294 PALEOZOIC TIME. 



The subdivisions of the Subearboniferous rocks are best exhibited in the limestones 

 of the era in the Mississippi valley or Interior Continental basin; and hence, in giving 

 further details respecting the formation, they are first considered. 



(a.) Interior Continental Basin. — In Illinois, the subdivisions, according to Worthen, 

 — partly following those of Hall, — are the following, beginning with the oldest. 



1. Kinderhook Group. — Consists of sandstones, grits, and shales, with thin beds of 

 oolitic limestone; 100-200 feet thick. The " Choteau limestone," " Lithographic lime- 

 stone," and " Vermicular sandstone and shales," of Missouri, are here included, and 

 also the "Goniatite limestone" of Rockford, Indiana. It rests on the Devonian Black 

 shale. 



2. Burlington Group. — Limestone, with cherty layers at top, and nodules of 

 hornstone through portions of the limestones; 25 to 200 feet thick. Much of it is ex- 

 cellent building stone. 



3. Keokuk Group. — Mainly limestone, with thin- bedded cherty layers below along 

 the junction with the Burlington limestone, gray limestone at middle, and a shalv, 

 argillaceous, magnesian limestone above, often abounding in geodes of quartz, etc., 

 called the Geode bed. The geodes vary from half an inch in diameter to twenty inches 

 or more; and many are beautiful for their agates, or druses of quartz crystals, and some 

 for crystallized calcite, dolomite, blende, pyrite, etc. 



4. St. Louis Group. — Evenly-bedded limestone of Alton and St. Louis; oolitic 

 limestone, three miles above Alton; and equivalent beds at Bloomington and Spergen 

 Hill, Indiana; blue calcareous shales and arenaceous limestone at Warsaw. In some 

 places 250 feet thick. 



5. Chester Group. — Limestone, in three or four beds, with some intercalated shale 

 and sandstone; occasionally 600 feet thick. Includes the " Pentremital " limestone, 

 and the " Upper Archimedes " limestones. The Upper Archimedes has also been called 

 the "Kaskaskia" limestone. 



The whole series in southwestern Illinois has a thickness of 1,200 to 1.500 feet: it 

 thins out rapidly to the north, and disappears before reaching Rock Island County, 

 leaving the Coal-measures resting on the Devonian limestones. 



In Iowa, according to C. A. White, the Carboniferous is the surface formation over 

 all the State, excepting the northeastern third where the rocks are older, and an area in 

 the northwestern part which is Cretaceous; and the Subearboniferous occurs along the 

 eastern portion of the Carboniferous area. It includes about 175 feet (maximum thick- 

 ness) of Kinderhook beds, consisting of alternating strata of sandstone and limestone, 

 the latter partly magnesian; 190 feet of Burlington limestone, in which are some 

 siliceous beds; 50 feet of Keokuk limestone, well developed about Keokuk; 75 feet of 

 St. Louis limestone, having magnesian limestone below, next a gray friable sandstone, 

 and above gray limestone. The Kinderhook beds reach farthest north; the Burling- 

 ton and Keokuk, much less so; the St. Louis, nearly to the limit of the Kinderhook. 

 The Chester group is not present, the Coal-measures resting directly on the St. Louis 

 limestone. 



In Missouri, the whole thickness of the Subearboniferous limestone is 1,150 feet. 



In Kentucky and Tennessee, the subdivisions of the Subearboniferous formation 

 observed in Illinois are not distinct. In Middle Tennessee, according to Safford, 

 there are two groups. The lower is the Siliceous group, consisting, commencing below, 

 of (1) the Protean beds, cherty and argillaceous beds, with some limestone, 250 to 300 

 feet, and (2) the Lithostrotion or Coral beds, an impure cherty limestone, the equiva- 

 lent of the St. Louis limestone, about 250 feet thick. The upper member is limestone, 

 400 feet thick on the northern borders of the State, and 720 on the southern. These 

 two divisions occur also in East Tennessee. 



The Upper member also extends into the northeast corner of Mississippi, where 

 it is overlaid by Cretaceous beds (Hilgard). At Huntsville, Ala., Worthen found it to 

 consist principally of gray limestones, partly oolitic, partly chert}', with some shaly 

 beds, in all about 900 feet. The larger portion of the series yields Chester fossils; but 

 characteristic forms of the St. Louis group mark the age of the lowest 250 to 300 

 feet. 



