GEOGRAPHICAL CHANGES DURING PENNSYLVANIAN 147 



the Tennessee coal field the Eockcastle is a more or less pebbly sandstone, 

 but on the west side, at the north, it becomes distinctly conglomerate 

 and it retains this character in Kentucky. It is notably conglomerate, 

 especially in its lower portion, everywhere in Ohio, and in the northern 

 150 miles of its extent it carries abundant fragments of chert or cherty 

 limestone containing Mississippian fossils; while near Lake Erie it holds 

 not only quartz and chert pebbles, but also very numerous flat and angular 

 fragments of rock. This distribution in Ohio clearly indicates a broad 

 gravel-filled valley like that of the upper Ohio. The nature of the frag- 

 ments shows that they were derived for the most part from the vicinity 

 of the present Lake Erie. The chert at one time was supposed to come 

 from the Maxville limestone of southern Ohio, but the deposit is far 

 to the north and west of the original limit of that limestone, which 

 is not reported anywhere as cherty; the source must be looked for in the 

 cherty Mississippian. limestone of Michigan. The valley holding the 

 Eockcastle lay for the most part west from the present area of Pennsyl- 

 vanian deposits, being well marked only near Lake Erie and in Pike 

 and Jackson counties of Ohio. In the intervening space one finds only 

 the eastern edge of the valley and in many places apparently the insignifi- 

 cant deposits laid down in lateral valleys. In Kentucky and Tennessee 

 the distribution of this sandstone is not confined to narrow areas. The 

 Sharon sandstone tells the same story as the Eockcastle, but has a wider 

 distribution. It, like the Eockcastle, is coarser at the sides than in the 

 middle of the great basin, so that the material forming it was derived 

 not from one but from both sides. 



At the close of the Eockcastle, water extended to but a little distance 

 within New York. Practically the whole of the Pennsylvania bituminous 

 area, the Northern Anthracite field, the northwestern third of West Vir- 

 ginia, and nearly all of the present coal area of Ohio were dry land. A 

 shallow, rather narrow arm of the interior, or Mississippian, sea extended 

 northward from Kentuclcy to the east side of what is now Lake Erie, 

 occupying the old valley partly filled with Eockcastle deposits, while other 

 valleys communicated with the Atlantic at the east. The elevation of the 

 land must have been insignificant throughout, for at the close of the 

 Beaver the whole region had received deposits. 



The passage from Eockcastle to Beaver is somewhat abrupt at many 

 places along the western and northern sides of the great basin. The sur- 

 face of the Sharon sandstone is irregular, so that in western Pennsyl- 

 vania and in Ohio the Sharon coal bed occurs in saucer-like patches, 

 while in Kentucky the interval from Sharon sandstone to Sharon coal 

 varies abruptly from 10 to 50 feet or even more. These variations may 



