THE IRON-BEARING MEMBER. 241 



abuudaut led luMuatite niMkiim tlicm i;is|M r. wiHi majinetite, and finally witli liiMiialir',' 

 and magnetite. Often tlic areas wliicli cdnlain litl !<■ m- no iron oxide and tiidseeon- 

 taining the largest aiaonnls are in Jiixtapusifidii. The siniple grains ol' tiauinenlal 

 quartz and the clierl areas alike are generally airaaged with iheir lunger axes in a 

 eoininon direction. The nnniis(akal)le fVagnieiital eharaeler of the simple (|iiartx., the 

 essentia] likeness to them of I he eliert areas in unllines. and I heir anani;(Mnenl with 

 longer axes in a comnKm direction, aie eonelnsive piools I hat they are all meehanical 

 sediments — not li a gments caused by a later intrusion ol silica, or to concretionary action 

 in connection with such inlroilnctioii of silica. In tact, th(>se areas show an entire 

 absence of any concretionary structure, exceiit l)eyond Ihcii- onl<'r liordcrs. Here 

 such a structure is at times seen, Imt the same thing is true of the eidarged frag- 

 meiital quartzes, and is in all cases plainly in the ceiuenting silica. In this lotdc is 

 therefore a sharp alternation of clastic and nonclastic sedimentation, the interhiMU- 

 nated l)elts being at times not more than an inch liroad. 



04. Sideritic! cherts, from low horizons. 8[)ecimens ftJilG (slide lL'21.*), from fiOO 

 N., 1085 W.; tt;307 (.slide 421!t), from ()I.-> N., 10.S5 W. 8ec. 17, T. 47 N., K. 44 W, 

 Michigan. 



A thinly lanniiatecl rock, in which line grained, dark gray hands alteiiiate with 

 redjaspery and clierty ones. S]). gi-. of tiie grayish liands. .i.dl. 



In the thin sections, narrow Itands are, composed of a nearly pure aggregale of 

 iron carbonate, but these grad(M)tf into otlier portions whii-li are mainly composed ot 

 minutely divided silica, whi(di contains a good deal of hematite and magnetite. In 

 slide 4222 these reddisli bands are in the main of tiu' peculiar t>pe described in (i.J, 

 although containing less fragniental ([uartz in simjilc grains. In IL'I'.I a liagmental 

 character is less plain. 



Gi). Femiginous chert or llinl. from a lower midille hori/on. Specimens i2<i.S<! 

 (slide .-.408), 12687 (slide .>40'.t); from s.-,() N., 1(m(I W., Sec. 17. T. 17 X.. K. 44 W., 

 ^lichigan. 



The rocks are ai.hauitic, dark reddish brown cherts, in which are seen aggre- 

 gated in certain portions immerous grains of lim])id ipiartz. Sp. gi'. of tlie more 

 Idghly hematitic portions, 3.(12. 



The thin sections are in all essential respects like the fnigmontal j.arts of 03. 

 The amount of simple fragniental (piartz in slide 5409 is less than that in ~>10S and in 

 03. The iron oxide in the fragments and matrix is mostly hematite, although mag- 

 netite is |iresent. The grains of simple cpiart/, ha\e been will iniinded. and are often 

 widely eidarged. They quite often contain within thi'ir cores crystals of hematite 

 and magnetite. In this respect they are closely like the ferruginous qnartzite 12f;S0, 

 described page 171. In fact, the chief ditfcrencc belw.een the two rocks is that ill 

 12080 the fragmeiilal material is more al>uii(laiit. 



310N XIX 10 



