200 ON THE CHEMISTRY OF METAMORPHIC ROCKS. 



but to the hypothesis of alteration by ascending heat, Naumami 

 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 apparent transition between the two. 

 This, taken in connexion with the well-known fact that in many 

 cases the intrusion of igneous rocks causes no apparent change in 

 the adjacent unaltered sediments, shows that heat and moisture 

 are not the only conditions of metamorphism. In 185*7, I showed 

 by experiments, that in addition to these conditions, certain chem- 

 ical 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 gene- 

 rating various silicates (1). Some months subsequently, Daubree 

 found that in the presence of solutions of alkaline solutions, at tem- 

 peratures 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 (2). These 

 observations were the complement of my own, and both together 

 showed the agency of heated alkaline waters to be sufficient to 

 effect the metamorphism of sediments by the two modes already 

 mentioned, — namely, by molecular changes, and by chemical reac- 

 tions. Following upon this, Daubree observed that the thermal 

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

 in the course of centuries, given rise to the formation of zeolites, 

 and other crystalline silicuted minerals, among the bricks and 

 cement of the old Roman baths. From this he was led to sup- 

 pose that the metamorphism of great regions might have been 

 effected by hot springs; which, rising along certain lines of dislo- 

 cation, and thence spreading laterally, might produce alteration 

 in strata near to the surface, while those beneath would in some 

 cases escape change (3). This ingenious hypothesis may serve in 



1. Proc. Royal Soc. of London, May 7, 1857 ; and Philos. Mag. (4) 

 xv., 68 ; also Amer. Jour. Science (2), xxii., and xxv., 435. 



2. Comptes Rendus de l'Acad., Nov. 16, 1857 ; also Bull. Soc. Geol. 

 de France (2), xv., 103. 



3. It should be remembered that normal or regional metamorphism is 

 in no way dependent upon the proximity of unstratified or igneous rocks, 

 which are rarely present iu metamorphic districts. The ophiolites, 



