220 CLARK AND MARTIN THE COAL MEASURES OF MARYLAND 



grinding or on exposure to the weather. The genetic difference between 

 the two varieties is not known, and there seems to be no regularity in 

 distribution between them. 



Mount Savage or Upper Mercer coal (9). — Immediately above the Mount 

 Savage fire-clay is a seam of coal varying in thickness from 2 to 4 feet. 

 It is the seam which has long been known as the Mount Savage coal in 

 the northern end of the Georges Greek basin, and is the same as the 

 Upper Mercer coal of Professor H. D. Rogers. The seam which was 

 named the Westernport coal * in the southern Georges Creek basin is the 

 same as this. The shales associated with this coal carry an abundant 

 flora, which Mr David White, after examination, informs the authors to 

 be identical with the Mercer flora. 



Homewcod sandstone (10). — A massive sandstone, varying in thickness 

 from 30 to 100 feet, is found a short distance above the Mount Savage 

 coal. This was formerly called the Piedmont sandstone. From its posi- 

 tion between the Mercer coal group and the base of the Allegheny for- 

 mation, it is evidently identical with the Homewood sandstone of Penn- 

 sylvania. 



ALLEGHENY FORMATION 



Composition and relations. — The Allegheny formation consists of a series 

 of sandstones, shales, limestones, and coal seams having a total thickness 

 in Maryland of from 260 to 350 feet. The thickness is greatest in the 

 southern and eastern parts of the area, in this respect corresponding to 

 the Pottsville. 



The name "Allegheny series " was proposed by H. D. Rogers in 1840 f 

 to include the strata from the lowest bed exposed at Pittsburg down to 

 the " sandstones and conglomerate at the bottom of the coal formation. 1 ' 

 The type section is along the Allegheny river between Pittsburg and 

 Warren. In later usage the formation was restricted by cutting off the 

 upper part, which now constitutes the Conemaugh formation. The Alle- 

 gheny formation was also known under the name of the Lower Product- 

 ive Coal Measures or Lower Productive Measures. The U. S. Geological 

 Survey in its Piedmont folio proposed the name Savage formation for 

 the lower part of the Allegheny formation, including the Davis coal, and 

 the name Bayard formation for the upper part of the Allegheny forma- 

 tion and lower part of the Conemaugh formation up to and including the 

 Four- foot coal of the Potomac valley. 



Brookville coal (11). — A seam of coal varying from 1 to 4 feet in thick- 

 ness sometimes occurs at or very near the base of the Allegheny forma- 



*The Physical Features of Allegany County, pp. 115, 170, 171. 



f Fourth Annual Report of the Geological Survey of the State of Pennsylvania, p. 150. 



