Devonian Shales of Northern Ohio. 201 



Unfortunately, however, the convenience of the geologist seems 

 not to have been a factor in the agencies of sedimentation and 

 we find an arrangement of unlike and intergrading sediments 

 not easy to describe accurately and consistently in any scheme 

 of taxonomic nomenclature. East of Cleveland the Ohio group 

 includes three formations (see chart of sections, fig. 2), a black 

 shale Avith interbedded gray shale at the base, followed by 

 gray arenacous shale with much interbedded sandstone in the 

 upper part, which is terminated above by a second black shale. 

 In going westward both the thickness of the Chagrin and the 

 amount of sandstone in it diminish. In the Huron River 

 section the moderately coarse sandstones of the Chagrin have 

 disappeared altogetber. While bands of blue argillaceous shale 

 interbedded with black shale are conspicuous in the middle 

 portion of the Huron River section, it appears impossible to 

 distinguish the exact limits in this section of the portion 

 equivalent to the Chagrin of the more easterly sections. 

 Although the Chagrin epoch of sedimentation is believed to be 

 as fully represented there as in the Cleveland section, it appears 

 better from the taxonomic viewpoint not to attempt to apply 

 the term Chagrin to any part of .the Huron River section but 

 to assign the whole of it to the two divisions which are pre- 

 eminently black shales as proposed on an earlier page. The 

 middle portion of the Ohio shale group as exposed along the 

 Huron River is probably one-third or one-fourth blue shale. 

 A representative example of this part of the shale is shown in 

 the following section of the exposures near the bridge 3 miles 

 southwest of Milan : 



Feet 

 Fissile black shale 12 



Blue clay shale and some interbedded fissile black shale 20 

 Black shale 3 



Interbedded blue gray clay shale and fissile black shale 4 

 Black shale with numerous spherical concretions 10 



The two elements, blue and black shales, of such sections as 

 this are not persistent for considerable distances if we may 

 judge from the high variability of their thicknesses in the 

 following section, which represents a horizonal exposure of 

 about 250 yards immediately above the bridge at Milan, Ohio : 



Feet Inches 

 Blue clay shale with 1-2 bands of black shale in 

 middle. Large irregular-shaped concretions 



abundant in thicker part.. _ 1-4 



Fissile black shale with numerous large spherical 



concretions _. 9 



Blue clay shale 6-18 



Fissile black shale .. 4-6 



