THE IRON -BEARING MEMBER. 193 



section. Compared with the sihceous ingredient of the rocks of the third 

 type, between which and those now under consideration there are distinct 

 gradation varieties, the siHca of the rocks of the second type is in general 

 very much finer in grain. Indeed, those phases in which the siUca is either 

 in so minute individuals that they can only with extreme difficulty be sepa- 

 rated under the microscope, or in which it is mingled with more or less 

 chalcedonic or even amorphous forms, greatly predominate. In addition to 

 this siliceous groundmass, to all of the phases of which we apply the general 

 term of chert, the principal ingredients of these rocks are the several iron 

 oxides: that is, magnetite, hematite, and a brown hydrated oxide. These 

 "are generally greatly subordinate in quantity to the silica, at times sinking 

 almost completely out of sight. In other cases they are present in consid- 

 erable quantity, and frequently occur so plentifully as to furnish interme- 

 diate phases between the cherts and the iron ores of the district. The 

 principal one of these oxides is hematite. When magnetite is present in 

 any considerable quantity, it is generally accompanied by more or less 

 actinolite in minute needles, and phases of gradation between the cherts 

 and the actinolitic rocks of the third type, subsequently described, are thus 

 produced. On the other hand, more or less iron carbonate is found in 

 renmants in the many sections of these cherts, and by its increasing 

 quantity leads us thi'ough phases of gradation into the rocks of the first 

 type above described. In color tliese rocks vary greatly, presenting red, 

 brown, gray, and white colors, depending upon the amount of iron oxide 

 present. Perfectly white nonferruginous phases are occasionally met with; 

 also light to dark gray kinds, and grayish kinds mottled irregularly with 

 black, in which phase the iron oxide is mainly magnetite. When hematite 

 is present in large quantity the rock may have, a uniform retl color. Occa- 

 siolially the I'ed iron oxide is present in just such quantity and condition as 

 to make of the chert a genuine jasper, but such phases are not common. 

 A rather unusual kind is quite black, apparently from the presence of 

 carbonaceous matter. 



In structure these rocks vary from the regular lamination of the first 

 type to those that are much less regular, being either without any uniform 

 banding, or, if the banding is present, the lamina; present the appearance 



JVION XIX 13 



