PORODITE, PORPHYRELLITE, ETC. 381 



Altered Tuff's and Mixed Rocks. — Though the argillites and their included 

 conglomerates are the most bulky -and conspicuous member of the semi- 

 crystalline schists, they are not the most characteristic feature of this sys_ 

 tern. Immediately eastward from Tower the proper argillites have a feeble 

 development, and they seem to be partially replaced by sericitic and chlo- 

 rictic argillites and sericitic schists, and the same conditions are found at 

 many places eastward as far as Ogishke-munice. But in the vicinity of 

 Vermilion lake certain clastic rocks represent very imperfectly the character 

 of sericitic schists. The transition from these to true schists is, many times, 

 along the line of strike, but it is also sometimes across the strike. These 

 nondescript rocks, when well developed, have often been designated 

 " porodite " by the Minnesota Survey. They are generally ashen colored, 

 mostly fine textured, generally rather soft, but with disseminated quartz 

 grains, which sometimes attain dimensions of a quarter of an inch, and they 

 show obscure tracings on weathered surfaces, suggesting an original conglom 

 eratic or agglomeratic constitution. They have only very obscure lami- 

 nation. Beds answering such a description are extensively iuterstratified 

 with characteristic schists. There are also occasional beds of this character 

 which cut, dike-like, at a small angle across the strike of the graywackes. 

 These probably, though similiar, have had a different origin. It seems to 

 be a rock allied to this porodite, which in places contains small quartzose 

 and granitic pebbles, and constitutes the "Stuntz conglomerate," which, but 

 for other evidence, might be regarded as occupying the horizon of the 

 Ogishke conglomerate. 



Porphyrellite. — The porodite of Vermilion lake holds lumps of serpentine, 

 and the recognized sericitic and chloritic schists are sometimes serpentinous. 

 These appearances increase eastward. It does not appear that the magnesian 

 formation is developed at the expense of the argillitic, though it is certain 

 that the magnesian character is sometimes superinduced on an argillitic 

 foundation. At Sucker lake, on the boundary, certain schists having a ser- 

 pentinous aspect begin to abound. This is in a zone somewhat north of the 

 argillites and nearer the granitoid rocks of the Basswood area, and there- 

 fore regarded as underlying. At Sucker lake these rocks possess a greenish, 

 argillitic aspect, but their edges transmit light, and the hardness and feel 

 are slightly magnesian. Traced to the eastern ramifications of Knife lake, 

 tliis formation attains an imposing development. It may, under some of its 

 aspects, answer to the " parophite " of Hunt, but I have called it, for the 

 purposes of description, " porphyrellite." The formation is unquestionably 

 bedded, but it is often imperfectly so, and it is intersected by a multitude 

 of irregular local fissures making acute angles with the bedding and with 

 each other and converting the rock sometimes into an infinite number of 

 small cuueate and lenticular forms closely packed together. 



L— Bum.. Geol. Soc. Am.. Vot,. 1, 1889. 



