1918] Davis: The Radiolarian Cherts of the Franciscan Group 337 



In simplest terms the iron-bearing formations of the Lake Superior region 

 consist essentially of interbanded layers, in widely varying proportions, of 

 iron oxide, silica, and combinations of the two, variously called jasper or jas- 

 pilite, where anhydrous and crystalline, and ferruginous chert, taconite, or fer- 

 ruginous slate, where softer and more or less hydrous. These rocks become ore 

 by local enrichment, largely by the leaching out of silica and to a less extent 

 by the introduction of iron oxide. There are accordingly complete gradations 

 between them and the iron ores. . . . Local phases of the iron-bearing forma- 

 tions are ampMbolitic and magnetitic cherts and slates, cherty iron carbon- 

 ates, ferrous silicate or greenalite rocks, pyritie quartz rocks, and detrital iron- 

 bearing rocks derived from older iron-bearing formations. One of the most 

 significant variations with reference to the origin of the ore is in the relative 

 abundance of greenalite rocks and siderite. 



In the iron-bearing formations the chert constitutes about seventy 

 per cent of the total volume. The cherts and jaspers are also inti- 

 mately associated with ordinary clastic rocks such as quartzites, gray- 

 wackes and slates. Jaspers often occur rhythmically interbanded 

 with slates. 



The following description of the jaspery rocks of the iron forma- 

 tions is taken from the description of the Soudan formation. Similar 

 rocks are described from all the other iron formations of the region : 93 



The "jaspery" j-ihase of the Soudan formation consists of interlaminated 

 bands of finely crystalline quartz, iron oxides, and various mixtures of the 

 two. With these preponderating minerals are various subordinate constituents, 

 among which amphibole is the most abundant, including actinolite, cumming- 

 tonite, and griinerite. Pyrite is also present in many places. The alternate 

 bands of material of different color, combined with the complicated fracturing 

 and brecciation of the formation, make it a striking rock which always 

 attracts the attention of the traveler, even if he is not accustomed to closely 

 noticing rocks. The bands of material of different color vary from a fraction 

 of an inch to several inches across. The quartzose bands have various colors — 

 nearly pure white, gray, red of various hues, including brilliant red, and black. 

 The difference in the color is chiefly caused by the contained iron. Hematite, 

 if in sufficiently fine particles, gives the brilliant red colors; magnetite and 

 hematite in larger particles give the grays and blacks. 



Between the bands dominantly quartzose are usually bands mainly com- 

 posed of iron oxide. This iron oxide may be either hematite or magnetite or 

 various intermixtures of the two. Occasionally also some limonite is present. 



The chief varieties of the "jasper" are (1) the cherty variety, (2) the 

 black-bedded variety, (3) the red banded variety, and (4) the white banded 

 variety. With these are subordinate masses of (5) the carbonated variety and 

 (6) the ore bodies. 



1. The cherty variety is characterized by the presence of a predominating 

 amount of gray chert, the iron oxide being subordinate. The rock is there a 

 slightly ferruginous well-banded chert. 



2. The black-banded form of the Soudan formation has dark-gray or black 



»3 Monograph 52, pp. 124-125. 



