ON THE FORMATION OF CRYSTALLINE ROCKS 289 



and of party-colored grit, without admixture of sand, and cemented 

 by lime. It rests sometimes on the granite itself, sometimes on 

 alluvial gravel. Under the prolonged action of the mineral water, 

 which continually penetrates this mass of concrete, I have found that 

 the calcareous cement and the bricks have been partially transformed.* 

 The new combinations appear chiefly in the cavities of the mass, 

 where they form mamliated and sometimes crystallized coatings. 

 The products, the most remarkable on account of their abundance, 

 are the silicates of the family of the zeolites, and especially apophyl- 

 lite, chabasite, and harmotome. Besides the presence of the zeolites, 

 which have crystallized in the cavities, the pieces of brick which form 

 a part of the Roman concrete have often acquired a very peculiar 

 aspect; they have been thoroughly impregnated with silicates, resem- 

 bling those which have crystallized in geodes; they have undergone 

 a genuine metamorphism. I have been able to determine accurately 

 all the conditions under which this metamorphism has taken place 



Notwithstanding its extreme hardness, the Roman masonry gives 

 access to the thermal water as well by its pores as by the fissures 

 and cavities that exist in it. The pressure from the springs, more- 

 over, forces the water to circulate slowly through the mass, which 

 is thus not only bathed but traversed by the mineral water. The 

 water, therefore, is not stagnant; there is a current, very slow it is 

 true, but this current is continuous. On the other hand, the mineral 

 water of Plombieres contains only a very small quantity of saline 

 matter, (4.6 grains to 60 cubic inches of water,) which is composed 

 in part of silica, potash, soda, lime, and alumina; but continual 

 renewing and indefinitely prolonged action permit deposits of these 

 substances to accumulate in notable quantities. It is in this way that 

 ver}^ feeble agencies are multiplied by the aid of time. Up to the 

 present moment this condition of time is wanting in the greater part 

 of the experiments attempted for the purpose of imitating nature: but 

 its capital influence over certain geological phenomena will be easily 

 appreciated. 



By means of the alkali which this water contains it gradually acts 

 upon some of the substances which it traverses, and perhaps even 

 without there having been a real solution, but only a kind of cementa- 

 tion. It then causes the formation of double hydrous silicates, which 

 belong to the group of the zeolites. It is to the union of these two 

 circumstances, circulation of the water and its chemical agency, that 

 these modern formations are due. In order that the silicates which 

 are formed should crystallize perfectly, there is really no necessity 

 for so high a degree of heat as has been supposed; a temperature of 

 from 60^ to 70° centigrade, which is that of the mineral springs of 

 Plombieres, is sufficient for the production of at least some of them. 

 Zeolites have consequently often been formed in this rock under simple 

 atmospheric pressure, and even on the surface of the soil. It is re- 

 markable to see these silicates crystallizing perfectly, in water, at a 



■^Annates des Mines, 5th Eeries, vol. siii, p. 227, 1858 ; Bulletin de la Socite Giologique de 

 France, 2d series, vol. xvi, p. 562. 



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