PENNSYLVANIAN. 455 



field was studied and mapped in detail by Joseph Squire "' and the greater portion of it has 

 recently been reexamined by Charles Butts. ^'"' "^ 



Squire divides the section, calculated to average 5,525 feet, into four parts, which, in descend- 

 ing order, are as follows : 



"Conglomerate group," 475 feet, four coals intercalated with conglomerates, and extending from the Montevallo 

 coal to the top of the section. 



"Productive group," 2,200 feet, sandstones, shales, and 28 coals, including the Helena coal at 240 feet, a conglom- 

 erate (called Thompson) at 310 feet, and the Little Pittsburg coal at 655 feet below the top of the "group; " the Adkins 

 coal, 790 feet lower; and the Wadsworth coal, 500 feet lower, or about 240 feet above the base of the "group." 



"Micaceous group," 1,055 feet, micaceous gray and coarse pink sandstones and grits, with seven coals intercalated; 

 the Harkness coal, 160 feet below the top, underlain by 275 feet of sandstones and grits; the Nunnally coal 165 feet above 

 the base. 



"Millstone Grit group," 1,795 feet, conglomerates, sandstones, and six coals; the main sandstone masses at the 

 top (360 feet), in the middle (650 feet), and just below the latter (340 feet); the Gould coals in the basal portion. 



The section given by Butts agrees in most points with that abstracted above, though 

 indicating an average total thickness of over 6,100 feet in the regions studied. It seems probable 

 that the deeper southeastern portion of the field will ultimately be found to have a depth of 

 7,000 feet or more. The "Millstone Grit group" is correlated by Butts with the Lookout 

 sandstone. On the fossil plant evidence at present available, the writer is disposed to include 

 the Nunnally and Harkness coals (in the lower part of the "Micaceous group") also in the Look- 

 out. At least they are of lower Pottsville age. The paleobotany of the Wadsworth coal group, 

 which is thought to be as old as the Horse Creek coal group of the Warrior field, will prob- 

 ably be found to place this division also within the limits of the lower Pottsville, the Little 

 Pittsburg-Helena coal group belonging to the middle Pottsville. Fossils of upper Pottsville 

 age have not yet been recognized in the Cahaba field. These preliminary correlations indicate 

 both a very great thickness and a tremendous expansion of the terranes to the southeast in 

 Alabama. 



The Coosa field, a short distance east of the Cahaba field, is about 40 miles long and 4 to 

 10 miles broad. The coal measures are overlain by overthrust Lower Cambrian on the east. 

 Though this field was the scene of the first coal mining in the State, at least 12 workable coals 

 being reported by the Alabama geologists, its deformation has been so violent and the folds 

 are so complicated and the exposures of the soft beds so meager that little successful development 

 has been accomplished, even in the north end of the field. Very little detailed geologic work 

 has been done in the basin. A report on the field by A. M. Gibson ^^ shows a section which 

 exceeds 6,000 feet and which appears to be stratigraphically similar to that of the Cahaba field, 

 though there is said to be further eastward expansion of the groups. The greater part of the 

 reported thickness apparently belongs to the lower Pottsville. The basin lies low and, like the 

 Cahaba, passes under the Cretaceous to the south. No paleontologic studies of the Pennsyl- 

 vanian of this basin have been made. 



I-K 14-15. WESTERN INTERIOR COAL REGION. 



The following statement was prepared for this volume by G. H. Ashley in 1909 : 



GENERAL FEATURES. 



The Pennsylvanian of the western interior coal region covers a large area in Iowa, Missouri 

 Nebraska, Kansas, Arkansas, Oklahoma, and Texas, the area exposed being a part, possibly 

 only a small part, of that originally covered. The area of exposure has been much reduced on 

 the southeast by folding and faulting, on most of the eastern edge by erosion, and along all of 

 the west and southeast sides by the deposition of a mantle of later rocks. In central Arkansas 

 the southeastern edge of the outcrop passes beneath a cover of Tertiary rocks. Farther west 

 and south the Pennsylvanian is hidden by Cretaceous and Permian rocks. The extension to 

 the west in Iowa and most of Kansas is concealed by Cretaceous rocks; across Kansas, Okla- 

 homa, and Texas by Permian rocks. Deep drilhng for oil and gas indicates a continuance to 

 the west of the Pennsylvanian rocks, so far as the series is reached by such drilhng, but they 

 may be assumed as extending indefinitely beyond. 



