THICKNESS OF THE SEDIMENTS 269 



not only the thickness of the beds at various points, but the shape of 

 the floor of the basin at the close of the Pottsville throughout the area 

 of the surviving coal basins. 



The measurements given on the map for the Pottsville in Pennsylva- 

 nia and Ohio are largely taken from the respective state survey reports. 

 For West Virginia I have drawn extensively from the wealth of carefully 

 measured sections published by Dr I. C. White in his very important 

 state report lately issued and from the valuable data presented by Mr 

 M. R. Campbell in his folios issued by the U. S. Geological Survey. 

 Special acknowledgment is due Dr J. J. Stevenson for his courtesy in 

 permitting the use of a number of his sections, or interpretations of 

 others' sections, in Kentucky and West Virginia. 



In some instances the measurements here given differ from those 

 commonly current. In cases of transitional beds from the Mauch Chunk 

 to the Pottsville the boundary line has been drawn* at the topmost bed 

 of red shale. This cuts off over 400 feet of transitional beds at the type 

 locality, Pottsville, Pennsylvania, which would be included were the 

 boundary placed at the lowest conglomerates in this vicinity, as advo- 

 cated by Doctor Stevenson. In the Northern Anthracite field the sec- 

 tions have been made to include the lowest red ash beds where the fossil 

 plants of the latter have been found to be of Pottsville age. The upper 

 limit of the Pottsville in several of these sections is not yet definitely 

 fixed, but it is certain that the fossils of the Dunmore coals numbers 2 

 and 3, in the northern end of the Northern Anthracite field, belong 

 below the top of the Pottsville, the plants from the Clifford bed in the 

 vicinity of Forest City being unquestionably in the Mercer group. f 



As elsewhere noted, $ the Bloss or " B " coals of Tioga, Bradford, and 

 Lycoming counties are referable to the Mercer group and are included 

 in these sections. Departure from the correlations by the Pennsylvania 

 geologists is also made along the Allegheny front in Blair and Clearfield 

 counties, where the fire clays generally placed in the Clarion group are 

 found to lie at the Mount Savage horizon, and so fall within the Mercer 

 groups 



Of a more serious nature is the reduction of the sections in the Alle- 

 gheny valley and on Red Bank creek in Armstrong and Clarion counties, 

 where the shales above the bed supposed to represent the Sharon sand- 



*Twentieth Ann. Rept. U. S. Geol. Survey, pt. 2, p. 831. 



fThe limits of these, as well as of the sections nearer Shickshinny and Ashley, will be dis- 

 cussed in connection with the full treatment of the fossil floras. The thickness of the Pottsville 

 beds at Campbells Ledge near Pittston is not far from the average for the basin, the measure- 

 ments by Dr I. C White quoted in my former papers being confined to the lower members only 

 of the series as deposited at this point. 



X Twenty-second Ann. Rept. U. S. Geol. Survey, pt. 3, 1902, p. 136. 



§Loc. cit., p. 136. See also Science, vol. xvii, 1903, p. 942. 



