26 THE GEOLOGY OP MINNESOTA. 



[Genesis of the Archean. The greenstones. 



Jukes on the other hand (1862) assumes a generally diffused uniform molten 

 mass, from which, by a process of segregation, the basic rocks are extracted under 

 certain conditions, and acid ones under others. This is essentially the idea of Iddings, 

 which (1892) has been discussed by him under the topic of "Consanguinity " of igneous 

 rocks. 



Dutton (1880) assumed a primordial basic primitive crust, allied to basalt or 

 doleryte, entirely homogeneous, and appealed to atmospheric forces to separate from 

 it the more acid parts and form of them the quartzytes, granites and gneisses. The 

 sedimentary products of erosion are heterogeneous, and by the rising of temperature 

 within certain subterranean horizons, or perhaps by relief of pressure, the deeply 

 buried materials are re-fused, to issue as igneous rock at the surface, with their recog- 

 nized heterogeneous characters ; or are reconsolidated in *itn. 



Under various theories for the cause of such differentiation, if it exist, this inves- 

 tigation lias been carried on by Rosenburch (1889), who maintains that beneath the 

 earth's solid crust the molten interior is separated naturally into certain reservoirs or 

 "kerns" in which are gathered different magmas of which he distinguishes, on 

 chemical grounds, five classes: by Brogger (1890), who believes tlmt on account of 

 the principle of Soret, certain silicates more basic than those with which they are 

 associated are concentrated upon the borders of laccolitic masses, and hence are also 

 the first to appear in cases of extrusion, followed by the magmas more acid; by 

 Iddings (1892), who considers the differentiation to be caused by physico-chemical 

 forces, largely inherent in the environment of the supposed primary or fundamental 

 magma ; by Geo. F. Becker (1897), who demonstrates that if such segmentations have 

 occurred under the operation of physico-chemical forces, the time involved is incon- 

 ceivably long, and the process so slow that fifty millions of years, or all the time 

 elapsed since the close of the Archean, would be insufficient, and that the probable 

 direction of the action of osmotic force would be to promote uniformity rather than 

 heterogeneity; by Michel Levy (1897), who, questioning the actuality of such differ- 

 entiation and noting the opposing tendency of numerous observed facts, reaches the 

 conclusion, from an exhaustive comparison of the chemical constitution of rocks 

 having the "air of family," that but two magmas of permanent and fundamental 

 importance can be recognized, viz., a basic or ferromagnesian one, and an alkaline 

 one, represented respectively by diabase and granite. Of these, Michel Levy finds 

 the ferromagnesian was the earlier, and that from it, by a process similar to the 

 "crenitic"of Hunt, the alkaline magma may have been produced, the basic lying below 

 the alkaline. He, however, supposes the transformation took place in deep-seated 

 reservoirs before solidification.* 



* Classification des magmas des roches ruptives. Kullrtin Knciete Geologique de France, xxv, 326, 1897. 



