3G2 THE PENOKEE IRON-BEARING SERIES. 



proaclies that here attaineil is west of Sunday lake, and here again the 

 apparent thickness is in part and perhaps Lii-gely due to eruptives. For 

 the distance between these two points the Keweenaw series directly overlies 

 the Iron-bearing member. In the eastern area the rocks belonging to tiie 

 Iron-bearing member constitute, as far as at present known, a naiTOw belt, 

 narrower than anywhere to the west. The causes of this chansre from 

 extreme width to extreme narrowness will be discussed later. 



PetrogmpMcal character. — Tlie locations of the exposures belonging to 

 the iron formation are found upon PI. xiii and indicated in the tabulations 

 to follow. The kinds there found include nearly every phase of rock 

 characteristic of the Iron-bearing member west of the Presque Isle. This 

 likeness of the rocks east and west of this stream is such that no question 

 can be entertained as to their identity of character and origin. A general 

 discussion of the original nature and subsequent modifications of the rocks 

 of the Iron-bearing member has been given in another place and need not 

 be hero repeated. It is, however, worthy of note that in some of the 

 ferruginous cherts in T. 47 N., R. 43 W., Michigan, are found very numer- 

 ous small geodic cavities which are lined with quartz crystals. That this 

 quartz is of a secondary nature, or at least has been rearranged since the 

 rock was originally formed, can hardly be doubted. The close association 

 of siderite, magnetite, hematite, actinolite, and quartz is finely shown by 

 one exposure. The relations here are such as to indicate that the mag- 

 netite has formed directly from siderite, as has hematite so extensively in 

 the iron-bearing belt to the west. Further, the actinolite appears where- 

 ever quartz is found, while the quartz present in the section is in irregular 

 veinlike forms cutting across the lamination. The conclusion is that this 

 quartz is secondary, and that at the time it formed a portion of the silica 

 in solution united with the bases present — calcium, magnesium, and iron — 

 to form the actinolite. We have here, then, another reenforcement of tlie 

 argument given for the derivation of the actinolitic slates from an original 

 clierty carbonate. 



Mingled fragmental and nonfragmentaJ sedimentation. — The one impor- 

 tant point in which the iron-bearing rocks east of the Little Presque Isle 

 difi'er from those to the westward is that they are iuterstratified with a 



