96 ILLINOIS ACADEMY OF SCIENCE. 



at Niantic. A very small arch is shown in the No. 6 coal. It is 

 noticeable that from this point eastward there is a rapid dip of 

 the coal into the basin. It is suggested that the Niantic deforma- 

 tion may be a continuation of the Sandoval-Duquoin "terrace." 

 All the formations show a decided rise into the La Salle anticline 

 at Tolono. The crest of this arch is thought to be west of Tolono 

 in the vicinity of Sadorous. 



Other sections, not presented here, show similar features in the 

 key horizon and seem to corroborate the presence of the previ- 

 ously mentioned minor folds along the broad western flank of 

 the Illinois basin. 



Besides structural cross-sections, detailed contour maps on the 

 No. 6 coal have been made of some areas, (plate VI, coal con- 

 tour map of Marion County.) That for the new Sandoval oil 

 field is presented here. The position of the coal in the several 

 mines and wells along the western side of the county were noted, 

 and also the surface elevations at each. Contours of 25 feet 

 interval were made from these data. The dips in the various 

 directions are portrayed and warrant definite conclusions regard- 

 ing the present and future drilling. The structure of the coal is 

 further shown by the use of profiles drawn between prominent 

 points on the contour maps, in such position that they are at right 

 angles to the dip. They are intended merely to make clear the 

 mental picture to those who are not familiar with contouring. 



The present active oil field near Sandoval seems to be bounded 

 by the-125-foot coal contour, and the best area to further develop 

 that field lies within the confines of that line. This includes the 

 town of Sandoval and the area southeast of it. The most prom- 

 ising area, however, is in and about the crest of the dome-like 

 structure in section 29. Oil has already been found in shallow 

 sands in this structure, but the lower sands are even more 

 promising. 



The similarity of the structure south of Centralia to that of 

 Duquoin leads to the conclusion that a structural "terrace" exists 

 slightly to the west of Centralia, and hence offers prospective terri- 

 tory. The so-called "terrace" is irregular but possibly continuous 

 throughout Fayette, Marion, Washington and Perry counties. 



Reference to plate I indicates the position of various deforma- 

 tions discussed. It will be readily seen that the western flank of 

 the Illinois basin holds promise of new oil fields and that possibly 

 the northern portion of the La Salle anticline will bear investiga- 



