GEOLOGICAL PAPERS 95 



have not been removed by erosion. It will be noticed, however, 

 that the eastern Maquoketa is considerably thinner than the west- 

 ern. (2) The amount of the displacement of the La Salle anti- 

 cline. The logs showing the full extent of this displacement are 

 Xos. 15 and 17a. In both cases we have data on the bottom of 

 St. Peter and top of the Lower Magnesian limestone. The Lower 

 Magnesian limestone, as it is exposed in the bluff above well 

 Xo. 17a, reaches practically to the base of the St. Peter sand- 

 stone, inasmuch as the top of the limestone contains the oolitic 

 flints that mark the contact between the St. Peter and Lower 

 ^lagnesian. The total displacement as indicated by these two 

 wells is 1.500 feet. (3) The time and amount of the displace- 

 ment. Inasmuch as coal veins are probably laid down on hori- 

 zontal surfaces, and as coal No. 2, or third-vein, extends complete- 

 ly across the anticline in places, and. in fact, as the complete "Coal 

 Measure"' section near La Salle is affected by the disturbance, it 

 seems unquestionable that the final folding is post-Pennsylvanian. 

 The amount of this displacement is about 500 feet. As the coal 

 beds dip slightly toward the fold on the west and away from it 

 on the east, there has probably been movement on both sides. 



The fact that there have been two movements shows in actual 

 outcrop at Split Rock, near La Salle, and elsewhere along tlie 

 anticline where the "Coal Measures" can be seen lying against the 

 St. Peter sandstone. The former has a dip of twelve or fifteen 

 degrees, the latter from twenty-five to thirty-five degrees. 



The date of the fold seems to be determinable from a study of 

 the contact of the Trenton-St. Peter formations and the Maquo- 

 keta-Galena formations east of the fold, as is shown by the 

 cross-section (Plate III). The straight line marking the contact 

 of the limestone and sandstone is a depositional surface and must 

 have originally been flat. Therefore the uplift to the west ending 

 in the anticlinal fold must have occurred in post-St. Peter time. 

 Further attention is directed to the Trenton-Galena lime- 

 stone east of the axis. It seems that in the well at Joliet, Xo. 25, 

 the thickness of the Trenton-Galena is about normal, conforming 

 with the condition west of the axis. It is thought that probably 

 originally the limestone extended across the position of the fold. 

 That this was the case seems probable because of its actual 

 occurrence at present in places between the St. Peter sandstone 

 and coal >>o. 2 near the axis of the anticline. The convergence 

 of the lines of deposition of the ^laquoketa on the Galena- 

 Trenton and the Trenton on the St. Peter indicates a trunca- 



