UNCONFORMITY AT BASE OF ONONDAGA LIMESTONE 319 
in which discordance of dip exists between the unconformable beds. 
This is limited to the Cincinnati geanticlinal region of northern 
Indiana, From this area in the Wabash valley of the maximum 
length of the stratigraphic break at the base of the Jeffersonville 
limestone, it decreases more or less regularly eastward by the 
appearance between the Silurian limestone of Guelph age and the 
Devonian limestone of Onondaga age of the successively younger 
formations known respectively as the Salina formation, the Coble- 
skill limestone, the Roundout limestone, the Manlius limestone, 
and the Oriskany sandstone. With the exception of the last- 
named formation the western border of each of these lies well to 
the eastward of the preceding, thus suggesting a shore line retreat- 
ing eastward during late Silurian and early Devonian time. The 
distinctive feature of the Oriskany in the New York—Ontario region 
is its discontinuous character. The western extension to Ontario 
of Oriskany sediments in irregular patches representing erosion 
remnants indicates that the easterly retreat of the shore line was 
reversed about the close of Helderberg time and a submergence of 
the land occurred in Oriskany time which carried its sediments 
westward as far as Ontario. Apparently the easterly retreat of 
the shore line which was reversed with the beginning of Oriskany 
sedimentation never passed to the eastward of the Hudson River 
valley in southeastern New York, for no evidence has been found 
in that region of a stratigraphic break at the base of either the 
Onondaga or the Oriskany. The disconformity which marks the 
base of the Onondaga is closely related in space and time with 
that below the Oriskany. 
The erosion of the Oriskany has given it a discontinuous: dis- 
tribution over an area extending from the Hudson River to the 
middle of southern Ontario. This has resulted in merging the 
post-Oriskany and pre-Oriskany disconformities into a single 
disconformity horizon separating the Onondaga and Silurian 
strata where the Oriskany has been removed. 
