. Ulrich—Paleozoic Systems in Wisconsin. 
97 
withdrawn at the close of the last of the Silurian deposits and 
that resubmergence of parts of the continent at the beginning of 
Devonian time left a bounding break of undetermined time value 
between the youngest of the Silurian deposits and the oldest of the 
succeeding Devonian sediments. The plane of separation between 
these is sharp and slightly undulating even when cementation has 
taken place so as to make it appear as passing through a single 
layer of rock—in such cases usually a limestone. In many places, 
among them east Wisconsin, the Devonian overlap was delayed to 
Middle Devonian time. Beds of the latter age there rest on some 
late or middle Silurian formation. At Louisville, Ky., the adjacent 
rocks of both systems are of limestone and the line of contact be¬ 
tween the two is sometimes so obscure that it is scarcely determin¬ 
able in a weathered slab two inches in thickness. However, the 
lower side of this slab contains Niagaran species of corals, the upper 
side Middle Devonian species. And yet nearly 2,000 feet of mainly 
limestone deposits were laid down in the northern Appalachian 
Valley during the time of the hiatus indicated by the fossils in the 
upper inch of this slab and those imbedded in its lower inch. 
In Wisconsin the Middle Devonian Milwaukee formation follows 
the Waubakee dolomite, which is regarded as of late Silurian age. 
No outcrop showing the contact of the two formations in which the 
lower bed was positively identified as belonging to the Waubakee 
has been observed. But it is quite probable that this contact is 
exposed in a small quarry three-quarters of a mile northwest of 
Port Washington. Whatever age may finally be assigned to the 
lower formation at this place it is certain that the contact between 
it and the uneven, clearly overlapping base of the overlying Mil¬ 
waukee dolomite is unconformable, with at least 1,000 feet of beds 
missing that were laid down elsewhere. 
The Siluro-Ordovician break .—That the contact of the Silurian 
and Ordovician formations in Wisconsin and adjoining States 
marks a stratigraphic hiatus of considerable magnitude and intro¬ 
duced great changes in the geography of the time is also shown 
mainly by absence of certain elsewhere important late Ordovician 
formations and by early Silurian sea transgressions of extraordi¬ 
nary extent and correspondingly great overlaps of deposits left by 
them. These earliest Silurian transgressions occurred during the 
Richmond or Lower Medina stage. In the Upper Mississippi Val¬ 
ley the first of the Richmond formations lies on the Galena dolomite 
