GEOLOGICAL PAPERS 85 



large sulphur content. The Tracey sand is so closely associated 

 with the underlying limestones that its oil and gas probably had 

 their origin from the included marine animal life of the limestones. 



The Ste. Genevieve limestone has a total thickness of eighty- 

 five feet and contains the ^IcCloslo.' sand, which has proven the 

 most prolific oil horizon in Illinois, because of exceptional initial 

 flow and steady yield. This pool has been instnmiental in 

 upholding the Illinois production when other sections were declin- 

 ing. The range of depth for the sand is 1,550 to 1,850 feet, or 

 about 446 feet lower than the top of the Chester and 104 feet 

 beneath the top of the Tracey sand. The oil is found twenty to 

 fifty feet in the limestone. 



The detailed structure of the Lawrence count}' oil field is 

 shown by five contour maps of oil sands and four cross-sections 

 of the field. The structure map of the top of the Kirkwood sand 

 is representative of the lay of the rocks in the area. The alti- 

 tudes of this sand are assembled and contoured with twent}--foot 

 interval. The most conspicuous feature revealed is a double 

 plunging anticline or elongated dome lying in section 30, Pett}- 

 township, which is 400 feet high and 3 miles wide. The 

 crest of this dome lies within the 680-foot contour. The 

 sand dips from the crest of the dome northward at the 

 rate of forty-one feet per mile ; southward, sixty-three feet per 

 mile : eastward, 194 feet per mile ; and westward, 228 feet per 

 mile. The sand dips southward from the dome into a small syn- 

 cline in the southwest comer of Lawrence township and then 

 spreads fanlike in its structure to the southeast through Lawrence 

 and Dennison townships on to a broad plateau-like crest of the 

 major fold. In this locality the sand lies at the 400-foot contour 

 level. The southern limits of the field seem to gradually dip 

 lower than the producing zone of the sand. Whether the major 

 fold continues to dip and merge into the southeastern side of the 

 Eastern Interior Coal basin, or the dip is local, as seems to be 

 the case between Crawford and Lawrence counties, is not known. 



The other structure maps of the Buchanan, **Gas," Tracey and 

 ^IcClosk}- sands corroborate and conform to the structure of the 

 Kirkwood sand, with the exception of ver\- minor irregularities. 

 In addition to these maps, four cross-sections were made to show 

 the nature of the crest of the La Salle anticline, as well as its 

 flanks. The A-A cross-section presents the structure of the 

 sands along the crest of the anticline and lengthwise through the 

 oil field. The section is valuable for its picture of the double 



