JPAPERS OX GEOLOGY AND GEOGRAPHY 391 



6. PlatfeviUe Limestone. The upper part of this 

 formation in the northwestern part of the state also 

 sometimes contains very thin seams of the oil rock, while 

 the lower part of the formation is dolomitic and more 

 porous. Eocks of this same age have furnished large 

 quantities of oil and gas in both Indiana and Ohio, but 

 not in Illinois. 



Geological Structure. The northeastern part of the 

 state consists of a low monocline dipping very gently to 

 the east or a little south of east, and is the eastern limb 

 of the main structural axis of the state, i. e., the LaSalle 

 anticline. Local irregularities may, and probably do, 

 occur in this monocline, but in order to furnish proper 

 structures for the accumulation of oil and gas there 

 should be well defined domes. While such structures 

 may possibly exist, they have not yet been found and 

 their locations, if they do occur, would be attended with 

 much difficulty and uncertainty because of the heavy cov- 

 ering of drift in most of the district. Sandstone lenses 

 would also be very difficult to locate. 



Well borings. Mention has already been made of a 

 number of shallow wells which have obtained gas from 

 the glacial drift or from the upper part of the Xiagara 

 limestone. Large numbers of deep borings for artesian 

 water both to the St. Peter sandstone and the Potsdam 

 sandstone. — the two main aquifiers of the district, — have 

 been made. Many of these wells have penetrated one 

 or more of the horizons noted above, and most of these 

 wells furnish fresh water of good quality, while a few 

 are somewhat charged with salts, but none have found 

 the decidedly briny waters which are so commonly as- 

 sociated with oil and gas. 



SUMMAKY 



Summing up the above, it may be said that small local 

 sui>plies of gas can be looked for in the drift and in the 

 upper part of, and perhaps at the base of, the Xiagara 

 limestone, but the chances of finding large amounts of 

 either oil or gas in these horizons are not encouraging. 



