4i8 A. C. LANE 



therefore, may indicate the uplift generally taken to mark the close 

 of the Devonian and beginning of Carboniferous. The upper part 

 of the Antrim is blue rather than black and frequently there are beds 

 of sand and grit. There is generally at least loo feet of the bottom 

 black shale, but since the blue and black shales alternate at times, 

 records may or may not show the Cleveland Chagrin and Huron 

 as one solid black shale, or may overlook the Cleveland and count 

 everything down to the Huron as blue shale. 



MISSISSIPPIAN 



28. Berea Grit (or sandstone'). 273 feet. — This is an Ohio forma- 

 tion 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 may well have once been con- 

 tinuous with its Ohio outcrops. Continuous past Ann Arbor and 

 Pontiac and Birmingham, Romeo, Utica, and Berville to the south- 

 east corner of Sanilac County it may be followed around the Thumb 

 in wells put down to tap its brine to Bay City. From Bay. City it 

 may be traced north to Harrisville, near which it comes to the under 

 surface of the drift. It thickens gradually from about 40 feet until 

 it is thickest near its western margin (over 300 feet). Then it dis- 

 appears suddenly. The brine is exceptionally salt, even near the 

 surface, and unusually free from sulphates. 



The sandstone is generally fine grained, micaceous, and overlain 

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



Now, if we take the Alma, Bay City, and Caseville wells and figure 

 from the top of the Marshall as a datum we shall have the form of a 

 deposit formed along a shore facing east and running nearly north 

 and south through the center of Michigan (compare Schuchert's 

 PI. 78). It is also true that it is coarser where it is thicker and not so 

 pure — more of a fine-grained grindstone to the east. 



This points to a marked line-between Carboniferous and Devonian. 

 We have something like the same question that arises as to the red 

 Richmond shales mentioned above, but there is a marked difference 

 in that above the Berea Grit we do not pass into limestone like the 



I Compare Oneonta Chemung and Catskill. 



