164 Association of American Geologists and Naturalists. 
the limestone of the northwest to be the equivalent of the Wenlock for- 
mation of Marchison in England, some of the Eifel limestones in Ger- 
many, and the Helderberg rocks of New York. 
Besides the rich lead veins which traverse this formation in the north- 
west, it contains a valuable copper ore and vast quantities of carbonate 
of zinc. 
The most remarkable feature in the paleontology of this great west- 
ern limestone, is the number of imbedded fossil corals, amongst which 
the chain corals hold a conspicuous place, especially i in the lowa ex- 
tension of this formation. — 
~ This great limestone formation rests on thin beds of a blue and grey 
renter alternating with marls, often a complete mass of agglutina- 
ted shells. It has yielded to the paleontologist more prolific subjects 
for Soldiers and research than any other group of western rocks, 
especially in the families of Trilobites, Brachiopoda and Encrinites, and 
has enriched our cabinets with numerous specimens of the marine in- 
habitants of our globe atvalmost the earliest period to which animal re- 
mains have been traced. Many of them are identical with those found 
in the lower Silurian rocks of England. 
_ This deposit is thickest near the centre of the Ohio ealeae and al- 
ternates towards the northwest. Though it occupies the surface only 
over a comparatively limited area, yet there is every reason to believe 
that it is coéxtensive with the whole mass of superincumbent rocks. 
The metallic yeins which are so wide in the overlying magnesian 
limestone of Iowa and Wisconsin, thin away on reaching the more yd 
tensible layers of this underlying shell limestone and marl. 
No inferior rocks are visible in the valley of the Ohio, but near the 
Wisconsin River are sections which show the relation of these lowest 
limestones of the Ohio valley with the inferior rocks. There the last 
described limestones are seen resting on a siliceous sandstone, beneath 
which we have again a magnesian limestone, so like the upper lead- 
bearing magnesian limestone as not to be distinguishable from it in hand 
specimens ; and near low water of the Mississippi, at Prairie du Chien, 
another sandstone is visible beneath this lower magnesian limestone of 
the Wisconsin River. No well defined fossils have been found in these, 
the sandstones and lower magnesian limestones of the northwest; so 
that it becomes difficult to pronounce on their equivalencies. Judging 
from the lithological character, absence of fossils, mineral crystalliza- 
tions, geological position, it seems probable that they correspond to the 
formation i in the lead region ‘of Missouri. It is highly probable, too, 
that the lower magnesian limestone of Wisconsin is cotemporaneous 
with ‘the calciferous limerock of the New York geologist, as well as 
with the magnesian limestone that forms the Natural Bridge in Virginia. 
