42 THE GEOLOGY OF MINNESOTA. 



[Oldest quartz-porphyry. 



greatly agitated, and hence that chemical precipitations would have been profuse, 

 such agitation probably being caused by local volcanic action. 



If we allow the chemical precipitation of the silica of the jaspilyte in the midst 

 of the greenstone Archean, or closely following the congealation of the earliest green- 

 stone crust of the earth, thus producing the jaspilyte masses which everywhere occur 

 in the greenstone, it is reasonable to presume that those conditions may have been 

 prolonged in time and intensified in degree as well as extended in area, and that under 

 favorable circumstances an enormous amount of siliceous mud, varying occasionally 

 to pure silica, may have been produced. There have been noted repeated instances 

 of the gradual passage, by inter-stratification, from jaspilyte to argillyte, and to a 

 chloritic schist, as well as to iron ore. In one remarkable instance such banded 

 jaspilyte has been seen to be at the same time a coarse conglomerate, showing that 

 violent agitation as well as chemical precipitation was an attendant of the Archean 

 ocean, both taking place, in some cases at least, at the same point and simultaneously. 



Under conditions producing chemical precipitation of silica, if the Archean 

 ocean was deep, and if the precipitation was rapid and abundant and the mass cooled 

 slowly (for the Archean ocean at this time must be considered to have been heated), 

 it might be that crystals of quartz of considerable size would be formed throughout 

 the mass, and that all the quartzes of this porphyry may thus have originated, in 

 some such manner as selenite, pyrite and other crystals form in a mud that holds the 

 elements of those minerals in saturated solution. The general absence of a banded 

 stratification, under this hypothesis, is the greatest obstacle; but if the precipitate 

 accumulated rapidly, it must have been subject to the same forces, whatever they 

 were, which excluded the banded structure from great thicknesses of fragmental 

 greenstone, and from greater thicknesses of the Ogishke conglomerate, and from the 

 Stuntz conglomerate. It is perhaps due to copious and quick accumulation that 

 the sedimentary structure is not seen in some large and important fragmental 

 terranes. A subsequent crystallization of the mass would also result in the obscura- 

 tion, or the obliteration of the sedimentary structure, a fact seen in many great lime- 

 stone strata of the Silurian. 



If, however, as seems to be proved by the presence of orthoclase crystals in this 

 porphyry, the precipitate was not wholly of silica, but included a considerable 

 amount of potassa, the crystallization of such a deposit of alkaline mud would not 

 only rearrange the molecular structure of the deposit throughout its whole thick- 

 ness, but would still more effectually destroy whatever sedimentary structure the 

 ocean may have stamped upon it. Elsewhere it has been shown by the writer that 

 potassium was probably retained as an element of the atmosphere, after the solidifi- 

 cation of the first crust, for a period long enough for the cooling of the crust and for 



