GEOLOGICAL PAPERS 89 



tions. The Pottsville lies below Xo. 2 coal. It is characterized 

 bv silicious. micaceous, g^ray or reddish shales, and sandstones 

 which are often micaceous. Occasionally there occurs a thin bed 

 of coal in the fonnation. In the main it is non-calcareous. The 

 texture and color, together with the lack of calcareous material, 

 are important criteria of discrimination. 



The Sweetland Creek shale, a dark colored shale, at least its 

 lowest part, is often colored brownish by an abundance of fossils 

 known as Sporangitcs huronense, supposed to be the spores of 

 some paleozoic tree. These are readily distinguishable, and 

 render the correlation of a shale containing them ver}- accurate. 

 The Hamilton limestone is a light gray, pure calcareous lime- 

 stone, not a dolomite. Considerable hesitancy should be shown 

 in correlating any dolomite with the Hamilton. 



The Niagara limestone is generally a dolomite of xtvy fine 

 texture, usually of a light color, and sometimes characterized by 

 layers of flint. In certain places the Xiagara limestone is knoAATi 

 to contain calcareous layers. 



The ^laquoketa shale is very variable in lithological character. 

 but bluish-gray shale with some streaks or beds of limestone and 

 black carbonaceous shale is the usual occurrence. The relation- 

 ship of the overlying and underlying formations are the readiest 

 means of identification. 



The Galena-Trenton formations are dolomite in the upper part, 

 the loAver sixty or 100 feet of Trenton, however, being usually 

 more calcareous. The upper part is usualh' of a characteristic 

 brownish-buft color, due to a large percentage of iron present. 

 The Trenton is often bluish. Both formations are often quite 

 flinty ; this is especially true of the Galena and the lower or 

 quarr}- beds of the Trenton. 



The St. Peter sandstone is characteristically a pure, soft white 

 sandstone, the individual grains being all quartz and of a rather 

 large size. Its purity, color and softness are almost unfailing 

 criteria for identification. 



The Lower Magnesian limestone, or Shakopee dolomite, is a 

 white flint}- dolomite, often having beds of fine laminated hydrau- 

 lic limestone. It sometimes contains silicious concretions. The 

 upper part in contact with St. Peter sandstone is usually marked 

 by the occurrence of oolitic flint. The beds lower than the 

 Shakopee dolomite are nowhere exposed in Illinois, and their 

 local characteristics are not known. 



The structural feature to be emphasized in the northern part 



