GEOLOGICAL SCALE AND STRUCTURE. 37 



found in the succeeding chapter, and the reader is, therefore, refered to it 

 for details as to the structure of the group. Its thickness can be counted 

 about two hundred and fiftv feet. 



15. The Lower Barren Measures. 



For the same reasons that have been given under the preceding 

 head, no detail will here be entered into as to the composition of this 

 formation, further than to say that it comprises from three hundred to 

 five hundred feet of strata which have been of little economic interest. 

 Though included in the Coal Measures they are well described by the title 

 which the} r bear, the Barren Coal Measures, the coal seams that are 

 included in them being thin and wanting in persistency. 



16. The Upper Coal Measures. 



This division is also to be treated in a succeeding chapter. Its thick- 

 ness may be taken as two hundred and fifty lo three hundred feet, but its 

 upper boundary is lacking in definiteness. 



17. The Upper Barren Measures. 



The upper Barren Measures complete the geological column of Ohio 

 so far as its bedded rocks are concerned. In other words, the latest 

 formed rocks of our scale are included in this division. They are to be 

 looked for in the southeastern portion of the state. Belmont and Monroe 

 counties contain the most of these exposures. While they are included in 

 the Coal Measures by the name above assigned to them, a question has been 

 raised by some geologists as to whether they should not be referred to the 

 succeeding age, viz., the Permian. The argument for this is based upon 

 the character of the fossil plants that they contain. The formation is but 

 poorly shown in Ohio, and no ground appears upon which an easily 

 recognized classification could be established. 



18. The Glacial Drift. 



Over the various bedded rocks of at least two-thirds of Ohio are 

 spread in varying thickness the deposits of the Drift. The boundary 

 which marks the farthest advance of the Drift formation, enters Ohio 

 in Columbiana county, passes eastward through Stark into Wayne county t 

 where it bends sharply to the southward, following in that direction as 

 far as Holmes county. From this point its general direction is south- 

 westerly. It leaves the state in Brown county, crossing the Ohio River 

 into Kentucky, a small part of which is included in the Drift formation. 

 A number of prominent points can be noted by which the boundary can 



