1893.) Geology and Paleontology. 939 
It is now definitely known through Irving’s researches that these 
ores, like many of a later age, are derived from carbonate of iron. 
The ores now mined occur in two geological series, the lower Huron- 
ian and the upper Huronian. The lower Marquette series may be 
taken as a type of the first, and the Penokee ores of the second. 
The Penokee ore deposits are roughly triangular in cross-section. 
They usually dip to the east. They rest upon impervious formations 
below, and generally grade upward into a porous ferruginous chert or 
slate of iron formation. The lower Marquette series vary greatly in 
shape, lie for the most part upon impervious formations in pitching 
troughs, and grade above into broken and porous material of the ore 
formation. 
As to the genesis of the ores, the author thinks that all evidence 
goes to show they are concentrations produced by downward percola- 
ting waters. These waters removed a part of the original material of 
the iron-bearing formations at the places where the ore-bodies occur, 
and introduced iron oxide almost simultaneously. This explains the 
forms, positions, and relations of the ore deposits. -They rest upon 
tilted walls or troughs of impervious formations because water has 
here been converged. They occupy the place of the original ore for- 
mation because this is easily penetrated by water, because it was Tich 
in iron carbonate, and because the constituents other than iron oxide 
are readily soluble. 
The interchange of silica and iron oxide is observable. The change 
from the ore bodies to the rocks above is a transition, and along this 
transition zone the silica bands die out by a gradual removal. In the 
iron formation proper the silica is frequently in solid bands alternating 
with bands richer in iron. In passing toward the ore the stratum is 
porous, due to cavities left by the removal of the silica, but before all 
the silica is removed, iron oxide begins to be introduced, and finally the 
solid body of iron ore occupies the place of the siliceous band. 
The iron ore does not appear throughout the Huronian rocks of 
Lake Superior, but only in definite formations which constitute a 
small percentage of the entire Huronian series.— Trans. Wis. Acad. 
Sci. Arts and Letters, Vol. viii. : 
The Geology of Nicaragua.—In an abstract of Notes from a 
Geological Survey in Nicaragua Mr. J. Crawford states that Nicara- 
gua, geologically considered, can be divided from north to south into 
five zones, differing from one another in lithological, mineralogical, 
and structural characters. 
