440 &. Irving—Primordial and Canadian Rocks of Wisconsin. 
limestone of the region. It appeared to have resisted decom- 
position in consequence of its compactness and . There 
is limonite of great thickness all about it, which ee been made 
y the decomposition of the limestone and i = included carbon- 
ate of iron, or its carbonate of iron an ; for an extensive 
ledge of limestone rises above the great ‘pit ‘or excavation on 
its north side, whose layers are conformable to that of the mas- 
sive carbonate of iron in the bottom of the mine. The material 
between the two must have originally been calcareous: it is 
now gone and there is limonite in its stead.” 
The fact here mentioned tends to prove that the limonite was 
ieebable originally present as carbonate of iron and that the 
outer and possibly less compact portion of the deposit became 
changed. 
It seems to me that in this instance, as in those at myo 
mine and Hellertown, we have a case of alteration of the li 
stone to carbonate of iron Leanne by particle, or, so to term i 
‘a pseudomorph by replacement. 
y objection to the original deposition of the carbonate of 
iron, as such, in the Calciferous epoch is threefold. In the first 
place, it should be uniformly distributed through the limestone 
and magnesia. or these were secreted by the marine animals 
to form their ee &e., which would not be the case with car- 
bonate of iro 
n orien oth I would once more point to the fact that what- 
ever the origin ‘of brown hematites may have been, the cause 
of their pon eee where they are now found, must have been 
the damourite-slat 
Lafayette College, hate Pa., Feb. 22, 1875. 
Art. XLVIL—Note on some New Points in the Hlementary 
Stratification of the Primordial and Canadian Rocks of South 
niral Wisconsin; by RoLAND IRVING. 
THE order always hitherto accepted for the Lower Silurian 
strata of Wisconsin has been as follows, beginning below: 
I. The Potsdam Sandstone—500-700 feet in thickness; in its 
upper portions somewhat dolomitic; gta in localities 
near the a nein! ds and St. Croix Rivers. 
estan Limestone 200-250 feet in thick- 
ness; oily a pure ety and almost without fossils. 
