338 University of California Publications in Geology [Vol. ll 



chert bands interlaminated with black iron oxide bands. The iron oxide is 

 commonly in large part magnetite. Usually associated with this magnetite are 

 some of the amphibole minerals already mentioned. 



3. In the red-banded kind the quartzose layers are stained with innumerable 

 minute flakes of hematite, which give the rock a red color, in many places a 

 brilliant red. The iron oxide between the red bands is ordinarily hematite, 

 usually specular hematite. With this hematite may be some magnetite. This 

 red-banded variety is a well known jasper of the Lake Superior region to 

 which Wadsworth has applied the name jaspillite. 



4. In the white banded kind the quartzose bands contain comparatively 

 little iron oxide. The iron-oxide bands between the layers of chert are gen- 

 erally hematite, but this hematite differs in many places from that of the jas- 

 pillite bands in that it is of the red or brown variety. With it also, in many 

 places, there is a certain amount of limonite. 



5. The banded carbonate variety, while subordinate in quantity, is impor- 

 tant in reference to the genesis of the formation. It is a gray-banded rock, 

 the light-colored layers of which consist largely of siderite. Between this 

 sideritic rock and the ordinary forms there are all stages of gradation. 



6. The positions of the iron-ore bodies will be fully discussed later. In 

 the iron ores the silica is very subordinate, the place of the quartzose bands 

 being taken by iron oxide. The iron ore is dominately hematite. 



The iron formations of the Lake Superior District also contain 

 considerable thicknesses of ferruginous slates. Some of these are 

 massive. Other varieties are cherty and contain numerous separate 

 bands of chert. Slate bands also occur as partings in some of the 

 jaspery portions of the iron formations. 



The greenalite rocks constitute the most unusual lithologic type 

 in these formations. They are dull, dark green rocks of rather fine 

 grain. They are generally well bedded with layers of slate or iron 

 ore. They may be seen to grade into quartzite or slate in going up 

 or down in the section. At times the greenalite rock may be seen to 

 grade laterally into slate. 



When the greenalite rock is examined closely, particularly when 

 the surface is wet, it may be observed to contain numerous ellipsoidal 

 granules of a green substance. These are slightly lighter green than 

 the matrix in which they are embedded. They are so small and so 

 much like the matrix in color that they are not likely to be noticed 

 unless specially looked for. They are rarely larger than one milli- 

 meter in diameter, and usually are above 0.1 millimeter. These green 

 granules are found to have a more or less definite composition, being 

 a hydrous silicate of ferrous iron and magnesia. 



The iron formations are remarkably well bedded. The beds are 

 thin, rarely being over five or six inches thick and commonly being 

 much thinner. Many of the jasper bands have oval terminations or 

 end in an irregular manner. They are frequently seen to end, even 



