24 THE GEOLOGY OP MINNESOTA. 



[Genesis of the Archean. The greenstones. 



below the ocean to allow of the enormous accumulations. This difficulty was met in 

 part by Dana (1843), who introduced and amplified the idea of volcanic origin of 

 much of the materials of the metamorphic rocks. He claimed that most of the 

 crystalline igneous rocks could be formed also by the recrystallization of their debris, 

 and that in the vicinity of volcanoes such debris was always abundant. According 

 to Dana, pyroclastic material enters largely into the constitution of the " hypogene 

 metamorphic rocks " of Lyell. 



Clarence King (1878) recognized volcanic action as a source of oceanic debris, 

 but showed that great depth alone (30,000 feet) is not a cause of metamorphism. He 

 explained the origin of olivine rocks, magnesian silicates and serpentine, by the 

 occurrence of volcanic olivine sand, etc. King also accounted for the variations 

 seen in igneous rocks, by a process entirely novel. He assumed the downward 

 increase in density of the mass of the earth, solid from pressure, and ascribed to 

 removal of pressure the occurrence of fused areas or lakes of lava. As these were 

 liable to occur at varying depths they would issue at the surface with the characters 

 proper to their various densities, and would be local and temporary. By this, and 

 by a presumed separation of such molten lakes through the action of gravity, into 

 two parts or layers, the more dense at the bottom, all the varieties of igneous rock 

 are explained. 



Marr (1883) urged the volcanic source for the origin of the ancient crystalline 

 rocks, i. e., the gneisses and schists, and this element has been admitted more and 

 more to the present day, except by T. Sterry Hunt. 



Dr. Hunt proposed (1884) the " crenitic hypothesis," for the origin of granite, 

 gneiss and all the other Archean rocks, including the schists, basalts and greenstones. 

 This hypothesis conceives the crystalline rocks to have been derived, directly or indi- 

 rectly, by solution from a primary stratum of basic rock, the last congealed and 

 superficial portion of the cooling globe, through the intervention of circulating 

 subterranean waters, by which the mineral elements were brought to the surface. 

 This circulation is supposed to have been caused by heat from below and surface 

 radiation. It is noteworthy that this hypothesis presumes a primeval basic layer, 

 which is wholly lost by the crenitic process, originally not differing much from the 

 composition of doleryte, and yet appeals to the " Huronian greenstones " as one of the 

 products of the crenitic action, the last step in the process. 



As metamorphism came to be recognized by most geologists as an essential 

 element in this investigation, the question assumed two branches, viz.: the origin of 

 the materials of the crystalline stratified rocks, and the origin of the differences in 

 the igneous rocks. On the latter, recent investigation has been centered, under the 

 terms segmentation or differentiation of magmas and their succession. At the same 



