LITHOLOGIC CHAEACTEK 181 



In regard to their lithologic characters, limestone dominates in both the 

 Galena and Trenton, although diversity of rock is found in each, ranging 

 from porous granular dolomite to lithographic limestone, clay-shale, and 

 even fat clay in places No considerable sand constituent and no recog- 

 nizable sandstone stratum occurs in any part, except just at the contact 

 with the Saint Peter sandstone. Intraformational conglomerate* occurs 

 in several zones from Minnesota to southwestern Wisconsin. This con- 

 glomerate appears to be the result of marine corrosion, and possibly ero- 

 sion in part, and tends to prove that intervals of non-deposition of sedi- 

 ment obtained over part if not all of the area between certain periods 

 of deposition. Again, there are carbonaceous bands of compacted 

 fucoids which are in some places — for example, at Faribault, Minne- 

 sota — capable of being ignited and might be called a low-grade coal. 

 These bands occur in diverse zones. Oolitic limonite occurs in places. 



With such lithologic differences that in the region about Beloit, Wis- 

 consin, the so-called Trenton is practically all limestone and the same 

 about Eochester, Minnesota, practically all shale; that the Galena about 

 the Beloit region has the typical granular dolomite or Galena phase, 

 while south of Eochester the Galena phase is very limited, there 

 is a notable contrast in the lithologic evidence as compared to that 

 of the great uniformity of thickness and wide extent of the formations. 

 The lithologic diversity may, in fact, lead a person to doubt that original 

 uniformity existed or that these deposits were made under widely uniform 

 conditions. 



Whether the lithologic differences which we now see in the Galena 

 and Trenton formations were largely original sedimentary differences or 

 not can not be positively asserted at present. The petrographic changes 

 which have taken place successively and in diverse parts of the so-called 

 Trenton limestone and the Galena limestone have not been fully worked 

 out. Discussions of their origin and special consideration of the ores 

 and other minerals of these formations have, indeed, been repeatedly 

 given, I yet much more on the whole remains to be traced out. For ex- 

 ample, it is an observed fact that the originally silicious skeletons of 

 sponges, as they occur at Minneapolis and in neighboring regions, are 

 calcified, while in northern Illinois and southern Wisconsin many orig- 

 inally calcareous shells are silicified. Besides such mineral alteration, 

 both exfiltration and infiltration, especially of calcium carbonate, have 

 taken place locally to some extent. Possibly most of the differences be- 



* F. W. Sardeson : Intraformational conglomerate of the Galena series, American 

 Geologist, vol. xxii, 1898, p. 315. 

 t T. C. Cliamberlin : Geology of Wisconsin, vol. i, 1883, pp. 151, 163, 169. 



