T. E. Savage — Stratigraphy of Southwestern Illinois. 431 



Art. XL VI. — On the Lovier Paleozoic Stratigraphy of 

 Southwestern Illinois; by T. E. Savage, University of 

 Illinois. 

 [ Contributions from the Paleontological Laboratory of Yale University.] 



The following paper is a preliminary statement concerning 

 the pre-Mississippian formations that occur in the southwest 

 portion of Illinois. A monograph on the stratigraphy and 

 paleontology of these terranes in the above mentioned area is 

 being prepared by the writer for presentation as a thesis for 

 the degree of Doctor of Philosophy at Yale University. 



The field work on which the report is based was done dur- 

 ing the summer of 1907, under the auspices of the Illinois 

 Geological Survey ; while the paleontological study was made 

 at the Peabody Museum under the direction of Professor 

 Charles Schuchert. The writer wishes at this time to acknowl- 

 edge his indebtedness to the Director of the Illinois Geolog- 

 ical Survey for assigning him to this very interesting piece of 

 work, and to Professor Schuchert for his invaluable assistance 

 in the study and interpretation of the faunas and the data that 

 were collected. 



The pre-Mississippian beds in this portion of the state under- 

 lie the surficial materials over an area 150 square miles in 

 extent. They appear in the southwest corner of Jackson 

 county, at the Back Bone and Bake Oven ridge ; at the south 

 end of Walker ridge ; and at Bald Rock, and southward on the 

 east side of the Big Muddy river. In Union and Alexander 

 counties they extend from the flood plain of the Mississippi 

 river eastward to the general line passing within about one 

 mile west of the towns of Alto Pass, Mountain Glen, Jonesboro, 

 and Mill Creek to a point nearly two and one-half miles south- 

 east of Elco, whence the line separating the Devonian from 

 ihe younger formations trends toward the southwest past the 

 Diswood postofnce, to near the middle of section 28, Township 

 15 South, Range 2 West. Eastward they are bordered by Mis- 

 sissippian beds, while along the southern edge sands and clays 

 of Tertiary age lie upon the flanks of these older formations. 

 Occasional patches of Tertiary gravels occur within the region 

 under discussion. 



This small area is exceedingly interesting geologically be- 

 cause of the fact that some of the formations liere represented 

 do not appear further north anywhere in the Mississippi valley. 

 The successive beds were deposited in a basin of the Interior or 

 Mississippian sea which, during a great part of the time, was 

 more or less separated from that in which the older strata in 

 other portions of the state were laid down. Owing to its prox- 



