18'6 F. W. SARDESON GALENA SERIES 



tion is nowhere entirely limestone and in places is more than half shale, 

 the designation of it as limestone was changed by me to that of formation. 

 It should, of course, be expected that the term "Trenton," having been 

 long employed, would be slow in dropping out of use, and also that con- 

 fusion from conflicting usage of terminology might lead to the use simply 

 of Galena series or group alone for most purposes of discussion, the terms 

 lower, middle, and upper Galena falling into use where exact correlation 

 is not made or is not practicable. For example. Galena lower limestone. 

 Galena lower shales, Galena middle limestone appear to be sufficiently 

 scientific terms for well logs and profile sections constructed from the 

 same. For the solution of other special problems, such as the origin of 

 lead and zinc — although the stratigraphic and paleontologic evidence at 

 the strongly mineralized spots where the mines are developed is gener- 

 ally obscured — too exact and detailed correlation of the entire area can 

 not be obtained and formational names would be required. 



The Platteville Limestone 



H. F. Bain, describing the Galena limestone in northwestern Illinois, 

 refers to Receptaculites ouieni as occurring 40 feet above the bottom of 

 the formation.* This reference is further explained on the preceding 

 page of his report when he says : 



"The Trenton In this area is about 40 feet thiclv. To the north a greater 

 thickness has been assigned to it, but this is due not to a thiclsening of the 

 strata, but to the reference of a portion of the overlying beds to this forma- 

 tion." 



His Galena is thus evidently extended downward 40 feet, leaving only 

 40 feet, or just half, of the Trenton or Beloit formation in his Trenton 

 limestone. Bain's language and action seemed to imply that others have 

 been in error when they included more than 40 feet of strata in the 

 Trenton. 



This description of the "Trenton" by Bain is of special significance 

 when taken in connection with a later report by him on the "Lead and 

 zinc deposits of northwestern Illinois, f Here appears what might be 

 meant by his expression "farther north." On page 21 of the Bulletin 

 cited he says : 



"North of the mining district the lower portion of the Galena is not dolomi- 

 tized and is lithologically similar to the underlying Platteville." 



* U. S. Geological Survey, Bull. no. 225, 1903, p. 205. 



t H. P. Bain : U. S. Geological Survey Bull., no. 246, 1905, pp. 18-21. 



