ON THE FOEMATION OF CRYSTALLINE ROCKS. 281 



CHAPTER III. 

 WATER CONSIDERED AS AN AGENT OF METAMORPHISM. 



But in volcanic exhalations there is a body which did not at first 

 attract attention, because under the dominion of former ideas it 

 seemed altogether inert, especially in the presence of minerals whose 

 formation was to be explained. It does not exist in minute quanti- 

 ties like the vapors of which we have just spoken ; it is, on the con- 

 trary, at the same time, the most abundant and the most constant 

 product in all the eruptions of the globe. This body is water, and 

 we shall see that the most important part was assigned to it in meta- 

 morphic phenomena, as well as in the eruptions of volcanoes. The 

 singular property which the incandescent silicates of lavas possess of 

 retaining for a very long period, even up to the time of their solidifi- 

 cation, considerable quantities of water, clearly proves that the 

 action of heat does not exclude that of water, and appears also to 

 show thiit water has even at high temperatures a certain affinity for 

 these silicates. 



Of the masses situated at a certain depth in our globe, we only 

 know what is brought up by volcanoes. Now, these ejections all, 

 without exception, contain water, either combined or mixed ; we 

 are, therefore, justified in thinking that water plays a very important 

 part in the principal phenomena which have their origin in the depths 

 of the earth. We have seen, in the historical part of this work, that 

 conclusive arguments have caused a most powerful action to be at- 

 tributed to water, such as the' formation of many metalliferous veins, 

 and an incontestable influence in the crystallization of the eruptive rocks 

 themselves, comprising granite. In truth, the hottest lavas, and those 

 which are most charged with water, as basalts and trachytes, do not 

 modify rocks to any considerable depth ; but this is, without doubt, 

 owing to the fact that as soon as they are exposed to simple atmo- 

 spheric pressure, the water escapes by being reduced to the state of 

 vapor. The numerous blocks of limestone in the tufas of Mount 

 Somma, which have come from the seat of volcanic action, show, in 

 their numerous geodes, adorned with such a variety of, and such well 

 crystallized minerals, what rocks may undergo, when, under pressure, 

 they are subjected to the permanent action of certain agents, without 

 which some of these agents would not have acquired their power, 

 and ethers not even have existed. 



Something altogether like this is seen in the little basaltic mass of 

 Kaiserstuhl in the Grand Duchy of Baden.* A strip of limestone, 

 torn oft' by the action of basalt from the formations which it has 

 traversed, has been modified by it most completely. This limestone 

 has become entirely lamellar, and contains cr^'stals of titaniferous 

 magnetic iron, iron pyrites, magnesian mica, perowskite, pyrochlore, 

 crystallized quartz, and innumerable needles of apatite. The very 



^ Annales des Mines, 5th series, vol. xii, p. 322 ; Naumann, Geognosie, vol. i, p. 791. 



