III.] THE CHEMISTEY OF METAMORPHIC ROCKS. 25 



of the change. The source of this has been generally supposed 

 to he from below; but to the hypothesis of alteration by 

 ascending heat, Naumann has objected that the inferior strata 

 in some cases escape change, and that, in descending, a certain 

 plane limits the metamorphism, separating the altered strata 

 above from the unaltered ones beneath, there being no ap- 

 parent transition Between the two. This, taken in connection 

 with the well-known fact that in many cases the intrusion of 

 igneous rocks causes no apparent change in the adjacent unal- 

 tered sediments, shows that heat and moisture are not the only 

 conditions of metamorphism. In 1857 I showed by experi- 

 ments that, in addition to these conditions, certain chemical 

 reagents might be necessary ; and that water impregnated with 

 alkaline carbonates and silicates would, at a temperature not 

 above that of 212 F., produce chemical reactions among the 

 elements of many sedimentary rocks, dissolving silica, and 

 generating various silicates.* Some months subsequently, 

 Daubree found that in the presence of solutions of alkaline 

 silicates, at temperatures above 700 F., various silicious 

 minerals, such as quartz, feldspar and pyroxene, could be 

 made to assume a crystalline form ; and that alkaline silicates 

 in solution at this temperature would combine with clay to 

 form feldspar and mica.t These observations were the com- 

 plement of my own, and both together showed the agency of 

 heated alkaline waters to be sufficient to effect the metamor- 

 phism of sediments by the two modes already mentioned, 

 namely, by molecular changes and by chemical reactions. 

 Following upon this, Daubree observed that the thermal 

 alkaline spring of Plombieres, with a temperature of 160 F., 

 had in the course of centuries given rise to the formation of 

 zeolites, and other crystalline silicated minerals, among the 

 bricks and cement of the old Eoman baths. From this he 

 was led to suppose that the metamorphism of great regions 



* Proc. Royal Soc. of London, May 7, 1857; andPhilos. Mag. (4), XV. 68; 

 also Amer. Jour. Science (2), XXII. and XXV. 435. 



t Comptes Rendus de 1'Acad., Nov. 16, 1857 ; also Bull. Soc. Geol de 

 France (2), XV. 103. 

 2 



