222 Wis [OORINAUL WF, GiB QILOG VY, 
kian. It is composed of three members designated in an ascending 
order as the Quartz-Slate member, the Iron-bearing member, and the 
Upper-Slate member. All of these lie conformably with each other, 
but differ considerably in the character and the mode of deposition of 
their constituents. The Quartz-Slate and Upper-Slate members are of 
clastic origin, while the Iron-bearing member is largely composed of 
chemically deposited materials. 
The Quartz-Slate member is about 500 feet thick, is composed of 
fragmental materials among which quartz predominates, and is capped — 
by a layer of pure quartzite, which has played an important part in the 
formation of the iron ore bodies. The Iron-bearing member averages 
about 800 feet in thickness, and is composed of cherty iron carbonates, 
ferruginous slates and cherts, and actinolitic and magnetitic slates. 
The cherty iron carbonates are the original form from which the other 
rocks mentioned were derived. The iron ore deposits of the region 
were also formed from the same sources and in a manner to be discussed 
more fully below. ‘The Upper-Slate member averages about 12,000 feet 
in thickness. It is of clastic origin, and composed mostly of gray- 
wackes or graywacke-slate, though sometimes altered to a crystalline 
schist. 
Numerous bodies of diabase have been intruded into the Penokee 
series, both in the form of dikes and of interbedded sheets probably 
contemporaneous with the dikes, both being presumably of Kewee- 
nawan age. ‘These diabase intrusions have had a most important effect 
on the formation of the iron ore deposits, as will be noticed later on. 
The Penokee series was subjected to a period of considerable ero- 
sion before the depositions of the overlying Keweenaw series, and this 
in turn was again subjected to disturbance and erosion before the depo- 
sition of the overlying Eastern Sandstone. It will thus be seen that 
the Penokee series is separated by marked unconformities, both from 
the underlying Southern Complex on the south and the overlying 
Keweenaw series on the north; while it is terminated on the east by 
the Eastern Sandstone and is abruptly cut off on the west by erosion. 
It occupies, therefore, an isolated area, unique among rocks of this age 
for the clear definition of its members and the simplicity of its structure. 
The iron deposits which have made the Penokee region (also known 
as Penokee-Gogebic region) celebrated as a mining district, occur in 
the lower horizon of the Iron-bearing member of the Penokee series, 
and generally immediately over the quartzite which forms the upper- 
most horizon of the Quartz-Slate member. The diabase dikes which 
