EEVIEW OF GEOLOGICAL STBUCTURE. 19 



tension of the upper half of the Portage Group — the Portage sandstones 

 of New York, and the overlying Chemung Group. These become 

 iiner and rapidly diminish 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. Thi De- 

 vonian Sea bad, 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 ot 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 Erie 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 Erie, 

 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 WAVEELY GROUP. 



The phenomena presentecj by the Waverly Group in different parts of 

 Ohio are so fully de'scribed 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- 



