H GEOLOGY OF OHIO. 



made by some geologists, is counted doubtful, to say the least, in view of 

 the facts given above. 



3. The Hudson River Group. 



The very important and interesting series now to be described ap- 

 pears in most of the previous reports of the State Geological Survey 

 under another name, viz. — the Cincinnati group. It is unnecessary to 

 review here the long discussions pertaining to the age of this series, or 

 the grounds on which the change in the name by which it is known has 

 been based. Suffice it to say that the change has appeared necessary to 

 many geologists on paleontological grounds, solely. But for those who 

 have not been convinced by the statements in regard to this subject, a 

 sure ground for the change has been found in the recent discoveries in 

 our underground geology, by which it appears that the Hudson River 

 series of New York can be followed almost continuously, and with but 

 little change in its general character, 1roin the eastward into the state of 

 Ohio, and thus to the outcrops of the formation in the southwestern por- 

 tion oi the state." 



The Hudson River group in southwestern Ohio consists of alternat- 

 ing beds of limestone and shale, the latter of which is generally known 

 as blue clay, but which was called in the earliest accounts of our geology, 

 marlite. The proportions of lime and shale vary greatly in different 

 parts of the series. The largest percentage of shale occurs in the two 

 hundred and fifty feet of the series that begin fifty or seventy-five feet 

 above low water in Cincinnati. The entire thickness of the series in 

 this proportion of the state is about seven hundred and fifty feet. 



The division that has been proposed and 1 adopted in the previous 

 reports of the Survey into an upper and a lower series, seems natural and 

 convenient, and accordingly likely to be maintained. The lower is 

 designated in the reports as the Cincinnati division and the upper as the 

 Lebanon division. The Cincinnati division has a thickness oi four hun- 

 dred and twenty-five to four hundred and fifty feet, and the Lebanon 

 division a thickness of about three hundred ieet. The divisions are 

 ' separated on both paleontological and stratigraphical grounds. Both of 

 them abound in exquisitely preserved fossils of Lower Silurian time; and 

 in fact the hills of Cincinnati and the vicinity have become classical 

 ground to the geologists of the world on this account. 



As the series takes cover to the northward and eastward it retains 

 for some distance the characteristics already described; but as it is fol- ' 

 lowed further it becomes less calcareous The limestone courses are 

 thinner and fewer in number, and inasmuch as they resist or delay the 

 drill but little in its descent, the entire series comes to be counted shale. 

 One other fact must be noted in this direction. The shale at certain 

 points, and especially on the western border in the northern portion of 

 the state, as noted in the previous section, grows dark in color, so that 

 the boundary between the Hudson River and the underlying Utica divi- 



