ON THE FOEMATION OF CRYSTALLINE ROCKS, 295 



CHAPTEK IX. 

 APPLICATION OF THE SAME FACTS TO ERUPTIVE ROCKS. 



Eruptive rocks present a great analogy of composition with meta- 

 morphic rocks; many minerals are, in fact, common to the one and the 

 other. It is thus that the elements of granite (feldspar, mica, and 

 quartz) are often found in the beds that it has traversed, and where 

 they are, as it were, extravasated.* "When granite or syenite have 

 enclosed fragments of pre-existing rocks, they have even been in some 

 sort assimilated, as I have elsewhere shown. t We find a very re- 

 markable example of this in the masses of compact limestone of Mount 

 Somma, in the interior of which, amphigene, sodalite, and anorthite, 

 have crystallized, as well as in the lavas which are adjacent to them. 

 The limestone of Kaiserstuhl, with its titaniferous oxydulatediron, its 

 pyrochlore, its perowskite, its apatite, manifests also its relation with 

 the doleritic rock, Avhich has furnished the principal elements of these 

 minerals. It is on this resemblance of composition, sometimes stri- 

 king, that the conclusion that the minerals of metamorphic rocks have 

 been produced in the dry way has often been based. 



I will reverse the argument by saying that if components, such as 

 feldspar, mica, quartz, amphigene, pyroxene, &c., are found in strati- 

 fied rocks, under conditions where they could not have been formed 

 but by the intervention of water, we ought to regard it as very 

 probable that water has acted in the same way in the crystallization 

 of the eruptive rocks themselves, a conclusion to which we have bo- 

 fore been drawn by other considerations. If it were necessar}- to 

 propose an hypothesis on this singular association of water with 

 eruptive rocks endued with a high temperature, we should be apt to 

 consider these hydrated masses as a very concentrated solution of the 

 silicates, a kind of watery fusion rendered persistent by pressure. 



When these silicates have crystallized, their mother-water, accom- 

 panied by various substances,:}: has disengaged itself from them, pre- 

 serving, however, a temperature and a pressure sufficiently great to 

 penetrate into the surrounding rocks and modify them extensively. 

 From this fact result, perhaps, the analogies which have been men- 

 tioned above between the eruptive rock and the rock which has been 

 traversed. Thus to recapitulate and follow out the part that we are 

 led to assign to water in eruptive rocks, I will say that we can distin- 

 guish three principal agencies, which it exercises under three condi- 

 tions: 1. Arriving in a state of combination with these rocks, of 

 which, with the concurrence of heat, it causes the softening; 2. Dis- 

 engaging itself from the rocks as they consolidate, traversing and 

 metamorphosing the neighboring rocks; 3. Escaping sometimes to 



* According to the numerous observations of Elie de Beaumont, de la Bfeche, Gruner, 

 Naumann, and many others. 



f Annates des Mines, 5th si-ries, voL xii, p. 319. 

 j Like the chlorides of the lavas. 



