978 THE GEOLOGY OF MINNESOTA. 



[Quartz-porphyry . 



indentation accompanied by chemical precipitation in Archean time. Its being pre- 

 ceded by a greenstone conglomerate* and followed by another conglomerate (No. 

 2230; also, plate Z, vol. iv) places it in the midst of rocks of fragmental origin. Its 

 feldspars and quartzes may have been derived by crystallization from an alkaline 

 and siliceous mud of great thickness, the same mud embracing more or less of the 

 regular detritus from the pre-existing Kawishiwin greenstone and jaspilyte. This 

 alkaline mud, following immediately after the primeval greenstones which are sup- 

 posed to represent the first crust of the earth, was deposited in a hot ocean. It may 

 be supposed that, up to this epoch, the crust had been too hot to allow the con- 

 densation of potassium from the surrounding atmosphere, as no potassium exists in 

 the oldest greenstones, nor in the greenstone conglomerate mentioned; and that, 

 cotemporary with the precipitation of potassium into the ocean, was its increased 

 siliceous content of silica, and, later, the copious precipitation of both, the former in 

 a silicated state. 



The partial resorption of the phenocrysts of feldspar, the mingling of orthoclase 

 with plagioclase, the fineness of the surrounding matrix, the greatly decayed state 

 of the feldspars, the bipyramidal forms of the quartz, most of these characters usually 

 accepted to indicate igneous origin of the rock showing them, may thus be explained 

 in conjunction with the existence of other features that are usually interpreted as 

 of clastic origin. f 



Notwithstanding these facts and inferences, it is necessary to hold the oceanic 

 origin of this quartz-porphyry as only hypothetical until further field examination 

 can be made. 



In numerous instances a porphyry which is somewhat granitic, but containing 

 porphyritic quartzes, has been noted cutting the upper part of the Lower Keewatin, 

 as at Ely (Nos. 2095, 2096), and at Vermilion lake (Nos. 2275, 2276), and occasionally 

 the Upper Keewatin, as at Snowbank and Kekequabic lakes. Such "quartz-porphy- 

 ries" are susceptible of two explanations as to origin, viz.: (1) A compacting and 

 recrystallization of a clastic rock embracing many fragmentary crystals of feldspar 

 and of quartz (Nos. 1062, 2184, 2187, 2189); (2) A transformation, through folding and 

 fracture, of the old quartz-porphyry of the Lower Keewatin (Nos. 2229, 2237), and 

 the production, in that way, of dikes in the later rocks. 



The other quartz-porphyry has an origin less doubtful. It constitutes one of 

 the forms of the " red rocks" of the Keweenawan. This is a class of rocks which the 

 writer, following J. C. Norwood, in 1878 and 1879 in several publications showed was 

 derived, as a group, from metamorphic action of the igneous traps on the clastic 



*Of this rock, at this place, no sample was taken, but it is represented by rocks Nos. 532H and 626E. 



tNOTE. Prof. W. O. Crosby has suggested a deep-sea origin for the red petrosiliceous rocks, comparing them with the 

 jaspilytes of the Lake Superior region. Proceedings of the Boston Society of Natural History, vol. xx, March, 1879. 



