182 F. W. SARDESON GALENA SERIES 



tween limestone and shale strata as we see them in the field are of such 

 origin. In other words, where different kinds of deposit, such as cal- 

 careous and non-calcareous clay, were made originally, exfiltration of the 

 former ma}^ have left both alike, or infiltration of the latter made a sim- 

 ilar likeness, or even both such changes obtaining may have reversed the 

 original difference. 



While I have observed the variations of differences of rock in the two 

 formations under consideration over the greater part of their extent, I 

 am not prepared to make an exposition of petrographic changes which 

 have taken place locally and successively in them. My observations have 

 been directed chiefly along the line of the occurrence of fossils, their 

 calcification, dolomitization, silicification, pyritization ; of the occurrence 

 of galenite and sphalerite in corals and shells; and of the exfiltration, 

 compacting, and infiltration of fossils and fossiliferous strata. I am 

 prepared only to express an opinion, on this partial evidence, that the 

 diversity of rock type has been increased as a rule ; that the original con- 

 dition of these formations was more uniform than their present condi- 

 tion. I may add that the internal casts of shells in the clay beds are 

 generally limestone still; also, as I have observed, the fossil shell of Lin- 

 gula ioivensis, Owen, proves local compacting of strata. The specimens 

 of that species may occur in any stratum of the series. They are gener- 

 ally in vertical position. They show a shortening from vertical compres- 

 sion of the matrix of from one- tenth to nine-tenths of their original 

 length. 



Eegarding paleontologic evidence, I have already aided in making 

 known facts tending to prove that there was general uniformity of faunal 

 conditions over the entire area from Green Bay, Wisconsin, to Dubuque, 

 Iowa, and from Beloit, Wisconsin, to Minneapolis, Minnesota. Under 

 direction of Professor C. W. Hall, I traveled over the entire area under 

 consideration and published the results as to the faunal and strati- 

 graphic succession in Minnesota* and gave some comparison of the same, 

 with the stratigraphic succession as seen in Wisconsin. Nine or ten dis- 

 tinguishable faunal zones or beds are recognizable in Minnesota, and the 

 same zones are evident over the entire area in Wisconsin, Illinois, and 

 Iowa as far as the formations are accessible or preserved. Later, a more 

 complete correlation was published,! and in this case special attention 



* C. W. Hall and F. W. Sardeson : Paleozoic formations of southeastern Minnesota, 

 Bull. Geol. Soc. Am., vol. 3, 1892, pp. 331-.338 ; P. W. Sardeson : The range and distri- 

 bution of the Lower Silurian fauna of Minnesota, with descriptions of some new species. 

 Bull. Minn. Acad. Nat. Sci., vol. 3, 1892 (excerpt April 6). 



t The Galena and Maquoketa series, American Geologist, vol. xviii, 1896, p. 356, and 

 vol. xix, 1897, p. 21. 



