INTERGRADUATTON OF VARIOUS ROCKS. 379 



the strike to beds petrosiliceous, felsitic, and pseudo-diabasic. A tei*rane 

 bearing the characters of an argillite passes in one direction into a siliceous 

 schist and in another acquires felsitic or serpentinous matter until it arrives 

 at the stage of a petrographic nondescript, which I have called " porphyrel- 

 lite," but which approaches somewhat to Hunt's " parophite." The same 

 terrane passes on one side into an obscure conglomerate and on the other 

 into a porphyroid condition, sometimes with pebbles added. In this system 

 are formations which may be styled volcanic tuffs, often light colored, with 

 imbedded angular fragments blending with the groundmass, often agglom- 

 erate and nondescript. 



It is evidently beyond the scope of this paper to furnish an elaborate ac- 

 count of these rocks, but I will endeavor to enumerate their leading strati- 

 graphic aspects. 



Argillite is one of the most persistent terranes. Its centre of characteristic 

 development in the trough between the Basswood and White Iron granitoid 

 areas is in the region of Moose and Newfound lakes, in the ninth range of 

 townships west of the Minnesota meridian. It is here prevailingly russet, 

 handsomely cleavable in immense vertical sheets, and strictly argillitic. In 

 places, both eastward and westward, it assumes a slaty color. On Ensign 

 lake, to the northeast it, becomes sericitic, and the same variation is wide-spread 

 westward, about Eagle-nest lakes. In both directions it sometimes takes a 

 greenish, chloritic-sericitic character. As far west as Tower it becomes 

 rather characteristically a sericitic schist, and it is in this that the great de- 

 posits of haBinatite occur. At a few localities on Ensign lake the soft sericitic 

 argillite contains a great abundance of quartz grains, and this character reap- 

 pears 18 miles father northeast, on Frog-rock and Town-line lakes. The same 

 feature is widely distributed in the vicinity of Vermilion lake.* North and 

 northeast of Ensign lake, on Sucker lake, and at the west end of Knife lake 

 it becomes siliceous and in places is simply a black siliceous or flinty schist, 

 here as everywhere standing on edge. In the more exact direction of the 

 strike it continues to Ogishke-muncie lake and along both shores of this lake ; 

 and throughout the vicinity this argillite undergoes a remarkable modifica- 

 cation by the inclosure of long-extended series of pebbles and bowlders, form- 

 ing what' we know as the Ogishke conglomerate/^ It is to be remarked in 

 this case that the slaty beds do not curve around the bowlders. On the north 

 and west of the lake the matrix of the conglomerate preserves well its slaty 

 character, but on the south it has been altered to a silico-diabasic aspect. 

 In this state the pebbles are inconspicuous, but they may be distinctly seen 

 on smoothed surfaces under water. This condition also exists on Crab lake 

 and the northwest part of Frog-rock lake. The bowlders and pebbles of 



* Fifteenth Minn. Rep., p. 20. 



t'fhis conglomerate, in the judgment of my brother, is embraced in the Animike formation. 

 Fifteenth Minnesota Rep., pp. 01, 97; SeventeentltRep., pp. 17,47- My views and reasonings are given 

 in' the Sixteenth Rep., pp. 344-350, 259-360. 



