346 ILLINOIS STATE ACADEMY OP SCIENCE 



that the dolomite and shale belongs in the Eau Claire 

 formations which is quite variable in its lithology. This 

 interpretation would make the Eau Claire considerably 

 thicker there than elsewhere. The Eau Claire in the 

 northwestern part of Illinois is a gray or pinkish sand- 

 stone with a thin shale showing in some wells. It is a 

 rather variable formation and in northeastern Illinois it 

 consists of shale, sandstone, or red marl. This red marl 

 is rather persistent. In the eastern part of the State the 

 formation is 65 to 175 feet thick. At Savanna there is a 

 conglomerate at the base. 



The Mt. Simon sandstone is an unsorted sand with 

 medium angular to subrounded grains, which are rather 

 coarse toward the base. This is the basal formation of 

 the Cambrian of Wisconsin, and probably of northern 

 Illinois also. F. T. Thwaites claims that the Dixon" and 

 Rockford wells both penetrate this formation and enter 

 the Huronian below*. If this is true the sediments below 

 the Mt. Simon are sandstones resembling those of the 

 Mt. Simon. They would probably be Keweenawan sedi- 

 ments instead of Huronian quartzites such as occur in 

 the Baraboo region and appear in some of the records of 

 Wisconsin wells. It seems more probable to the writer 

 that those lowest sandstones are of Cambrian age and be- 

 long to the Mt. Simon formation. More definite informa- 

 tion will be available after a larger number of well sam- 

 ples have been sent to the Geological Survey office and 

 studied so that the data will be more complete. 



* Norton, W. H., Hendrixon, W. S., and Simpson, H. E., Muzner. U. S. 

 G. S. Water Supply paper 2P3, pp 67-78. 



