INFLUENCE OF IMPERVIOUS STRATA. 1209 



tration in which impervious layers are of the greatest importance. Taking 

 this region as a whole, and considering only the lithology, we have the 

 following snccession in descending order: (1) Thick shale formation; (2) 

 limestone, bearing ore bodies; (3) shale; (4) limestone, bearing- ore bodies; 

 (5) shale, often interstratified with limestone ; (6) sandstone; and (7) pre- 

 Cambrian. As illustrating the influence of the impervious strata in the 

 formation of the ores, we can consider more particularly two districts, the 

 Missouri-Kansas district and the upper Mississippi Valley district. 



In the Missouri-Kansas district the descending succession of strata is: 

 (1) Undifferentiated Carboniferous sandstones and shales; (2) Lower Car- 

 boniferous (Mississippian) limestones; (3) Devono-Carboniferous shales; 

 (4) Cambro-Silurian limestones; (5) Cambrian sandstones; (6) pre- 

 Cambrian. In this area there was an artesian circulation in the Cambro- 

 Silurian limestones and sandstones under the Devono-Carboniferous shales, 

 and in the Lower Carboniferous limestones under the Carboniferous 



Fig. 30. — Illustrating mingling of circulations of two limestones separated by a shale. 



sandstones and shales. (See fig. 30.) Probably the deposition of the ores 

 began in consequence of a connection of the circulations of the Cambro- 

 Silurian and the Carboniferous limestones by faults cutting the impervious 

 Devono-Carboniferous shales. The ores of the district are deposited in the 

 Carboniferous limestone, which is not dolomitic. The Cambro-Silurian 

 limestone is strongly dolomitic. A very important gangue mineral deposited 

 with the ores in the Carboniferous is dolomite. It is therefore believed 

 that the metals for the ores and the dolomite alike were derived from the 

 Cambro-Silurian limestone and were precipitated in the Lower Carbon- 

 iferous limestone in consequence of the reducing action of the solutions 

 furnished by it and by the direct action of the organic matter in the shales 

 and limestones. At an early stage of erosion, when the Carboniferous 

 shales and sandstones were removed, the artesian water under pressure 

 arose along joints, faults, and other trunk channels to the surface, and thus 

 the first concentration of the sulphides in the Carboniferous limestone took 



