CHEMISTRY OF THE EARTH. 1\)1 



chemical reagents might be necessary, and that water impregnated with 

 alkaline carbonates and silicates would, at a temperature not above 100° 

 centigrade, produce clieiuical reactions among the elements of many 

 sedimentary rocks, dissolving silica and generating various insoluble 

 silicates.* Subsequent experiments by Daubree contirmed these results 

 of mine, and both together showed the agency of heated alkaline waters 

 to be sufticient to effect the metamorphism of sediments by thi^ two 

 modes already mentioned, namely, by molecnlar changes and by chem- 

 ical reactions. 



§ 3l>. Daubree further showed, by his observations on the thermal 

 alkaline spring at Plombieres, that its waters, at a temperature of 70° 

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

 zeolites and other crystalline silicated minerals among the bricks and 

 mortar of the old Roman baths. The influence of similar waters may 

 account for many cases of local metamorphism, but is utterly inadequate 

 to explain the complete and universal alteration of great areas of sedi- 

 mentary rocks, embra(;ing many hundreds or thousands of square miles. 

 On the other hand, the study of the origin and distribution of mineral 

 springs shows that alkaline waters, whose action in metamorphism I 

 tirst pointed out, and whose efficient agency Daubree has since so well 

 shown, are confined to certain sedimentary deposits and to definite 

 stratigraphical horizons, above and below which saline waters wholly 

 different in character are found imi)regnating the strata. This fact 

 offers a simple solution of the difficulty advanced by N aumann, and a 

 complete exi)lanation of the theory of metamorphism of deeplj -buried 

 vStrata by the agency of ascending heat, which is operative in producing 

 chemical changes only in those strata in which soluble alkaline salts are 

 present. 



§33. ^Yehave said that the metamorphism of sediments includes both 

 hemical and crystallogenic changes. The gradual transformation of 

 amorphous precipitates under water into crystalline aggregates, so often 

 observed in the laboratory, appears to depend upon partial solution and 

 re-deposition of the material, which must not be entirely insoluble in the 

 surrounding liquid. If the solvent power of this be reduced, the dis- 

 solved iiortions are deposited on certain particles rather than others. 

 By a subsequent exaltation of the solvent ])ower of the liquid, solution 

 of a further portion takes place, and this, in its turn, is deposited 

 around the nuclei already formed, which are thus augmented at the 

 expense of the smaller particles, until these at length disappear, being 

 gathered to the crystalline centers. Such a process, which has been 

 studied by H. Deville, suffices, under the influence of the changing tem- 

 perature of the seasons, to convert many fine precipitates into crystal- 

 line aggregates, by the aid of liquids of slight solvent powers. A simi- 

 lar agency may be supposed to have effected the crystallization of buried 

 sediments, and changes in the solvent power of the permeating water 

 might be due either to variations of temperature or of pressure. Simul- 

 taneously with this process one of chemical union of heterogeneous 

 elements may go on, and in this way, for example, we may suppose the 

 carbonates of lime and magnesia become united to form dolomite or 

 maguesian limestone. (§ 20.) 



§ 34. When the sedimentary^ strata have thus been rendered crystal- 

 line by metamorphism, their permeability to w^ater and their alterability 

 thereby become greatly diminished ; and it is only when again broken 

 down by mechanical agencies to the condition of soils and sediments 

 that they once more become subject to the chemical changes which have 



T. S. Hiiut, Americau Jonruul of Scicuce, [2,] xxiii, 407 ; xsv, '287-437. 



