22 THE GEOLOGY OP MINNESOTA. 



[Genesis of the Archean. The greenstones. 



has been discussed as if it were a unit in structure and origin, as well as in date. 

 Under the general term "crystalline rocks," they have been supposed to be due to 

 original congealation of the sui'face of the earth, or to the crenitic action of water. 

 At one time they have been assigned to a metamorphic origin, from sediments, and 

 again considered as original precipitates in their present mineralogical constitution 

 from a peculiar primeval ocean, a chaotic liquid which surrounded the globe, holding 

 them as silicates in solution. By some extensive volcanic action has been invoked 

 and by others specially excluded. 



The following historic sketch of the progress of opinion is essentially condensed 

 from T. Sterry Hunt,* the author of the "crenitic hypothesis," but with additions. 



Newton, Descartes, Leibnitz, Buffon and others, seeing a resemblance between 

 consolidated volcanic rocks and some of the crystalline rocks, and assuming a former 

 molten condition of the globe, inferred that the crystalline rocks, whether massive or 

 schistose, were the cooled and consolidated outer crust of the globe. 



Lehmann (17J7G), called them "primitive," and regarded them as "parts of 

 the original nucleus of the globe." which had not been altered since. Pallas and 

 De Luc had the same opinion, though De Luc lastly admitted that they are due to 

 " aqueous deposition." Pallas also regarded them as " due to the agency of water," 

 and believed that volcanic phenomena were but "local accidents" (1777). 



According to Werner (1785), "the crystalline rocks" were separated from 

 the primeval ocean as chemical precipitates, and granite was the first laid down. 

 This was followed by the gneisses and the mica schists. Thus Werner accounted also 

 for basalt, and all the forms of the Archean. He saw no indications in these rocks 

 of volcanic activity, or of subterranean heat. Actual volcanoes were by him explained 

 as caused by internal combustion of carbonaceous deposits. 



According to Hutton (1785), and Playfair, his interpreter (1802), who argued 

 from injected granitic veins and from analogies with basalts and volcanic rocks, 

 granite is of igneous origin, cooled from fusion. He supposed that quartz, 

 feldspar, tourmaline and other minerals in granitic veins are formed from cooling 

 from igneous fluidity. But Hutton considered granite of later date than the strata 

 that lie on it, agreeing, however, with Werner that granite is the lowest known rock. 

 Hutton, however, considered the gneisses and the schists as of different origin, viz.: 

 detrital, distributed by water, consolidated by heat and by pressure, re-fused, injected 

 in the strata, destroyed and again re-formed by the same process, in a ceaseless 

 round, without beginning or end. He was attacked by theologians, and Werner's 

 hypothesis, which began in chaos and invoked a universal ocean, was espoused by 

 them. Wernerism. however, could not withstand the evidences of observation, 



*HiTNT. Mim nil I'lnmiology and Physiography, pp. 68-115. 



