538 F. T. THWAITES 



states that the distribution of these members is erratic and that 

 those below the Brainard shales are missing in some counties. 

 There also appears to be considerable irregularity at the top of the 

 Maquoketa. Ulrich' divided the Richmond of Iowa into several 

 formations : ascending (a) Dubuque dolomite, {b) Wykoff limestone, 

 (c) Clermont shale, {d) Maquoketa shale, (e) Fort Atkinson lime- 

 stone, (/) Brainard shale, and {g) unnamed dolomite. In later 

 unpublished work the succession is given as (ascending) Dubuque 

 dolomite, Elgin shale. Fort Atkinson limestone, and Brainard 

 shales. Whatever may be the final disposition of these divisions 

 as members or as separate formations, it is clear that the irregularity 

 of sequence in the Richmond group is due to the disappearance of 

 certain lithologic units, both at the base and the top of the group. 

 Most well records in northern Illinois show only blue calcareous 

 shale; some disclose limestone, or dolomite layers, at either the 

 top or the bottom of the group. 



MoHAWKiAN Series 



TRENTON AND BLACK RIVER GROUPS 



Distribution. — The Trenton group of Wisconsin is called the 

 Galena formation, and the underlying Black River group was 

 incorrectly termed Trenton in older literature. The revision of the 

 formation names within these groups has not yet been entirely 

 completed for Wisconsin, and since the two groups constitute a 

 lithologic unit of limestones and dolomites a detailed discussion of 

 this subject would be of little value in this paper. 



The Trenton and Black River groups (Galena-Trenton of older 

 reports) cap a broad cuesta in eastern and southern Wisconsin, 

 whose escarpment is well marked from Oconto southwest to Green 

 Lake, and from Cross Plains west along the north side of Military 

 Ridge to Mississippi River. The groups constitute the surface 

 rock of most of the southern Driftless Area and of much of north- 

 western Illinois. 



Character. — In southwestern Wisconsin and northwestern Illinois 

 the Black River group consists in ascending order of the Platteville 

 calcitic limestone and the Decorah shale and shaly limestone. 



' E. O. Ulrich, "Revision of the Paleozoic Systems," Geol. Soc. Amer. Bull., Vol. 

 XXII (i9ii),pp. 281-680. 



