THE IRON-BEARING MEMBER. 203 



constituents, are detected under the microscope iron ctirbonate, wliidi 

 occurs very f'refpiently ; actiuolite, which is of less frequent occurrence; 

 carbonaceous matter, and chlorite, these two l)eing- of unusual occurrence. 

 The predominating- siliceous groundmass of these rocks varies from very 

 niiuutely though completely ciystalline to ])artly amorphous (PI. xxii, Fig. 

 2), often presenting the intermediate condition characteristic of chalcedony. 

 In some sections none of the chalcedonic or amorphous phase of silica is 

 present, but in others all three of these phases occur together. In general 

 there is in these rocks a much greater tendency toward the crystalline 

 kind of silica than in those of the first type, the cherty iron carbonates; 

 but as compared with the silica in the rocks of the third iy\w, or actinolitic 

 slates, there is a greater tendencj' toward the noncrystalline kinds. It has 

 been shown that the actinolitic slates have tlieir main distribution in the 

 western portion of the iron belt, occurring again in considerable develop- 

 ment far to tile east, while in the intervening' space the ferruginous cherts 

 and carbonates have their great development. Correspondingly those 

 phases of the ferruginous cherts, wliich in their content of small (juantities 

 of magnetite and actiuolite present lis with a gradation into the actino- 

 litic slate type, are found particularly toward the west as one approaches 

 the region of actinolitic slates proper, and again at the extreme east as the 

 actinolitic rocks of tliat area are reached. Accompanying the presence 

 of magnetite and actiuolite in these gradation phases is usually found the 

 most completely crystalline condition of the siliceous groundmass. 



The ferruginous cherts, as indicated in the general description, pre- 

 sent two widely different phases; the ferruginous slates in wliicli tlic iron 

 oxides and silicates are quite uniformly mingled, and rocks in which the 

 iron oxide is concentrated to a greater or less degj'ee in bands, rings, and 

 shots, leaving the silica comparatively or almost wholly free from iron 

 oxide. The rocks of the first phase are" composed of intimately mingled 

 chert and brown, somewhat hydrated hematite, red hematite, and occa- 

 sionally magnetite. In the ferruginous slates tliere has also been a concen- 

 tration of the iron oxide to a small extent in layers. At times these laj-ers 

 are very regular ones, Ix'twccn whicli alternate layers of chert, containing 

 comparatively little oxide of inm Fiom these perfectly lauu'nated phases, 



