OEIGIlSr OF CEYSTALLINE EOCKS. IS 



heated waters, at great depths in the earth. " All this is but a repetition of the hypo- 

 thesis put forward forty years since by Dana, and subsequently abandoned by him. 



Tornebohm has also lately advanced a similar hypothesis to explain the origin of 

 the primitive granite, and of the gneiss into which it seems to graduate. The material of 

 these rocks came up as lava now does, and a portion of it, disintegrated, rearranged by 

 water, and recrystallized, assirmed the form of gneiss. Reusch, in like manner, according 

 to Marr, supjposes that the gabbros, diorites, and dioritic and hornblendic-schists of the 

 Bergen district, in Norway, are but altered tufas and erupted rocks. 



§ 31. Mr. Marr, in a recent paper, urges the claims of the volcanic hypothesis to explain 

 the origin of the ancient crystalline rocks, seemingly unaware of its earlier advocates. It 

 is apparent that if we accept the doctrine of the permanence of continents and of oceanic 

 depressions, the metamorphic-detrital theory of the Huttonians, which builds up series 

 of crystalline rocks beneath the sea from the ruins of an older land, which had itself been 

 formed beneath the sea, is no longer tenable. The difficulty of getting the thirty thousand 

 feet of sediments required to spread over a continent, as in Dana's later hypothesis, is, as 

 Marr perceives, overcome if we suppose this material to have been derived not by the 

 superficial waste and disintegration of former land, but by ejection from reservoirs beneath 

 the earth's crust. Hence, with the advocates of the doctrine of the permanence of con- 

 tinents, the volcanic or exoplutonic hypothesis is again coming into favor. "' 



Similar considerations appear to have led C. H. Hitchcock, in 1883, to a like conclusion. 

 The continents, in his scheme, are built up from beneath the waters of a universal ocean. 

 He writes : — " We start with the earth in the condition of igneous fluidity. It cools so as 

 to become encrusted and covered with an ocean. Numerous volcanoes discharge molten 

 rock, building up ovoidal piles of granite [beneath the ocean], which change gradually into 

 crystalline schists. When the hills are high enough to overlook the water, they constitute 

 the beginnings of dry land." All this is intelligible, but it seems strange to one familiar 

 with the geological literature of the last forty years to read, in this connection, the remark 

 that "few have ventured to speak of anything like volcanic action, except as it has been 

 manifested in the formation of dykes, in the early periods." ^ 



To all of these speculations as to the exoplutonic or volcanic origin of the crystalline 

 rocks, the language of Naumann, in criticizing the original volcanic hypothesis of Dana, is 

 applicable. " The perfect and thoroughly crystalline character of the gneiss, the enormous 

 extent which the primitive formations occupy in so many districts, the architecture of 

 these great gneissic regions, and their occurrence wholly indejpendent of larger granitic 

 masses, are all incompatible with this idea." 



§ 32. The view of the igneous and eruptive origin of crystalline limestone, admitted in 

 Dana's former scheme, was familiar to the geologists of forty years since. Emmons and 

 Mather in America, and von Leonhard, Rozet and Savi in Europe, among others, then 

 held to the belief that many crystalline limestones were igneous, and Savi had even 

 attempted to point out the centres of eruption of the Carrara marbles.^ It is hardly neces- 



-^ Neues Jahrbuch fur Minéralogie, 1872, pp. 388 and 490. 



'' Marr, Tlie Origin of Archaean Rocks ; Geological Magazine, June, 1883. 



''' Hitchcock, The Early History of the North American Continent.— Proc. Amer. Assoc. Adv. Science, 1883. 



2« See for references Hunt, Chem. and Geol. Essays, p. 218; also Boue, Guide du Géologue Voyageur, ii. 108. 



