REVIEW OF GEOLOGICAL STRUCTURE. 19 
tension of the upper half of the Portage Group—the Portage sandstones 
of New York, and the overlying Chemung Group. These become 
finer and rapidly dimish in thickness westward, and finally run out 
to a feather-edge in the central part of the State. On the Penn- 
sylvania line they have a thickness of at least twelve hundred 
feet. With the commencement of the deposition of these strata a new 
round of physical changes began in this part of the continent. The De- 
vonian Sea had, up to this time, been gradually shallowed and dimin- 
ished until its bottom formed land, which came west to the border of 
Ohio. After a longer or shorter period this land began to sink with an 
influx of water which received the wash from the continent on the east 
and spread it in the form of shales, sandstones, and conglomerates, that 
in their accumulation kept pace with the subsidence, so that North- 
eastern Ohio was maintained in either a shore or an off-shore condition 
until all the Hrie and succeeding Waverly series had been deposited. 
The open and clear water of the new sea that spread these sediments 
reached no further than the central line of the State, there leaving its 
record in the thin edge of the Lower Carboniferous Limestone. Then 
the process of retreat began again and the whole eastern half of 
the continent was slowly raised out of the ocean, with many oscil- 
lations and local subsidences. In this latter period of continental 
elevation and local subsidence, the Conglomerate and Coal Measures 
were formed, and finally the elevation of the Allegheny Mountain 
belt. took place, and this portion of our country was lifted above the 
ocean level, so to remain until the present day. If such was the progress 
of events in geological history—and abundant proof can be furnished for 
each of the steps reported—it is evident that the Erie Group is the record 
of the introduction of a new geological age; and that there are, there- 
fore, reasons for removing it from the Devonian system, where it has 
hitherto been placed and attaching it to the Carboniferous. This change 
of classification is also favored by the character of the fossils of the Krie, 
which are generally different from those of the Hamilton, and resemble 
and probably shade into those of the Carboniferous system. Hence, 
it seems that the geological record would be best interpreted by consider- 
ing the Erie Group as the base of the Carboniferous. 
THE WAVERLY GROUP. 
The phenomena presented by the Waverly Group in different parts of 
| Ohio are so fully described in the volumes of our report already published, 
that no further reference to it would be needed here, but that Professor 
J. P. Lesley, Director of the Geological Survey of Pennsylvania, has ad- 
