ON THE CHEMISTRY OP METAMORPHIC ROCKS. 201 



some cases to meet the difficulty pointed out by Naumann ; but while 

 it is undoubtedly true in certain instances of local metamorphism, 

 it seems to be utterly inadequate to explain the complete and uni- 

 versal alteration of areas of sedimentary rocks, embracing many 

 hundred thousands of square miles. On the other hand, the study 

 of the origin and distribution of mineral springs, shows that alka- 

 line waters (whose action in metamorphism I first pointed ou 

 and whose efficient agency Daubree has since so well shown), are 

 confined to certain sedimentary deposits, and to definite strati- 

 graphical horizons ; above and below which saline waters wholly 

 different in character are found impregnating the strata. This fact 

 seems to offer a simple solution of the difficulty advanced by Nau- 

 mann, and a complete explanation of the theory of metamorphism 

 of deeply buried strata by the agency of ascending heat ; which 

 is operative in producing chemical changes only in those strata in 

 which soluble alkaline salts are present. (4). 



When the sedimentary strata have been rendered crystalline 

 by metamorphism, their permeability to water, and their altera- 

 bility, 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 just been described. Hence, the mean com- 

 position of the argillaceous sediments of any geological epoch, or 



amphibolites, euphotides, diorites, and granites of such regions, which 

 it has been customary to regard as exotic or intrusive rocks, are in most 

 cases indigenous, and are altered sediments. I have elsewhere shown 

 that the great outbursts of intrusive dolerites, diorites, and trachytes in 

 south-eastern Canada are found, not among the metamorphic rocks, but 

 among the unaltered strata along their margin, or at some distance 

 removed ; and I have endeavoured to explain this by the consideration 

 that the great volume of overlying sediments, which, by retaining the 

 central heat, aided in the alteration of the strata now exposed by denuda- 

 tion, produced a depression of the earth's surface, and forced out the still 

 lower and softened strata along the a lines of fracture which took place in 

 the regions beyond. See my paper " On some Points in American 

 Geology," Amer. Jour. Science (2), xxxi. 414., and Can. Nat. vi. 81. 



4. See Report of the Geological Survey of Canada, 1853-6, pp. 479, 

 480 ; also Canadian Naturalist, vol. vii., p. 262. For a consideration 

 of the relations of mineral waters to geological formations, see " Gene- 

 ral Report on the Geology of Canada," p. 561 ; also chap. xix. on " Sedi- 

 mentary and Metamorphic Rocks ;" where most of the points touched 

 in the present paper are discussed at greater length. 



