GEOGRAPHICAL CHANGES DURING PENNSYLVANIAN 157 



appear frequently during the upper half of the Conemaugh, several of 

 them widely, though irregularly, distributed, but in no case are they dis- 

 tinctly marine. Some are crowded with minute univalves of undeter- 

 mined relations; others are associated with carbonaceous shales, filled 

 with fragments of plants and fishes, which point rather to fresh-water 

 conditions. 



The most notable feature of the Conemaugh is the red and green 

 shales, in color resembling those of the Catskill and Shenango but 

 deeper. The greatest development is in west central West Virginia and 

 the adjacent part of Ohio, where at times nearly the whole section is 

 red shale. The greatest geographical expansion was just preceding the 

 deposition of the Ames limestone, when the reds reached southeast 

 nearly to the outcrop and northward to the outcrop in Pennsylvania ; but 

 they did not reach into northern Ohio and they are practically wanting 

 east from the line of Chestnut hill in Pennsylvania. From that time 

 to the end of Conemaugh the area contracted and reds occur in irregular 

 patches. These beds frequently contain nodules of limestone, which, at 

 least in the Pittsburg reds, underlying the Ames limestone, are usually 

 fossiliferous. The red shales in some cases mark horizons elsewhere 

 carrying limestone and they may indicate a marine condition. 



The exceeding shallowness of the water and the long periods <of quiet 

 during the Conemaugh are indicated by the coal beds, which, though 

 extremely thin, have great extent. The most remarkable is the Harlem, 

 which underlies the Ames limestone. It rarely exceeds 15 inches, yet is 

 present in much of Pennsylvania, Maryland, and Ohio, persisting in the 

 interior of the basin within West Virginia, where it is brought up under 

 the Cowrun anticline. The peculiarities of this and other Conemaugh 

 coal beds are of much importance in any discussion respecting the ac- 

 cumulation of coal in beds, and they will be examined carefully in 

 another connection. 



Toward the close of the Conemaugh the streams bringing in materials 

 had become sluggish and the deposits, except within limited areas, are 

 fine in grain. The Monongahela began with a long period of exceed- 

 ingly slow subsidence, during which the Pittsburg coal bed gradually 

 extended across the northern part of the great basin and southward along 

 the east and west sides; but from all sides it became thinner toward the 

 central part of the basin and it is practically wanting in a great part 

 of West A-^irginia and eastern Ohio, where it occurs only in widel}^ 

 separated patches. The bed may have been almost continuous around 

 the basin. The singular uniformity of conditions and the extreme slow- 

 XII — Bull. Geol. Soc. Am., Vol. 18, 1906 



