194 



F. W. SARDESON GALENA SERIES 



The new name Decorah shale is of interest here. Of this he says : 



"The persistent body of shale between the two parts of what has generally 

 been called the Trenton limestone is named Decorah shale, from the city in 

 which it is typically developed. Heretofore it has been recognized as a distinct 

 geological unit under the name 'Green shales,' . . . ."* 



On page 85 of this report, figure 7 represents a photograph of 

 "a very typical exposure" of the Decorah shale, with overlying basal 

 ledges of the Galena limestone at Decorah, Iowa. This exposure in 

 "the Dugway," shown in figure 7, is a familiar one to me, and 

 I can therefore at once concur in designating as Galena the strata seen 

 in the little old quarry at the top of this exposure. Moreover, the 

 "Decorah shale" as here shown is the Upper Blue bed (Pucoid bed) 

 which is the top of the Beloit formation or "Trenton." This being the 

 case, the term Platteville stage, which Calvin has employed in the synop- 

 tical table, cited above, is synonymous with the term Beloit formation, 

 and not with the Platteville as defined by Bain. 



The "undisturbed Decorah shale" which is represented as the base of 

 this shale in figure 6, page 83, he. cit., I do not recognize, but from the 

 general description which is given in the report, it is evident to me that 

 Calvin here includes in the Decorah shale the strata which I had in- 

 cluded in both the Pucoid bed (5) and the Stictopora bed (4). The 

 Stictoporella bed (3), which includes the lowest strata in which Plectam- 

 honites sericea Sowerby is known to occur, he evidently places in the 

 "Platteville limestone." The term Platteville limestone, as usedf here 

 by Professor Calvin, appears therefore to be the exact equivalent of 

 Bain's section of the Platteville at the type locality. This being true, 

 the horizon of the "green shales" in the "Platteville" of Wisconsin is in- 

 cluded in the "Platteville limestone" at Decorah, Iowa, and not in the 

 "Decorah (Green) shales." 



The following table represents the formational divisions which still 

 appear to accord most nearly with historical and geologic conditions, in 

 relation to the Galena stage : 



Series 



Stage 



Formation 



Zones 



Trenton 



Maquoketa 





11-14 



Dubuque formation 



10 



Galena 



Galena formation 



6-9 



Beloit formation 



1-5 



Saint Peter 



Saint Peter sandstone 





*Loc. cit., p. 61. 



tOp. cit., pp. 60, 76, 80-84. 



