84 ILLINOIS ACADEMY OF SCIENCE. 



Illinois, and has produced more large wells with steadier yield 

 than the rest of the fields combined. 



The explored rocks of Lawrence county lie in the Pennsyl- 

 vanian and Mississippian series. The former are from 800 to 

 1,300 feet thick; the latter are, as a rule, penetrated only to a 

 depth of about 475 feet. 



The Pennsylvanian rocks of Lawrence county include a shallow 

 producing sand at the south end of the field, probably in the 

 McLeansboro formations; the Bridgeport sand, of three lenses, 

 in the upper part of the Pottsville formations, and corresponding 

 to the Robinson sand of Crawford county; and the Buchanan 

 sand in the basal portion of the Pottsville rocks. 



The Mississippian rocks underlie the Pennsylvanian and con- 

 tain the most important oil sands. The upper part is known as 

 the Birdsville and Tribune (Chester), followed by the Ste. Gene- 

 vieve and the St. Louis limestones. The Chester beds include the 

 so-called "Gas," Kirkwood and Tracey sands, while the Ste. 

 Genevieve contains the rich McClosky sand. The sand names are 

 those of land-owners upon whose farm the sand was first tapped. 

 The Chester rocks average 365 feet in thickness, in comparison 

 with 700 feet in southwestern Illinois. 



The "Gas" sand, or first sand in the Chester, produces gas 

 locally over Petty township of Lawrence county. It is 125 feet 

 beneath the top limestone of the Chester and 198 feet lower than 

 the Buchanan sand of the overlying Pottsville. 



The Kirkwood sand is the most widespread producing horizon 

 in Illinois, as well as in Lawrence county. It is correlated with 

 the Sparta sand of Randolph county, the Carlyle sand of Clinton 

 county, the Lindley sand of Bond county, the Benoist sand of 

 Sandoval in Marion county, all of Illinois, and the Oakland City 

 sand of Pike county, Indiana. The Kirkwood sand shows excel- 

 lent initial production and long continued yield. It is the most 

 reliable of all the sands. The oil is "sweet," or with small per- 

 centage of sulphur. The top of the Kirkwood sand is 192 feet 

 beneath the top of the Chester, sixty-seven feet below the top 

 of the "Gas" sand, and usually lies about in the middle of the 

 Chester rocks. 



The Tracey sand is stratigraphically equivalent to the Cypress 

 sandstone, and is usually about 317 feet lower than the top of 

 the Chester and 114 feet beneath the Kirkwood sand. This sand 

 yields gas under high pressures and some oil. The gas has a 

 rank odor and the oil from this •sand is "sour," both due to the 



