THE 



AMERICAN JOURNAL OF SCIENCE 



[THIRD SERIES.] 



Art. LYIII. — On Subdivisions in Archcean History; by 

 James D. Dana. 



1. Subdivisions based on Kinds of Hocks. 



"Werner's idea that kinds of rocks and grade of crystal- 

 lization afford a basis for the chronological subdivision of 

 crystalline rocks is more or less apparent in nearly all attempts 

 that have since been made to lay down the general subdi- 

 visions of Archasan terranes. The "fundamental gneiss" has 

 gone to the bottom and the thinner schists to the top. There 

 is a degree of truth in the idea. But the assumptions are so 

 great that at the present time little reason exists for the 

 earnestness sometimes shown by advocates of such systems. 

 The idea has little to sustain it in the known facts of geology. 

 The following are sufficient to decide the question. 



According to the thorough penological and geological study 

 of the rocks of the Bernardston region by Prof. B. K. 

 Emerson* — a region in the Connecticut valley, in the towns 

 chiefly of Bernardston, Massachusetts, and Yernon, Yermont — 

 there are the following rocks : granite, largely feldspathic ; 

 dioryte, so like intrusive dioryte that it had been pronounced 

 trap ;. quartz-dioryte ; granitoid gneiss faintly foliated with 

 biotite and passing into the granite ; hornblende schist ; 

 quartzyte ; quartzyte porphyritic with feldspar crystals ; 

 staurolitic and garnetiferous mica schist ; hydromica schist ; 

 argillyte ; massive magnetite, making a bed of magnetite rock ; 



* A description of the "Bernardston Series " of Metarnorphic Upper Devonian 

 Rocks, by Ben K. Emerson, this Journal, III, xl, 263, 1890. 



Am. Jour. Sci.— Third Series, Vol. XLIIT, No. 258.— June, 1892. 

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