218 CLARK AND MARTIN THE COAL MEASURES OF MARYLAND 



In the more western Castleman and upper and lower Youghiogheny 

 basins the Potts ville, Allegheny, and Conemaugh alone are represented, 

 the latter formation being only in part present. The Castleman basin 

 contains, however, only a few rods north of the Pennsylvania line, the 

 basal beds of the Monongahela formation. 



The detailed description of the individual members of the Coal 

 Measures series is presented in the succeeding pages. The grounds for 

 the correlation of these several divisions of the Coal Measures of Mary- 

 land with the type sections of Pennsylvania and West Virginia are 

 briefly given. The fuller discussion of this subject, together with the 

 complete description of the floras and faunas found at the different 

 horizons, will be presented in a forthcoming monograph of the Maryland 

 Geological Survey on the Carboniferous formations of Maryland. 



The Formations and their Equivalents 

 pottsville formation 



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

 formation consist of conglomerates, sandstones, shales, fire-clays, and 

 coals which reach from 330 to 380 feet in thickness. The thickness is 

 apparently greatest in the southeastern part of the region under discus- 

 sion and decreases toward the north and west. 



The Pottsville formation in Maryland is of the western Pennsylvania 

 type and lacks the greatest thickness shown in the southern anthracite 

 field of Pennsylvania, where the formation was named. Comparison of 

 the formation, both as a whole as well as the individual members, with 

 the strata exposed there can at present be made only on paleontological 

 evidence. This is not at this time sufficient for complete correlation, 

 and consequently the present discussion will be restricted to a consid- 

 eration of the relations of the Maryland deposits with those exposed 

 and named in western Pennsylvania. The U. S. Geological Survey in 

 its Piedmont folio adopted the name Blackwater formation for the 

 deposits of this horizon. 



Basal contact (1).* — The Pottsville formation everywhere rests on the 

 red and green shales and sandstones of the Mauch Chunk formation. 

 There is some evidence which suggests an unconformity, but it is not as 

 yet conclusive. 



Sharon sandstone (2). — A sandstone which probably nowhere in Mary- 

 land exceeds 25 feet in thickness, and is sometimes absent, is generally 



♦The numbers used in this paper correspond to those used on the plates. There is no inten- 

 tion to give the coal seams and other members of the Coal Measures of Maryland a permanent 

 numbering. 



