OEIGIN OF CRYSTALLINE EOCKS. 9 



§ 18. That such a primitive grauite had been the source of gneiss, was taught by Berol- 

 diugeu, " who maintained that all the rocks of granitic character having an appearance of 

 stratification, are granites of .secondary formation, or regenerated granites, similar in their 

 origin to sandstones ; " a notion which wa.s vigorously combatted by Saussure, " who 

 held, as we have seen, to the neptunian theory of the origin of these rocks. The detrital 

 hypothesis, which he opposed, was however strenuously defended by Hutton and his 

 school, and especially by Bone and by Lyell. To the former belongs the first definite 

 attempt to explain how uncrystalline sediments like grayvvacke and clay-slate might be 

 changed into crystalline rocks such as gneiss and mica-schist. Of his views, put forth in 

 1822 and 1824, Naumann remarks, "Boue first understood how to bring this theory into 

 more decided harmony with the details of geological phenomena, and besides invoking the 

 internal heat, brought to his assistance emanations of gases and vapor from the earth's 

 interior to explain the alteration of sedimentary slates into gneiss and mica-schist." He 

 imagined under these conditions " a sort of igneous liqiaefactiou, followed by a cooling- 

 process, which permitted a crystalline arrangement, and a development of new mineral 

 species, without destroying or deranging notably the original laminated structure." " 



§ 19. These views were adopted in 1838, in his " Principles of G-eology," by Lyell, 

 who designated strata supposed to have been thus transformed by the name of hypogene 

 metamorphic rocks, a title intended to indicate a metamorphism which took place in the 

 depths of the earth's crust, and proceeded i"rom below upwards. Under this name, Lyell 

 first popu.larized the Huttonian view as extended by Boue, which may be conveniently 

 designated as the metamorphic hypothesis of the origin of crystalline rocks. 



Its plausibility has led to the adoption of this theory by many geologists diiring the past 

 fifty years. Some, unwilling to admit the influence of a high temperature in such change, 

 have imagined it to result from causes operating at ordinary temperatures during very long 

 periods. As regards " the nature of these transforming processes, Gustaf Bischof and 

 Haidinger were inclined to suppose that a long -continued percolation of water through the 

 rocks produced an alteration of their substance, and a recrystallization, in the same way as 

 must have taken place in the production of certain pseudomorphs by alteration." "' Hence 

 the significance of the often repeated dictum that "metamorphism is x:>seudomorphism on 

 a broad scale." 



By a further application of the notions derived from the study of epigenic or replace- 

 ment-pseudomorphs, which show in many cases the partial or even the total replace- 

 ment of the original elements of a mineral species, constituting what has been appro- 

 priately designated metasomatism, a METASOMATIC hypothesis of the origin of crystalline 

 rocks has been arrived at, to which we shall revert farther on. 



§ 20. Regarding the metamorphic hypothesis, we may remark, as Naumann has done, 

 that the very transformation assumed, namely that of mechanical sediments into crystalline 

 rocks, remains to be proved. In his " Lehrbuch der G-eognosie " in 1857, while still 

 admitting the metamorphic origin of certain limited areas of crystalline schists, Naumann 



" Voyages dans les Alpes (1796), vol. vili., pp. .S.î, 64. 



'^Bouë, Annales des Sciences Naturelles, Aug., 1824, p. 417, cited by Xauuianu. 



"Xaumann, Lehrbuch der Greognosie (1857) 2nd éd. vol. ii.,pp. 160-170. We shall have frequent occasion in 

 these pages to quote from this section of Naumann's Ijehrbuch. 



Sec. lU., 1884. 2. 



