I9I2] STEVENSON— THE FORMATION OF COAL BEDS. 505 



southwestern Pennsylvania, but apparently most of them are mere 

 lenses and few of them can be regarded as definite members of the 

 section, though they are the striking feature in the lower half of 

 the formation. The Nineveh limestone, however, is persistent, hav- 

 ing been recognized along a line of more than lOO miles, and is 22 

 feet thick at its last outcrop in West Virginia — strangely persistent 

 in an area where every other bed in the column is changeable. 

 Although the beds have small extent, the quantity of limestone in 

 the Greene is very great. Fossils are rare; minute forms, mostly 

 ostracoids, occur in many beds and occasionally one finds in the 

 shales a form resembling Naiadites. Some of the more argil- 

 laceous limestones contain much finely comminuted vegetable 

 matter. 



The composition and structure as well as the fossils in many 

 limestones above the Ames horizon have led to the belief that they 

 were deposited in freshwater basins; and evidence of like character 

 is not wanting in some of the Allegheny limestones. 



I. C. White°- found that the Upper and Lower Freeport lime- 

 stones are very often brecciated in some of the layers and con- 

 cluded that they had been deposited as muds in inland lakes — con- 

 clusion very like that of E. B. Andrews presented in 1873. Ostra- 

 coids are abundant in the Upper Freeport. Farther south in Penn- 

 sylvania, the brecciated limestones appear toward the end of the 

 Conemaugh and the fossils are ostracoids along with forms resemb- 

 ling Spirorbis. The limestones of the Monongahela, Washington 

 and Greene are brecciated at very many localities and some of them 

 rarely show normal structure. Ostracoids and Spirorbis-Vike forms 

 are extremely abundant in some.^^ Hyde's''* observations in Ohio 

 prove that conditions are the same in that state. A limestone in 

 Noble county, below the Ames, about 10 feet thick, has an irregular 

 top owing to erosion prior to deposition of the overlying sandstone. 

 The upper surface is mud-cracked and the cracks are filled with 



°" I. C. White, Sec. Geo!. Surv. Penn., Rep. Q, 1878, contains numerous 

 illustrations ; Rep. Q2, 1879, p. 220. 



''^J. J. Stevenson, Sec. Geo!. Surv. Penn.. Rep. K, many references. 



"J. E. Hyde, "Desiccation Conglomerates," Amer. Journ. Set., IV., Vol. 

 XXV., 1908, pp. 400-408; letter of January 12, 1912. 



