134 J. J. STEVENSON — CARBONIFEROUS OF APPALACHIAN BASIN 



ion is about 500 feet thick — conglomerate, sandstone, and gritty slates. 

 The middle conglomerate is identified by Mr Gibson with his second 

 conglomerate of Blount mountain, the Bonair. If this identification be 

 correct, the important coal bed above is at the Sewanee horizon. The 

 immense mass of conglomerate at the top is thought by Mr Gibson to 

 be the same with his fourth conglomerate of Blount mountain, but this 

 conclusion awaits confirmation * 



The Coosa coal field, embracing parts of Saint Clair and Shelby coun- 

 ties, is only 3 or 4 miles east from the last, being separated from it by the 

 Cahaba valley. Mr Gibson gives 5,750 feet as the thickness of Coal 

 Measures in the deepest portion of the basin. He has traced his First 

 and Second conglomerates (Etna and Bonair) through a great part of the 

 field, but they are greatly increased in thickness and are separated by 

 about 200 feet of shale, the total being nowhere less than 1,200 feet, 

 while in one part it may be 1,500. He recognizes his Fourth conglomer- 

 ate in 500 feet of conglomerate and quartzitic rocks near the top of the 

 section, evidently the top conglomerate of the Cahaba field. Mr Gibson 

 especially emphasizes the thickening of the coarser beds; the First 

 (Etna) conglomerate, only 80 to 100 feet in Blount mountain, is 400 to 

 500 feet here, and the Fourth conglomerate shows an almost equal in- 

 crease. The coal beds, almost twenty in number, are persistent, and 

 half of them are workable, while one of them attains a thickness of 13 

 feet of solid coal.f 



The thickness assigned to the sandstones in the Cahaba and Coosa 

 fields may be excessive, but numerous opportunities for direct measure- 

 ment were found, and such measurements prove sufficiently that Mr Gib- 

 son is correct in asserting so firmly that the coarser deposits thicken 

 rapidly toward the east. 



The western part of the Warrior coal field is properly the southern 

 termination of the great Indiana-Illinois coal field, from which, however, 

 it is separated by an interval of more than 200 miles. The general con- 

 ditions were the same throughout this southern part of both basins, as 

 appears from the close resemblance of the sections obtained in the sev- 

 eral counties. The work in this field was performed by Mr McCalley. 



The elevated ridge, forming the eastern edge of the field along Browns 

 valley and the northern edge across Marshall, Morgan, Lawrence, and 

 Franklin almost to the Mississippi border, is known as Sand mountain. 



The continuation of the Raccoon area lies east from the Warrior river, 

 in Jefferson and Tuscaloosa counties, and passes under the Cretaceous 



*J. Squire: Geol. Survey of Alabama, Cahaba Coal Field, 1890, pp. 4,5,14. Diagrams of sections 

 on the map. 

 t A. M. Gibson : Geol. Survey of Alabama, the Coosa Coal Field, 1895, pp. 26, 55, 79, 81, 125. 



