132 THE PENOKEE IRON-BEARING SERIES. 



lite is known to 1)e such by its usual microscopical characters, which need 

 not be here enumerated. Fig. 1 of PI. xvi is from a section of one of these 

 limestones magniiied sixty diamaters, in which there is shown one of the 

 larger tremoHte blades, through which in places are diud}' seen the out- 

 lines of the dolomite individuals. 



Many of the sections of these limestones show moi-e or less of a sili- 

 ceous ingredient, which is found in varying (piantities up to an amount 

 which very largely predominates over that of the carbonate. In fact, these 

 siliceous varieties furnish us with a complete gradation into the chert rock, 

 which, as already indicated, at times excludes the limestone completely. 

 Now and then there are apparent in the thin sections of these siliceous 

 limestones a few grains of quartz, whose fragmental nature is demonstrated 

 by their rounded contours and by the secondary enlargements they have 

 occasionally received. Bvit these fragmental particles are relatively sparse 

 and unimportant, the most of the silica having plainly solidified in situ. Of 

 the latter silica there may be distinguished two varieties, which, however, 

 grade into one another." The first of these presents itself in the shape of 

 an interlocking mass of (juartz individuals of finer or coarser grain. As 

 these become finer and finer, there is found intermingled with them more 

 or less of a fine spotty and chalcedonic silica with the characteristic aggre- 

 gate polarization and radiating structure ; and finally the}" })ass into kinds 

 containing a good deal of a completely amorphous opaline material. In 

 Fig. 2 of PI. XVI the thin section of one of the more sihceous varieties of 

 limestone is represented as seen in polarized light and magnified sixty 

 diameters. In the nnddle band of this figure the dolomite predominates 

 greatly over the silica. On the upper right-hand side of the figure is a 

 band composed of the finely crystalline silica, and at the lower left-hand 

 corner is an area of the more coarsely crystalline (piartz. 



Prtrofii-aphical character of the chert. — As already indicated, the cherty 

 material of this limestone belt is often in layers of consideraljle thickness, 

 at times ap})arently making up the greater part of the whole belt. At 

 Penokee gap, for instance, it has a thickness of some 45 feet. In other 

 cases it is scattered through or is interstratified with the limestone in narrow 

 seams. As in the case of the siliceous material just described as occurring 



