96 ILLINOIS ACADEMY OF SCIENCE. 



tion of the limestone by erosion to a flat surface, upon which 

 the shale was locally deposited. The convergence of these 

 two lines of deposition seems reasonable proof that the age 

 of the chief fold can be placed as post-Galena and pre-Maquo- 

 keta, or about contemporaneous with the formation of the Cin- 

 cinnati arch. The displacement at the time amounted to about 

 1,000 feet.^ 



CORRELATION OF THE DEVONIAN SYSTEM OF THE 

 ROCK ISLAND REGION. 



W. Elmer Ekblaw. 



This paper is an attempt to correlate the strata of the Devonian 

 system of the northwest part of Illinois with those of the same 

 system in other parts of the State and in adjacent States. It is 

 the result of detailed field work done in that region in the summer 

 of 1910, under the direction of Professor T. E. Savage, and in 

 accordance with a general plan for the study of different forma- 

 tions of the State as formulated by Mr. F. W. DeWolf, director 

 of the State Geological Survey. 



Not much work has been done on the Devonian of this region. 

 A. H. Worthen,^ F. B. Meek,^ James Shaw^ and J. A. Udden* 

 have devoted some attention to it, but no detailed account of its 

 paleontology and stratigraphy has yet been given. 



The Devonian exposures in northwest Illinois are confined to 

 a rather narrow belt in Rock Island and Henry counties, border- 

 ing the bluffs of the Mississippi and Rock Rivers, and extending 

 from a point opposite Montpelier, Iowa, to Green River, tribu- 

 tary to Rock River, and to Campbell's Island on the Mississippi 

 River. 



This narrow belt embraces the margin of the highlands front- 

 ing upon the flood plains of the Rock and Mississippi Rivers, and 

 that portion of the flood plains immediately adjacent to the bluffs 



' Since the presentation of the foregoing paper the well No. 27 at Cedar Point 

 has been completed, so that the section at that place has been extended by the 

 addition of iio feet of Galena-Trenton limestone and 140 feet of St. Peter sand- 

 stone to the top of the Lower Magnesian limestone. The total depth is now 1,749 

 feet and 8 inches. These data have been included in the diagram on Plate II. 



1 Worthen, A. 11., Geol. Survey of Illinois, Vol. I, 1866, pp. 120-122. 



^Worthen, A. H., and Meek, F. B., Geol. Survey Illinois, Vol. II, 1866, p. 423- 

 Idem, ^'ol. in, 1868, pp. 419-449. 



' Worthen, A. H., and Shaw, James, Geol. Survey Illinois, Vol. V, 1873, pp. 217- 

 234. 



♦ Udden, T. A., Rept. Illinois Board of World's Fair Commissioners, 1893, pp. 117- 

 177. Idem. "Jour. Cincinnati Soc, Nat. Hist., Vol. XIX, 1896, pp. 93-94- 



