442 THE CEYSTAL FALLS IROjST-BEARraG DISTRICT. 



PETROGRAPHICAL CHARACTERS. 



The exposures tlirougli sectiou 10 and the uortheru part of sec. 15, 

 T. 44 N., R. 31 W. — the southern river section — give us a nearly complete 

 sequence across the Hemlock formation, the j)i'incipal gaps being on the 

 extreme east and west, thus leaving the details of the relations with the 

 dolomites below and the iron formation above undisclosed. In this section 

 of 3,000 feet in length, the rocks are chiefly chloritic and epidotic schists, 

 with which are associated schists bearing biotite, ilmenite, ottrelite, and 

 am]3hibole, greenstone conglomerates or agglomerates, and amygdaloids. 

 These rocks are characterized by a generally fine and even grain, by a lack 

 of sedimentary characters, and by a double structure. In most of the 

 varieties minerals, which have formed quite independently of and later than 

 these structures, are raacroscopically conspicuous. The prevailing color is 

 green, passing to dark purple and black in the varieties in which biotite, 

 hornl^lende, and magnetite abound. 



The distinction made in the field between the several varieties of the 

 schists is a rough one, indicating the predominating minerals rather than 

 implying the absence of the others. In fact all the varieties are intimately 

 related. The chlorite-schists are very fine-grained green rocks, usually 

 from their color evidently very epidotic; they weather to greenish or pink- 

 ish white. The cleavage surfaces are often plentifully sprinkled with little 

 flakes of biotite. Frequently also black needles of ilmenite, brilliant plates 

 of ottrelite, and large clusters of actinolite run irregularly through them, 

 quite independent of the cleavages. The biotite-schists are much darker, 

 and lack the green coloring. Through them also the same metamorphic 

 minerals are frequently interlaced. By an increase in these minerals the 

 passage to the other varieties in limited exposures is a very easy one. 



Greenstone-conglomerates and amygdaloidal rocks occur in a few 

 exposures. In the former, light green or gray aphanitic inclusions, of 

 angular shapes, ranging from an inch to 2 or 3 feet in long diameter, are 

 inclosed in a matrix of chlorite-schist or biotite-schist. The chlorite-schists 

 often hold round or lens-shaped eyes of epidote, and epidote and quartz. 

 That these are filled cavities can in most cases be shown only by the micro- 

 scope, yet some of the larger amygdules have a banded structure evident 

 to the naked eje. These rocks are of structural interest since they are the 



