CONEMAUGH FORMATION 223 



mont folio of the U. S. Geological Survey and the " Report on the Geol- 

 ogy of Allegany County " it is called the Thomas coal. 



CONE MA UGH FORMA TION 



Composition and relations. — The strata here referred to the Conemaugh 

 formation consist of a series of sandstones, shales, conglomerates, lime- 

 stones, and coal seams. The total thickness varies from 600 to 700 feet. 

 The average thickness in the Georges Creek basin is about 630 feet. In 

 the Potomac basin it is slightly greater. In the Castleman basin the 

 only complete measurement obtainable in Maryland gave about 700 

 feet, which, however, is 100 feet in excess of the thickness obtained by 

 the Pennsylvania survey farther north in the same basin. The thick- 

 ness in the lower Youghiogheny basin is slightly over 600 feet. 



This formation, generally known hitherto under the name of the 

 Lower Barren Coal Measures, or Lower Barren Measures, was called the 

 Conemaugh formation by Franklin Piatt in 1875 * from the typical 

 development of these rocks along the Conemaugh river, in western 

 Pennsylvania. This formation has also been known under the name of 

 the Pittsburg Coal series and the Elk River series, while that portion 

 above the Four-foot coal of the Potomac valley was called by the U. S. 

 Geological Survey in its Piedmont folio the Fairfax formation. 



Lower Mahoning sandstone (26). — A very massive and persistent sand- 

 stone from 25 to 50 feet in thickness occurs at the base of the Conemaugh 

 formation. It corresponds to the lower part of the Mahoning sandstone 

 of western Pennsylvania and eastern Ohio. 



Mahoning limestone (27). — Overlying the Lower Mahoning sandstone 

 is sometimes a bed of limestone corresponding to the Mahoning lime- 

 stone. It has been recorded by I. C. White f from a bore-hole at Fairfax, 

 where it has a thickness of 20 feet, and occurs at a distance of 42 feet 

 above the base of the formation. In the bore-hole at Jennings mill, in 

 the Castleman basin, it is apparently represented by a bed of carbonate of 

 iron 3 feet in thickness and is about 20 feet above the base of the forma- 

 tion. This corresponds to the Johnstown iron ore, which, in Pennsylva- 

 nia, is recognized as occurring at the horizon of the Mahoning limestone. 



Mahoning coal (28). — In the absence of the Mahoning limestone there 

 frequently appears in its place a thin seam of coal, known in Pennsylvania 

 and West Virginia as the Mahoning coal. 



Upper Mahoning sandstone (29). — Immediately above the Black roof- 

 shales of the Mahoning coal, or, in the absence of the coal or of the lime- 



* Second Geological Survey of Pennsylvania, H, p. 8. 

 fBull. 65, U. S. Geol. Survey, p. 82. 



XXXIII— Bull. Geol. Soc. Am., Vol. 13, 1901 



