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Wisconsin Academy of Sciences , Arts and Letters. 
being for the most part at the base of the iron-bearing formation, are 
either within this formation or at its upper part; that is, just below the 
base of the Upper Huronian. The formation above the iron-bearing for¬ 
mation is a quartzite grading downward into a conglomerate, the mater¬ 
ial of which, is largely derived from the subjacent Lower Huronian series. 
The iron-bearing formation is cut by numerous dikes at various inclina¬ 
tions and by great bosses of basic eruptives. These latter sometimes break 
across the bedding of the iron formation, at other times the intrusives 
bow the bedding upward so as to make the formation dip away from the 
greenstone ridges. Not in frequently a ridge of eruptives forms a more or 
less complete semicircle, the iron formation constituting a valley within 
it. 
Now, the ore deposits are found either at the contact plane between 
the Upper and Lower Huronian, or else they rest upon or along the 
eruptives (PI. VII, fig. 4); when fresh called diorite, when altered called 
soapstone. The dikes may be vertical or inclined. One may be alone or 
two or more may be adjacent. The greenstone bosses may have a uni¬ 
form dip or may be folded so as to form a complete synclinal trough (PI. VII, 
fig. 5). A dike may unite with a larger greenstone mass to form a trough 
(PI. VII, fig. 6). Whether the ore deposit rests upon a wall, a synclinal or 
an irregular trough of eruptive rock, the body is not horizontal, but fol¬ 
lowing the greenstone, pitches at an angle of 20° to 40° or even more. As 
in the Penokee district, the ore-deposits have an abrupt termination at 
the underlying eruptive rocks and grade upward somewhat gradually 
into the chert and jasper. In the case of those ore-deposits which are at 
the contact of the Upper and Lower Huronian, the bodies usually occur 
at places where the ©reformation has been sharply shattered or bent; or 
where an intersecting intrusive serves as an inpervious basement; or 
where both are combined. Such deposits may terminate abruptly along 
a joint or may grade into the chert or jasper (PI. VII, fig 7). 
While the Marquette ore-deposits may rest upon formations of differ¬ 
ent characters, and may vary greatly in shape, in common with those of 
the Penokee district, they lie for the most part upon impervious forma¬ 
tions in pitching troughs, and grade above into the broken aud porous 
material of the ore formation; or, if at the contact horizon, into the re¬ 
composed ore formation of the Upper Huronian. 
Ores at base of Upper Marquette series .— The contact deposits of the 
Lower Huronian may continue upward into the Upper Huronian, a 
single ore-body belonging in part to the Lower Huronian and in part to 
the Upper Huronian, when the two series, although unconformable, are 
welded together by subsequent infiltrations, as a consequence of which 
the non-fragmental iron formation of the Lower Huronian appears to 
grade up into the mechanical sediments of the Upper Huronian. In 
other places the ore-deposits near the contact plane may lie wholly 
within the Lower Huronian or wholly within the Upper Huronian. In 
