414 A Geological Section at Todd's Fo?% 0. — Foerste. 
of Orthis occidentalism and 0. hifomta are found imbedded in 
tlie clay, but of an unhealthy appearance when compared with 
■similar forms from the strata immediately below. As a rule 
the b!ue clay layer is unfossiliferous, excepting at its very base. 
The blue clay seems to have been the result of erosive action 
and to have been deposited under the influence of litoral cur- 
rents. At any rate at the close of the deposition of the layer 
of blue clay, the bottom of the ocean was no longer flat here, 
but subject to many undulations, caused by depressions in the 
blue clay layer. 
The so-called Medina is found also at Fair Haven, here how- 
ever reduced to two feet in thickness. A large annelid shell 
formerly seen at this locality suggested that the sandy rock was 
synchronous neither with the Ciucinnati nor Clinton deposits, 
being distinct from the species of either group, but it was not 
otherwise determined. In addition to this, the annelid teeth of 
Todd's fork are the only fossils. Too little is known of annelid 
teeth at present to be of value as means of identifying the hor- 
izon. It will be seen therefore that our correlation was based 
entirely upon lithological grounds. A similar layer in Indiana 
has been identified with the Clinton group, a nomenclature 
which we are not inclined to follow, seeing that the true Clinton 
rocks are also found there in many places. 
The Clinton group of the state usually reposes directly upon 
the blue clay layer. The Medina is only occasionally found. 
Owing to the irregular surface of the blue clay, the Clinton 
rocks, though of fairly uniform thickness, often appear as ir- 
regular, often isolated lenses, with thinned edges, these lenses 
forming the lower parts of the series, filling the hollows of the 
blue clay, the rest having been worn away by the denuding 
action of glacial and other phenomena. Owing to the irregu- 
larity of the blue clay, and the consequent irregularity of all 
superposed strata, thicknesses of the Clinton, not measured, but 
calculated by altitude of base in one locality and summit at 
some neighboring exposure, have probably given rise to some 
of the high estimates made by Ohio geologists. Fifteen to 
twenty feet will express the hight of all exposures actually 
measured by us, but these belong to the summit of the Cincin- 
nati anticlinal. It is not unusual to find all except the upper 
parts of the Clinton group comparatively barren. This is true 
