MISSISSIPPIAN. 421 



The formations are deeply covered by drift and are known chiefly by drill 

 records. Lane comments on the sequence according to the sections drilled in many 

 localities. 



The base of the Carboniferous in Michigan is usually drawn in practice at the 

 top of the Antrim shale, between it and the overlying Berea sandstone, although it 

 is theoretically drawn in the Antrim shale itself, so as to separate from it a portion 

 equivalent to the Bedford shale of Ohio, regarded as the lowest Carboniferous of 

 that State. Ulrich would draw the line below the Antrim, removing that formation 

 entirely from the Devonian to the Carboniferous. Where the Berea sandstone is 

 not present the Antrim shale is succeeded by the Coldwater shale (Carboniferous) 

 and a practical distinction is not possible. 



With reference to the line between Devonian and Carboniferous, G. H. Girty 

 has prepared the following discussion " for this paper : 



In northwestern Pennsylvania I have discriminated a group of rocks between the Berea 

 ("Corry") sandstone and the top of the typical Chemung under the name "Bradfordian." In 

 Ohio I am tentatively assigning to this group the Bedford shale, the Cleveland shale, and the 

 Chagrin ("Erie") shale, or at least the upper portion of the Chagrin shale, from which our 

 fossil faunas are obtained. Because the "Bradfordian" faunas were very different from the 

 Waverly faunas (inclusive of the Berea sandstone but exclusive of the Bedford shale), I orig- 

 inally assigned the "Bradfordian" without hesitation to the Devonian. After comparing the 

 "Bradfordian" faunas with the typical Chemung, however, I find that there exists a difference, 

 almost equally marked, and feel somewhat doubtful whether the "Bradfordian" would not 

 better be placed in the Carboniferous. 



In northern Ohio the most convenient horizon at which to draw the base of the Carboniferous 

 'would doubtless be immediately below the Berea sandstone. If the Bedford shale is included 

 in the Carboniferous, so must also be the Cleveland. A more or less conspicuous change of 

 fauna occurs in passing from the Chagrin into the Bedford. 



If the Berea sandstone is taken as the earliest formation of the Carboniferous in Ohio, then 

 in Michigan the Devonian would naturally close with the top of the Antrim. If the Bedford 

 and the Cleveland of Ohio are included in the Carboniferous, then probably a corresponding 

 portion at the top of the Antrim would have to be taken out of the Devonian. If the entire 

 "Bradfordian" is assigned to the Carboniferous (on the assumption that that term includes in 

 Ohio the whole of the Chagrin shale), then more or possibly all of the Antrim would have to be 

 placed in the Carboniferous. 



Regarding the Berea sandstone, Lane ^^^^ states : 



This is an Ohio formation and has never been seen at the surface in Michigan, but may be 

 traced very well along the flanks of the Cincinnati anticlinal, from near Adrian north. Westward 

 it seems soon to disappear and to be spotty in occurrence. Eastward it is persistent in Ohio, 

 but thin, about 50 feet or less. 



It continues to the northeast and north, as determined in wells at several points 



cited by Lane. 



Its thickness is usually about 100 feet and, as is well shown in Huron County, it thickens 

 gradually from about 40 feet until it is thickest near its western margin (over 200 feet), where 

 it disappears suddenly * * *. It is generally fine grained, micaceous, a grindstone, and 

 overlain by a black shale (the Berea or Sunbury shale). 



The Coldwater shale of Michigan directly overhes the Berea sandstone where 

 that formation is present; elsewhere it succeeds the Antrim shale. The base of the 



o Notes, December, 1910. 



