634 A. W. GRABAU-^-TYPES OF SEDIMENTARY OVERLAP 



up a new fan, but this time of fine materials. However this may be, the 

 presence of the Greenbrier limestone shows a widespread subsidence, and 

 the fossils of this limestone show that this took place at the beginning of 

 Chester time. 



It seems likely that the post-Logan elevation, which in Michigan was 

 accompanied by the formation of gypsum beds (lower Grand Eapids), 

 gave an impulse to the southeastward transgression of the Mississippian 

 sea, and that during this period the Cumberland ridge above outlined was 

 finally submerged. The elevation was felt in the Mississippian valley, 

 since a pronounced line of erosion exists at the top of the Warsaw, which 

 is followed by only the highest Saint Louis limestone, resting with a basal 

 breccia on it. Southward in the Mississippi region this interval becomes 

 less; and southeastward, in the southern Appalachians, it disappears 

 altogether, the Saint Louis and succeeding Chester beds being of great 

 thickness. Since this seems to be the case, the Greenbrier-Maxville re- 

 advance apparently came from the southeast, where continuous deposition 

 was going on. The readvance reached the Iowa-Mississippi region and 

 northern Kentucky in Saint Louis time; but central Ohio, Michigan, and 

 Pennsylvania only in Chester time. This explains the greater thickness 

 of the Greenbrier in Maryland and West Virginia. The retreat of the sea 

 after the temporary advance into Penns}dvania was followed there by the 

 formation of the upper non-marine Mauch Chunk fan, but transgressive 

 movements seem to have continued over the southern Appalachian region. 



The Pottsville.— -The Pottsville of the Appalachians represents even a 

 better case of non-marine progressive overlap than either of the preceding 

 examples. The series has been worked out in great detail by the Pennsyl- 

 vania geologists, and the relationships of the beds to each other have been 

 fully discussed by Stevenson,* and, with special reference to the overlap 

 shown by them, by David White, f 



One of the pronounced characteristics of the Pottsville is the well 

 known abundance of conglomerates and coarse cross-bedded sandstones. 

 Coal beds and layers of coal plants are common, but marine fossils are 

 rarely found. As has been convincingly demonstrated by Stevenson and 

 David White, the lowest beds of the series — that is, the Pocahontas divis- 

 ion — is found only in two areas, and those the most easterly of the series. 

 The first is in the type region of central eastern Pennsylvania, and the 

 other is in southwestern West "Virginia and adjoining Virginia. The 

 next higher series, the Ealeigh-Bon Air (middle Pottsville), is especially 

 well developed in the southern Appalachians, where it extends through 

 Alabama, eastern Tennessee, Kentucky, and West Virginia. In eastern 



* Stevenson : Loc. cit. 



t David White : Bull. Geol. Soc. Am., vol. 15, 1904, pp. 267-282. 



