THE EASTERN AREA. 398 



cai'honate arc almost free from any fragmental material, hut the rock coni- 

 posnig most of thi^ exposure has a hackg-round of chert and carbonate which 

 contains abundant fragmental material. Almost in contact with the crys- 

 talline rocks to the south, in the north part of Sec. 23, is a l)and a few feet 

 thick of nearly pure cherty ferro-dolomite, and the fragmental rocks to the 

 north contain qiu'te a quantity of chert and carbonate. In the ex])osures 

 along the east side of Sec. 14, T. 47 N., R. 43 W., Michigan, ferro-dolomite 

 is a chief constituent. Fi-om the west side of Sec. 13, T. 47 N., R. 43 W., 

 Michigan, to the center of Sec. 28, T. 47 N., R. 42 W., Michigan, chert and 

 feiTO-doloiuite (chiefly the former, except locally) constitute the Imckground 

 in which the abundant fragmental material is contained; while in some of 

 the quartzites and conglomerates the nonfragmental material sinks to an 

 insignificant quantity. 



It will thus be seen that there is a marked chanw in the nonfra"-- 

 mental sediments in passing from west to east. Throughout its eastern 

 portion this material is cherty silica and a ferriferous carlHtnate; in its 

 central jjart it is cherty carbonate and iron oxides, and here in most of the 

 sections the iron oxides are seen in actual process of foi-mation from the 

 carbonate; in the western part of the belt the iron present is almost 

 wholly in the form of oxides. Considering these facts by themselves, it 

 seems a natural conclusion that it is probable that all of the iron was 

 oi-iginally jjresent as a cai-bonate, and that the iron oxides now found have 

 been fonned from the alteration of such material. This probaljilit}' is 

 made almost a certainty when the facts are taken in connection with what 

 has sjone before in reference to the iron oxides of the Iron-bearinsr member 

 in the main area to the westward. We, then, have here another belt in 

 which the widespread occurrence of iron oxide is due to the presence of an 

 origiiud ferriferous carbonate. 



From the foregoing it is evident that this belt of rocks is different 

 from any considerable area foinid in the main western area. The scries 

 there consists of four great formations, which are alternately nonfragmental 

 and fragmental sediments, and which are separated from each other with 

 surprising sharpness. In only a few localities are fouml transition belts 

 between the two clas.ses of sediments, and these are iiiosth- narrow: but 



