The Iron Ores of the Lake Superior Region . 221 
sistent quartzite. It is covered by a great formation of clay-slates, gray- 
wacke-slates, etc. These formations constituting the Penokee series are 
a simple monocline, dipping northward from 50° to 80° (PI. VII, fig. 1.) 
The series has been cut before tilting by numerous basic dikes nearly at 
right angles to the bedding, but in such a direction as to now make the 
outcrops of a dike and the iron-bearing formation form the two sides of 
an acute angle which usually faces to the east. The unaltered phase of 
the iron-bearing formation, i. e., the lean cherty carbonate of iron, is 
most frequently found immediatly under the overlying slate, which has 
protected this material from percolating waters, while near its base rarely 
is found this sideritic phase, it having here usually been decomposed. 
The intersections of the dike rocks and the quartz-slates form numerous 
right angled troughs tilted somewhat to the northward, (PI. VII, fig. 2.) 
These generally have a pitch toward the east (PI. VII, fig. 3), consequent 
upon the relations already described. But if the outcrop of the dike is 
parallel to that of the ore formation the ore-deposit will have no pitch, 
while if the outcrops of the dike and quartz-slate form a westward-facing 
acute angle the deposit will have a pitch toward the west. 
Now, with few exceptions, the ore-bodies of the entire Penokee dis¬ 
trict occur at the apices of these troughs (PI. VII, figs. 2 and 3), having 
roughly a triangular section or a V shape the lower part of which is re¬ 
latively heavy and pitching with the altered underlying diabase dikes, 
usually called by the miners “ soapstone.” The boundaries of the ore 
formation are usually sharp along the dike-rocks and the quartz-slate, but 
vary upward often by imperceptible stages into the ferruginous rocks of 
the iron-bearing formation. It follows from the above that each ore-deposit 
may be traced to the surface in one direction,and in the other direction will 
pass deeper and deeper. It is not uncommon for one dike some distance 
below another dike to also carry an ore-body. In this case a shaft will 
pass through its first ore-body, its basement dike and a greater or lesser 
thickness of lean ferruginous material, when another ore-body, resting 
upon another dike will be found (PL VII, fig. 3). This latter body may 
have been previously discovered at or near the surface at some point 
east or west of where the uppermost body reaches the surface and there¬ 
fore ceases. In other cases the dikes may be so close together as to have 
the entire space between them filled with ore. In still other cases more 
than two dikes bearing ore-bodies have been found in vertical section. 
Summing up, the Penokee ore-deposits are then roughly triangular in 
cross section. They usually pitch to the east. They rest upon impervi¬ 
ous formations below, and generally grade upward into a porous ferrugin¬ 
ous chert or slate of the iron formation. 
The Lower Marquette Ores .— In the Lower Marquette* series the ores 
have somewhat greater variety of occurrence. The deposits, instead of 
* Van Hise, C. R. The Iron Ores of the Marquette district of Michigan. Am. Jour. Sci., 
3d series, vol. 43, 1892, pp. 116-132. 
