Devonian Era in Ohio Basin. — Clay pole. 337 
occasional minglings of the faunas took place on the neutral 
territory between their two special places of permanent abode, 
and further still, that conditions allowing of these interming- 
lings endured throughout the time required for the deposition 
of thirty or forty feet of nearly pure limestone in Ohio, or dur- 
ing all the upper part of the Black shale and probably a con- 
siderable portion of the succeeding Hamilton in the east. These' 
faunas, then, were not successive, as often supposed, but real- 
ly contemporary, and must be distinguished, not by their hori- 
zon, but by the sediment in which they are found, the latter br- 
ing apparently the determinant condition. 
Evidently a good and characteristic Corniferous fauna 
lived on in Ohio through the whole Marcellus period and until 
the Hamilton was far advanced, if not nearly complete, in 
Pennsylvania and New York, thus bringing the Lower Devon- 
ian (Corniferous, Marcellus, Hamilton) species up almost to 
the base of the Upper Devonian (Tully?, Chemung. Catskill). 
It did not finally fail until the permanent change in the nature 
of the sediment, from argillaceous to arenaceous, rendered 
longer tenure of the ground impossible, save to those forms 
which could adjust themselves to the new conditions. Further 
west, where a similar change is unknown, it is not possible to 
determine how long the Corniferous fauna presisted before it 
could be said to have entirely changed its facirs to that of the 
Hamilton. The 50 to 100 feet of calcareous shale, of which 
the whole Devonian series there consists, is marked by changes 
so slow and minute that all sub-division has proved imprac- 
ticable. When longer study and larger collections more care- 
fully located by the geologists of the west shall have given 
us the whole life-history of the period, we shall probably be en- 
abled to trace the evolution along the lines of species and to 
recognize the slow changes of form and fauna from horizon 
to horizon. 
There is no difficulty in ascertaining where each of. the two 
faunas held its headquarters while thus mutually and altern- 
ately invading the middle ground. The Corniferous fauna 
evidently maintained itself in the open sea to westward, where 
clear water and calcareous deposits continued uninterrupted. 
while the Marcellus and Hamilton faunas tenanted the shores 
and the shallows, whose wash kept the water clouded and the 
bottom muddy. An inquiry into the origin of the Marcellus and 
