T. F. Savage — Ordovician and Silurian Formations. 519 



chert. This chert-bearing phase is succeeded by thicker layers 

 of pink or reddish, mottled, subcrystalline limestone. 



In the upper part the cherty limestone contains Favosites 

 favosus, Haly sites cate?mlatus, Atrypa marginalise Ortliis 

 flabellites, Plectambonites transversalis, StricMandinia triple- 

 siana, Triplecia ortoni var. and Illcenns cf. daytonensis. The 

 strata are well exposed along" Sexton creek, one and one-half 

 miles north of Gale. They appear in the river bluff 

 between Gale and McClure ; and they may also be seen in the 

 bank of the river two and one-half miles south of Thebes. The 

 maximum thickness of the formation is about seventy feet. 

 The species of fossils listed above indicate that the Sexton 

 Creek limestone represents the westward extension of the 

 Clinton strata occurring in Indiana and Ohio. 



The Post- Sexton Creek Unconformity. 



After the deposition of the Sexton Creek beds, land condi- 

 tions prevailed over this area for the greater portion of the 

 time during which the Niagara limestones in the northern por- 

 tion of the state were laid down. The strata that occur next 

 above the Sexton Creek beds, in this region, represent the 

 Helderbergian series of the Devonian. 



Oscillations of level. — Frequent strand-line movements are 

 clearly recorded in the Coal Measure deposits of Illinois, and 

 elsewhere, where a number of coal seams occur in vertical suc- 

 cession, and separated one from another by marine beds of 

 shale or limestone. The numerous oscillations that are shown 

 to have occurred in southwest Illinois, during the late Ordovi- 

 cian and early Silurian times, would indicate that frequent 

 movements were not peculiar to the Pennsylvanian Period. 

 It seems probable that oscillations of level may not have been 

 uncommon throughout the Paleozoic era. The scarcity of such 

 records may be largely due to the fact that the deposits made 

 in shallow water, near shore, are not present over large areas ; 

 and that such deposits would be most t likely to be removed 

 during subsequent periods of erosion. 



University of Illinois, Urbana, 111. 



