530 F. T. THWAITES 



furnished free to both owners and drillers. This policy, combined 

 with visits to drillers in the field, has resulted in a constantly- 

 increasing percentage of good records of new wells. 



The present paper is not intended to treat the subject of the 

 paleontology nor to outline the history of the separation of the 

 formations. For these subjects reference may be made to the works 

 of Owen, Hall and Whitney, Wooster, Chamberlin, Strong, Irving, 

 Hall and Sardeson, Sardeson, Ulrich, Walcott, Savage, Savage and 

 Ross, Twenhofel and Thwaites, Alden, Bassler, and others.^ 



' D. D. Owen, Report of a Geological Survey of Wisconsin, Iowa, and Minnesota, 

 Philadelphia, 1852. James Hall and J. D. Whitney, Report of the Geological Survey of 

 the State of Wisconsin, Vol. I, 1862. G. A. Shufeldt, "On an Oil Well Boring at Chi- 

 cago," Amer. Jour. Sci., Vol. XL (1865), pp. 388-89. H. M. Bannister, "Cook 

 County," Illinois Geol. Survey, Vol. Ill (1868), pp. 239-56; Economical Geol. of Illinois 

 (1882), pp. 180-201; "Dekalb, Lake, and Other Counties," Illinois Geol. Survey, 

 Vol. IV (1870), pp. 111-89; Economical Geol. of Illinois, Vol. II (1882), pp. 361-449. 

 T. S. Hunt, "On the OU-Bearing Limestone of Chicago," Canadian Naturalist, Vol. VI 

 (1872), pp. 54-59. James Shaw, "Geology of Northwestern Illinois," Illinois Geol. 

 Survey, Vol. V (1873), PP- 1-216; Economical Geol. of Illinois, Vol. Ill (1882), pp. 

 1-226. Moses Strong, "Geology and Topography of the Lead Region," Geology of 

 Wisconsin, Vol. II (1878), pp. 668-88. R. D. Irving, " Geology of Central Wisconsin," 

 ibid., pp. 525-607. L. C. Wooster, "Geology of the Lower St. Croix District," ibid., 

 Vol. IV (1882), pp. 104-28. Moses Strong, "Geology of the Mississippi Region 

 North of the Wisconsin River," ibid., pp. 38-91. R. P. Whitfield, "Paleontology," 

 ibid., pp. 163-349, plates. T. C. Chamberlin, Geology of Wisconsin, Vol. I (1883), 

 pp. 119-212. A. S. Tiffany, "Record of Deep WeU at Dixon, Illinois," Amer. Geol., 

 Vol. V (1890), p. 124. C. W. Hall and F. W. Sardeson, "Paleozoic Formations of 

 Southeastern Minnesota," Geol. Soc. Amer. Bull., Vol. Ill (1892), pp. 331-68. J. A. 

 Udden, "A Geological Section Across the Northern Part of Illinois," Illinois Board of 

 World's Fair Commissioners Report, 1895, pp. 117-51. C. W. HaU and F. W. Sarde- 

 son, "The Magnesian Series of the Northwestern States," Geol. Soc. Amer. Bull., Vol. 

 VI (1895), pp. 167-98. F. W. Sardeson, "The Galena and Maquoketa Series," Amer. 

 Geol., Vol. XVIII (1896), pp. 356-68; Vol. XIX (1897), pp. 21-35, Qi-i", 180-90; 

 "Nomenclature of the Galena and Maquoketa Series," ibid., Vol. XIX (1897), pp. 

 330-36. Stuart Weller, "A Peculiar Devonian Deposit in Northeastern Illinois," 

 Jour. Geol., Vol. VII (1899), pp. 483-88. F. W. Sardeson, "The Lower Silurian 

 Formations of Wisconsin and Minnesota Compared," Minnesota Acad. Nat. Sci. 

 Bull., Vol. Ill (1901), pp. 319-26. C. W. Rolfe, "The Geology of Illinois as Related 

 to Its Water Supply," Illinois Univ. Chem. Survey of the Waters of Illinois, 1903, pp. 

 41-56. H. F. Bain, "Zinc and Lead Deposits of the Upper Mississippi VaUey," U.S. 

 Geol. Survey Bull. 2(14, 1906; Wisconsin Geol. and Nat. Hist. Survey Bull, ig, 1907. 

 U. S. Grant, "Lead and Zinc Deposits of Southwestern Wisconsin," Wisconsin Geol. 

 and Nat. Hist. Survey Bull, g, 1903; Bull. 14, 1906. C. P. Berkey, " Paleogeography 

 of St. Peter Thne," Geol. Soc. Amer. Bull., Vol. XVII (1906), pp. 229-50. F. W. 

 Sardeson, "Galena Series," ibid.. Vol. XVIII (1907), pp. 179-94. Stuart Weller, 



