UPPER HURONIAN. 383 



splierulites, and around each one of these spheruhtes there occurs also a 

 fihu or thicker layer of limonite, with, in a few cases, some small quantity of 

 hematite. This structure is interpreted to mean that the ferrous silicate 

 originally occupying these areas has been altered into its constituents, iron 

 oxide and silica, the silica forming the radiating areas above mentioned, and 

 the limonite having been retained either at the centers of these areas or 

 forced outward during the processes of crystallization, so as to form a ring 

 now surrounding these areas (see PI. XII, B). That a large part of the 

 silica of the granules is a secondary deposit is shown by the fact that an 

 imperfect agate structure is not uncommon (see PL XII, B). A similar 

 agate structure also occurs between the large rounded gramiles referred to 

 Projecting from the sides toward the centers of the spaces between these 

 granules occur also segments of or complete quartz spherulites. This 

 spherulitic structure showing black cross is reproduced in PI. XII, A. 



The rocks briefly described above are the least altered forms of the 

 rocks of the iron-bearing formation, and when weathered exhibit on the 

 surface a brown ferruginous crust. As we follow these rocks westward we 

 find that they change somewhat, passing into ferruginous cherts and cherts 

 which have been more or less completely recrystallized into relatively coarse- 

 grained rocks that might be spoken of almost as quartzites — although they 

 are not, as should be clearly understood, metamorphosed clastic sandstones — 

 actiuolite, griinerite, and magnetite rocks, in which there is practically no 

 carbonate, or but very little. These rocks, of course, vary greatly in color, 

 ranging from white or gray to brownish, light green, dark green, and prac- 

 tically to black, the color depending on the quantity and kind of the minerals 

 mentioned which are present in them. This is especially true of those 

 rocks that occur in the narrow belt extending from a short distance east of 

 Paulson's mine west nearly to Gobbemichigamma Lake. Here the gabbro 

 is either in immediate contact with or but a short distance from these rocks. 

 The rocks in this area are made up of coarsely crystalhne bands of quartz 

 of varying width in alternation with coarsely crystalline bands of magnetite 

 ore, reported to range from 1 inch up to 10 or 12 feet in thickness, and 

 bands of dark-green, brown, or black rocks, which consist of combinations 

 of quartz, augite, hypersthene, hornblende, olivine, and magnetite as the 

 principal minerals, associated occasionally with some ferruginous carbonate, 

 actiuolite, and griinerite. Tliese bands, consisting largely of ferromagnesian 

 minerals, vary from mediuni grain to cofirse grain. Occasionally they 



