ON THE FORMATION OF CRYSTALLINE ROCKS 291 



CHAPTER VII. 

 CONCLUSIONS TO BE DRAWN FROM THE FACTS OBSERVED AT PLOMBIERES. 



It has been justly remarked that there are but few insoluble sub- 

 stances when the solvents circulate by millions of gallons. Yet we 

 must not conclude from this that the insoluble minerals formed by 

 water in the interior of rocks have been purely and simply deposited by 

 it after the action of centuries. One of the newest and most important 

 facts which reveals what is going on at Plombieres is, that in general 

 only a small part of the elements constituting these minerals is fur- 

 nished by water. The other elements pre-existed in the rocks, and 

 appearing to obey an energetic tendency to crystallization they seize, 

 as it were, upon what first passes, according to their affinities, and 

 the mineral is, so to speak, formed in situ. 



In metalliferous veins, on the contrary, almost everything that is 

 deposited in the channel of circulation of the spring appears to be 

 foreign to the rock forming its sides. These are the very different 

 effects of the same cause, and their union in the same place, at Plom- 

 bieres, leaves no doubt of their common origin. 



There is a striking analogy betn-een the formation of the crystal- 

 lized silicates of the concrete of Plombieres and the formation of the 

 silicates which are found in a great many metamorphic rocks ; such 

 are wernerite, garnet, feldspar, and pyroxene in limestones, often 

 hardly modified at all ; and made or staurotide in argillaceous schists. 

 The production of mica in rocks is not more difficult to understand 

 than that of apophyllite at Plombieres, which is also a silicate con- 

 taining fluor. 



When a dislocation produces a group of hot springs is it not prob- 

 able that the greater part of the formations traversed by these 

 springs will undergo an action of which an idea is given by what has 

 happened at Plombieres ? This action, gradually extending with the 

 aid of time, Avould occasion metamorphism over zones of very great 

 extent. At Plombieres, before the hollowing out of the valley gave 

 issue to the springs, the thermal water already arrived from the in- 

 terior of the earth,* and if it appeared on the surface it was, without 

 doubt, only by a scarcely apparent transudation. In diffusing itself 

 through the beds of the lower triassic formation, which are in contact 

 with the granite, it deposited there jasper, crystallized quartz, and 

 several other products. Thus water circulating in the interior may 

 cause a very energetic metamorphic action without evidence of its 

 existence being shown on the surface by thermal springs. It is 

 probable that in many cases the silicification of polyps and wood in 

 certain beds, the precipitation of crystallized quartz in others, such 

 as those that are found in the tertiary basin of Paris, the complete 

 silicification of some beds that were formerly limestonet have no 



* See the memoir on Plombieres, referred to above. — {Annales des Mines, 5th series, vol. 

 xiii, p. 232 ; Bulletin de la Society Giologique de France, 2d series, vol xvi, p. 562.) 

 ■\ Description Geologique du Baa Rhin, pp. 325 and 326. 



