THE VOLCANIC ROCKS OF ICELAND. 59 



from the contact of the limestone with the lava does not leave 

 any doubt as to the nature of the process by which the palago- 

 nite has been formed. Wherever the lava is in contact with the 

 Hmestone, it is converted into a mass presenting all the mineral- 

 ogical characters and chemical reactions of palagonite ; and this 

 metamorphism^ characterized by a gradual transition into the 

 unaltered rock, is more fully developed where the calcareous 

 substance preponderates over the other constituent of the mass. 

 The analysis of this metamorphic lava showed it to have the 

 following composition, which scarcely differs from that of the 

 pure palagonite : — 



I have been able to recognize perfectly analogous, although 

 not identical relations, by observation and experiment on basaltic 

 dykes traversing sedimentary limestone. Direct observation is 

 therefore sufficient to prove that a palagonitic substance may 

 result from the action of Hme upon pyroxenic rocks at a high 

 temperature. And, in fact, a number of tuffs from our basaltic 

 formations, and from the volcanoes of the Galapagos islands, 

 which I have had an opportunity of examining, prove in the 

 most decisive manner that some masses of this kind have actually 

 been formed in this way. The following analysis of a tuff from 

 Chatham island, mixed in the most intimate manner with car- 

 bonate of lime, and forming a crater, furnishes a proof of this 

 statement : — 



* After deducting the oxygen of the carbonates. 



