374 T. E. SAVAGE ALEXANDRIAN SERIES IN MISSOURI AND ILLINOIS 



Although the basin in which the Sexton Creek limestone of the Mis- 

 sissippi Valley region and the Brassfield limestone of the Ohio region 

 are both thought to have had a southward sea connection, the difference 

 in the faunas of this time in the two areas is interpreted as indicating 

 the presence between them of a land barrier (the enlarged Cincinnati 

 anticline), extending sufficiently far to the south to prevent the easy 

 migration of the organisms into this localized basin, in which there de- 

 veloped a somewhat provincial fauna. 



For this basin connecting southward with the wider sea of the Ten- 

 nessee and Arkansas region, in which the Alexandrian strata in Illinois 

 and Missouri were laid down, the name Illinois basin is here proposed. 

 This basin is thought to have been a northwardly extending embayment 

 and a distinct area of sedimentation during the time of deposition of all 

 of the strata in this region from the beginning of the Thebes sandstone 

 to the end of the Sexton Creek limestone. 



During iilexandrian time the Illinois basin had a widely open typically 

 marine southern connection with the Gulf region, and from the Tennes- 

 see area another bay, known as the Brassfield, extended widely east of the 

 Cincinnati axis, in Ohio and Kentucky. The early Silurian sea gained 

 access to the Illinois and Missouri region earlier, and probably continued 

 later, than in the region east of the Cincinnati axis. Several of the 

 genera and a number of the species of fossils that occur in the Brassfield 

 limestone in Ohio were introduced a little earlier into the Illinois basin, 

 appearing in late Edgewood strata. In the upper part of the Sexton 

 Creek limestone there are a few species that do not appear east of the 

 Cincinnati axis until post-Brassfield time. 



In the fauna of the Edgewood limestone there can be recognized an 

 element that was derived from that of the Girardeau limestone, and in 

 the Sexton Creek fauna there appears a still larger element that was 

 present in the earlier Edgewood limestone. The Alexandrian series in 

 the Illinois basin is thus shown to consist of a number of closely related 

 early Silurian formations which record a succession of oscillatory north- 

 ward sea advances that are seemingly separated one from another ln- 

 sedimentary breaks due to temporary sea withdrawal. The earliest of 

 these invasions, represented by the Girardeau limestone, was the least ex- 

 tensive. The deposits of each succeeding invasion reached farther north 

 and were spread more widely than those of the preceding, the cycle of 

 early Silurian sedimentation in this region culminating in the Sexton 

 Creek submergence. 



The relations of the Alexandrian formations above described are shown 

 in the following composite section : 



