PROGRESSIVE OVERLAP 611 



ably containing feldspar. It also contains some sandy shales." The age 

 is Oriskany, as shown by the occurrence of some poorly preserved fossils. 

 It has a thickness of 1,200 feet and over. On the north side of the 

 Coosa valley the Armuchee chert replaces the sandstone with a thick- 

 ness of 50 feet. The chert is bedded and contains fossils similar to those 

 of the Frog Mountain sandstone, of which it probably represents an off- 

 shore deposit. 



There is a marked time break and erosion interval above these forma- 

 tions, followed by the Chattanooga Black shale. This consists of two 

 divisions. The lower, with a maximum thickness of 40 feet, but de- 

 creasing to 1 or 2 inches in places, is jet black and rests on the 

 Armuchee chert or directly on the Rockmart sandstone. The upper 

 member consists of blue or greenish clay shales, usually with phosphatic 

 concretions, which are generally perfectly round, when small; but when 

 sometimes they reach a diameter of a foot or more, they are oval. The 

 green color of the formation is due to the presence of glauconite. This 

 upper member varies from 1 to 3 feet in thickness and apparently repre- 

 sents the Maury shale of central Tennessee. The Chattanooga is suc- 

 ceeded by the Fort Payne, from 20 to 200 feet thick, and this by the 

 Floyd shale, 2,000 feet or more in thickness, or by the Bangor lime- 

 stone. 



At the base of the Black shale opposite Rome, Georgia, a few fossils 

 have been found suggestive of Hamilton age, but the evidence is scarcely 

 conclusive.* 



Summing up the facts so far determined, it becomes apparent that 

 there is in the interior area a progressive overlapping of the Mississippian 

 formations southward and eastward, beginning in Kinderhook time and 

 continuing, practically without interruption, throughout that epoch; for 

 Mississippian strata are wanting in central Texas, where the mid-Car- 

 bonic strata rest directly and unconformably on earlier Paleozoics. A 

 southward transgression also took place in the area east of the old Cum- 

 berland land ridge, which was eventually submerged in later Saint Louis 

 time. Whether or not land conditions existed throughout the southern 

 parts of the Gulf states is not determinable from the data at hand. 



Nearly everywhere resting directly on the surface of the slowly sub- 

 siding old land lies a bed of highly carbonaceous shale. In its basal 

 portion, in many localities, it contains fossils weathered out of the under- 

 lying Ordovicic strata. Sometimes it is replaced by a sandstone or 

 conglomerate; sometimes it carries worn fish bones; in many places, too, 

 it carries remains of land plants. Several, observers have been struck 



* Schuchert : American Geologist, vol. xxxii, p. 152. 



