CO ILLINOIS ACADEMY OF SCIENCE. 



study appear either in the detailed cross-section of northern Illi- 

 nois prepared for the Report of the Illinois Board of World's 

 Fair Commissioners or in his correspondence with the Director 

 of the State Geological Survey, Following the interpretation of 

 the new logs, a comparative study of the records of a number of 

 the wells in the vicinity will be undertaken. In conclusion, as a 

 summary of the data presented, and in order that a broad appli- 

 cation to conditions in the State may be made, a revision of the 

 Udden cross-section from Rock Island to the east side of the 

 State will be presented and discussed. 



PRESENTATION AND INTERPRETATION OF LOGS. 



In way of preparation for the discussion and interpretation of 

 the two new logs, a general understanding of the geological 

 sequence in northern Illinois and of the structure in the vicinity 

 of La Salle is necessary. 



The northern Illinois section, summarized by R. S, Blatchley 

 in his paper on Illinois Oil Resources in Bulletin No. i6. State 

 Geological Survey, page 60, is in brief as follows : 



Feet. 



Coal measures 575 



Devonian 



Sweetland Creek shale 40 



lowan Hamilton limestone 150 



Silurian. Niagara dolomite 335-388 



Ordovician 



Maquoketa shale 68-250 



Galena-Trenton 300-440 



St. Peter sandstone 150-275 



Lower Magnesian limestone 450-811 



Unconformities exist between all the divisions, except between 

 the St. Peter sandstone and Trenton limestone, and possibly even 

 locally there. 



On page 28 of the same volume, G. H. Cox describes the rocks 

 of the Lower Magnesian formation and separates it into the fol- 

 lowing members : 



Feet. 



Shakopee dolomite 40 



New Richmond sandstone 15-130 



Oneota dolomite 200-225 



The Lower Magnesian rests on the Potsdam formation of the 

 Cambrian. 



The Pennsylvanian of northern Illinois is separated into three 

 members : the McLeansboro, Carbondale and Pottsville forma- 



