CHAPTER IX.
UPPER SILURIAN AGE.


Subdivisions. The Upper Silurian age may be regarded as em-
bracing two periods: I, the Niagara, and II, the Helderberg, each
of which presented subordinate phases which are classified as epochs.
The epochs of the Niagara period are, (1) that of the advancing sea,
marked by the Oneida conglomerate and Medina sandstone, (2) an
epoch of oscillating transition, characterized by the Clinton shales
and ore beds, (3) a stage of advanced sea in which was formed the
Niagara limestone, and (4), a period of shallow and retiring sea,
embracing a portion of the Salina deposits, as we would classify
them.
The Helderberg period may be regarded as embracing (1) a stage
of advancing and deepening sea, including the later deposits com-
monly referred to the Salina group, (2) an epoch of advanced sea
marked by limestone accumulation - the Lower Helderberg epoch,
and (3) a period of retiring sea, during which the Orisk(ny sand-
stone was in part, at least, deposited. In tabular form the classifi-
cation may be presented thus:
Age.          Periods.          Epochs.
r Oriskany.
lHelderberg. . Lower llelderberg.
1 Salina.
Upper Silurian.               Niagara.
Clinton.
Niagara.     Medina.
i Oneida.
Terrestrial Interval. The terrestrial conditions above sketched
seem to have continued for a considerable period, during which,
obviously, no marine deposits were added to the surface of the State.
The period, so far as Wisconsin was concerned, was not one of
growth, like those which had preceded, but rather one of loss,
because of surface waste.
Although no definite record was left in Wisconsin, yet this was
not a lost interval in geological history. The ocean was elsewhere