CAKBONIFEROUS AGE. 309 



County, it gives no good reason for concluding that the upturn of the 

 Silurian formation took place directly before the era of the Coal-meas- 

 ures ; but simply teaches that the disturbance occurred at some timo 

 between the Niagara period, in the Upper Silurian, and the Carbon- 

 iferous period. A geographical change, however, occurred in the 

 region of the Upper Mississippi, as remarked upon by Hall, which 

 gave the Coal-measures a northern extension beyond the Chester 

 limestone, the last of the Subcarboniferous, and even beyond tho 

 Kinderhook beds ; and thus was produced an overlapping of the latter 

 by the former, instead of perfect conformability. Hall says, in his 

 Report on Iowa (1858), " I have ascertained, in the most satisfactory 

 manner, that the coal-fields of Iowa, Missouri, and Illinois rest un- 

 conformably upon the strata beneath, whether these strata be Car- 

 boniferous limestones, Devonian, Upper Silurian, or Lower Silurian 

 rocks."' As unconformability by overlap is all that is certainly known 

 to occur between the Coal-measures and the Subcarboniferous forma- 

 tion, this was apparently the foundation for including this formation 

 in the above general statement. 



In Great Britain, Russia, and the most of Europe, the Carbon- 

 iferous and Subcarboniferous beds, when occurring together, are con- 

 formable. But, in central and southern France, as Murchison says, 

 the two are always unconformable. In Bavaria also, at Hof, the Sub- 

 carboniferous limestones and Devonian follow one another regularly, 

 though inclined together at a large angle ; while the Coal-measures 

 of Bohemia lie in horizontal strata, over their tilted edges. 



• 



2. CARBONIFEROUS PERIOD (14). 

 1. Distribution of the Carboniferous Rocks. 



The areas of Carboniferous rocks, and of the Coal fields of North 

 America, have been pointed out on page 291, and also on the map on 

 page 292. 



The principal coal-producing fields are (1) the Appalachian ; (2) 

 the Eastern Interior, or that of Illinois and the adjoining States ; (3) 

 the Western Interior, or that of Missouri and the States adjoining on 

 the north, west, and south, and reaching, though with some interrup- 

 tions, into Texas; (4) the Michigan; (5) the Rhode Island-, (6) the 

 Acadian, or that of Nova Scotia and New Brunswick. 



The thickness of the Coal-measure rocks in these regions varies 

 from 100 to 1,000 feet in the Interior coal areas, to 4,000 feet where 

 greatest in Pennsylvania, and over 8,000 feet in Nova Scotia. The 

 maximum thickness of the rocks of the Carboniferous age in Penn- 

 sylvania is about 9,000 feet, though not over 6,000 feet in any one sec- 



