68 BUNSEN ON THE FORMATION OF 



cal consequences, I must defer for the more extended considera- 

 tion of this subject, that the palagonitic and zeolitic metamor- 

 phism, which we generally find to succeed one another, may 

 even take place at the most elevated temperatures under the 

 simultaneous and subsequent influence of water. For it is only 

 necessary to add to the red-hot superbasic mixture of caustic 

 potash some powdered basalt, in order to obtain by subsequent 

 treatment with water a mixture of palagonitic substance with 

 those zeolitic crystals of hydrated silicate of lime. And indeed 

 zeolitic druses, traversed by a palagonitic tuff, presenting none 

 of the indications of such a second plutonic metamorphism, are 

 actually found in Iceland, and very abundantly in the Faroe 

 Islands. I have, for instance, a specimen of this kind from the 

 Faroe Islands, which consists of concentric radiated desmin sub- 

 stance, enclosing a nucleus of unaltered palagonitic tuff, and 

 surrounded externally by unaltered tuff. 



After these experiments and observations, the occurrence of 

 olivine and augite in sharply defined crystals, together with 

 zeolitic minerals in the midst of a hydrated palagonitic mass, is 

 easily accounted for. Those anhydrous minerals are crystalline 

 plutonic products, which were not influenced in their composi- 

 tion by the subsequent neptunian metamorphism. They are 

 consequently found in their original form, together with the 

 zeolitic and palagonitic products of this metamorphism. Simi- 

 lar phaenomena present themselves at the Pferdekopf in the 

 Rhon mountains, with the single exception, that there the pro- 

 ducts of the last stage of rock formation, caused by aqueous 

 vapour and volcanic gases, predominate. The formation of zeo- 

 litic minerals in pyroxenic rocks becomes equally intelligible 

 through the aid of these experiments and observations. They 

 may be produced in the melted rock when this is sufficiently 

 alkaline and superbasic, as in their artificial preparation. And, 

 in fact, the palagonitic constituent so characteristic of the meta- 

 morphosis of superbasic silicates is scarcely ever wanting in the 

 zeolitic pyroxenic rocks. It is indeed the amorphous part of 

 basalt which gelatinizes with acids, and which it is customary to 

 regard as the zeolitic substance of this species of rock. 



