INTRODUCTION. 3 



distribution from the series of parallel belts northward. These belts, four 

 in number, follow in regular succession, with the excej)tJon of a few miles 

 at the east end of the area, rhey are separated from each other upon the 

 principle of fragmental and nonfragmental character. This memoir treats 

 of the rocks between the gneiss-granite areas and the Keweenaw series, and 

 to them is applied the name Penokee series. 



The Wisconsin geologists, as will be seen by the literature, called the 

 iron-bearing rocks and associated slates the Penokee series. On the Michi- 

 gan side of the boundary the course of travel was largely by the way of 

 Agogebic, or as it is now contracted, Gogebic lake, and the geologists, 

 explorers, and miners gave to that part of the area the name Gogebic district. 

 As the areas are parts of continuous geological series, one or the other of these 

 two names must be accepted for the whole, or else a new name be coined 

 by their combination. The latter course would be perhaps the less objec- 

 tionable if the resultant name Penokee-Gogebic were not so awkward. The 

 only systematic geological treatment of any part of the district is that by 

 Irving and Wright in the Geology of Wisconsin; hence, under the law of 

 priority, the term Penokee series is in this volume used to cover the whole 

 area. 



The southernmost of the belts of the Penokee series is called the Chei'ty 

 limestone. This name sufficiently indicates its character. Whether it is of 

 direct chemical or of organic clastic origin, it now gives no evidence of 

 having been fragmental. The next belt to the northward is called the 

 Quartz-slate member, because cjuartz is the preponderating constituent and 

 a slaty structure the normal one. It is sharply separated from the under- 

 lying Clierty limestone member by the fact that it everywhere reveals in 

 thin sections its essential fragmental character, and also by tlie fact that it 

 bears debris from the Cherty limestone. Next to the north is the Iron- 

 bearing member, so called because all the known ore-bodies occur within 

 it. Whatever its origin, it, like the Cherty limestone, never gives any evi- 

 dence of a fragmental character. North of this belt is the Upper slate mem- 

 ber. This is in places several times as thick as the three lower members 

 combined, but the whole width is included within a single belt because it is 

 everywhere substantially alike. It is a gray wacke, graywacke-slate, mica- 



