. The anhydrous silicates be- 

 long exclusively to the pyroxenic rocks treated of above, and 

 are never accompanied by trachytic masses, or even replaced by 

 them ; the hydrated silicates, which generally cement together 

 the fragmentary rock, may, on the contrary, be regarded as a 

 mixture or combination of two silicates, of which one is repre- 

 sented by the formula 3R0 2SiO^ + Aq, and the other by 

 3 AP O^, SiO^ + Aq. These silicates appear to combine together 

 in definite proportions ; at least I consider it necessary to regard 

 palagonite, for which I have proposed the formula 3R0 2SiO^ 

 -|-2APO^SiO^ + Aq, as well as a mineral occurring in the tuffs 

 of Chatham island, one of the Galapagos, which I found to con- 

 sist of 3RO 2Si03 + A12 O^ Si08 -f Aq, as being such com- 

 binations. 



