P.\PERS OF GE,\ER-\L INTEREST 31 



for a short period only. After Pottsville time similar condi- 

 tions persisted in southern Illinois through the period of 

 deposition of the Carbondale and McLeansboro formations. 

 The main coal beds which are so widely mined in southern 

 Illinois are all included in the Carbondale formation. The 

 McLeansboro also contains several thin coal seams, none of 

 which, however, are workable. During all of this time the 

 elevation of the whole of the Illinois basin must have been 

 near sea level. During the intervals of coal formation 

 great swamps covered the area, where the coal plants 

 grew and where the beds of peat accumulated which later 

 became changed to coal. At times these coal swamps were 

 widely distributed and of long duration. At other times 

 the coal swamps were local and of comparatively short du- 

 ration. Between the periods of coal accumulation the basin 

 was sometimes occupied by shallow, marine waters in which 

 shale beds of impure limestones were fomied, the marine 

 origin of such beds being established by the presence of the 

 marine fossils which are included in them. Other members 

 of the Carbondale and McLeansboro formations doubtless 

 were terrestrial in origin, similar to much of the sandstone 

 and shale of the Pottsville formation. 



After the close of Pennsylvanian time there was an ex- 

 ceedingly long period which has left no sedimentary* record 

 of the events which transpired, but during this time there 

 was a period of notable deformation of the rocks of the 

 earth's crust. This defonnation resulted in uplift, and 

 locally in the development of great fractures or faults 

 through the rock strata, with differential movement of the 

 blocks adjoining the fractures. The presence of the elevated 

 belt of land across Illinois from the Mississippi to the Ohio 

 river, south of Carbondale, is due to this deformation. The 

 rocks constituting the summits of this range of hills are 

 Pottsville in age, but at Carbondale and throughout the level 

 country- north of the hills the same Potts\ille strata lie many 

 feet beneath the surface. The amount of uplift along this 

 belt must be equal to the difference between the elevation 

 of the summits of the hills and the depth of the same Potts- 

 ville beds beneath the surt'ace, to the north, and must 

 amount to 1000 feet or more. The geologj' of the belt has 



