40 THE GEOLOGY OF MINNESOTA. 



[Characteristic elements of the alkaline magma. 



primeval ocean, tending to work over the volcanic ejectamenta and the prior-formed 

 ferro-magnesian crust. The essential inquiry, in this search, is to ascertain whether 

 there was any source for potash and for an excess of silica, such that these could 

 accumulate in the fragmentals in greater proportions than in the earlier rocks. 



At ordinary temperatures, and under ordinary conditions, silica is quite insolu- 

 ble, but in an alkaline water it is soluble and is carried from one place to another in 

 the crust of the earth, in large amounts. In a heated alkaline water larger amounts 

 may be held in solution. As it is known that throughout the Archean fragmental 

 rocks there is a very large amount of microscopic silica, sometimes constituting 

 important beds due to oceanic precipitation, as in the jaspilytes and other flinty 

 deposits, the inference is natural that the ocean was in suitable chemical condition 

 to hold in solution very large amounts of silica, and therefore that it must have been 

 alkaline and probably heated.* 



Such alkaline quality, residing in the ocean, could not have been derived, so far 

 as it consisted of potassium, from the pre-existing rocks, for, as already shown, they 

 contained no potassium. It must therefore have been resident in the ocean, from 

 which it was precipitated, like the silica, and took its place, in form of such chemical 

 combinations as its environment allowed, amongst the other sediments. Its ultimate 

 origin might be further considered, but at this point it is necessary simply to call 

 attention to the nature of the precipitate which would result from an alkaline ocean 

 in which were numerous active volcanoes. 



The acid exhalations of volcanoes are well known. They are sulphuric and 

 hydrochloric acid, and also carbonic. But as the last may not have existed in the 

 acid products of the Archean volcanoes, it may be omitted from this inquiry. These 

 acids, mingling with the oceanic waters, would seize on the alkaline bases, liberating 

 the silica. The soluble salts thus formed would be disseminated in the ocean, and it 

 would result that the alkalinity would be locally intensified. In general, however, in 

 case of the disturbance of the chemical equilibrium, there must have been a precipi- 

 tation of silica and of an alkaline silicate, and even without any distui'bance from the 

 addition of volcanic acids, the ocean may have given everywhere a slow accumulation 

 of such silicates. Owing to the avidity with which potassium seizes on oxygen, and 

 the insolubility of the potash feldspars, it is apparent that the silicates of the alka- 

 line feldspars would be immediately and perhaps copiously precipitated. This seems 

 at least to have been the case. The Archean ocean, therefore, was unfit for animal 

 life, and was characterized by the prevalence of the lightest of the alkaline bases 

 (potassium) in some such way as, later, the Silurian was charged with lime and 

 magnesia, and, as the present, with soda. It resulted that the world's great stock of 



* On the precipitation of silica from solution in the Archean ocean the reader may consult " On a possible chemical origin 

 of the iron ores of the Keewatin in Minnesota." N. H. and H. V. WINCHELL. American Geologist, vol. iv, p. 391, 1889. 



