40 J. J. STEVENSON — CARBONIFEROUS OF APPALACHIAN BASIN 



as far as does the Southern. It is made up of a number of small de- 

 tached basins, some of which, in spite of limited extent, possess great 

 economic importance. 



The Northern field, wholly isolated, its southwestward point being at 

 least 12 miles from the nearest portion of the eastern Middle, is a curved 

 canoe, the curve being due to change in direction of the Appalachian 

 strike, so that toward its northeasterly end the trend is almost east- 

 northeast. 



The structure is complex in these fields. At the western end of the 

 Southern field the folds are normal and the dips are comparatively gentle, 

 30 degrees on the northerly and 20 on the southerly side; but east- 

 wardly the disturbance increases; within 15 miles on the southerly side 

 the folds are overturned, the beds are shown with southerly dips of 70 to 

 80 degrees, and an important fault appears near the origin of the basins. 

 Thence the complication becomes more marked ; overturned folds as 

 well as faults are of common occurrence, and the consequent pinching and 

 crushing of the coals renders mining problems serious. This close fold- 

 ing is especially notable along the southern border of the field, and the 

 folds become more open toward the northern border.* The conditions in 

 the Middle fields are scarcely less complicated than in the Southern, but 

 in the Northern, overturned folds, though not unknown, are less frequent 

 and the disturbances are less severe.f 



Southern field. — The thickness of Pottsville in the southern prong of 

 this field has been estimated at from 1,200 to possibly 1,700 feet, accurate 

 measurements being out of the question and the calculations being made 

 from cross-sections which in most cases are incomplete. The upper half 

 of the formation is very massive, contrasting in this respect with the 

 lower half. The northerly prong shows a thickness of 1,400 to 1,475 

 feet, the upper 600 to 700 feet, being a coarse massive conglomerate 

 which, according to Rogers, is practically barren of coal, containing only 

 " a few thin and profitless seams," while the lower portion, consisting of 

 conglomerates, sandstones, and some shales, holds important coal beds. 

 Rogers calls attention to the character of the Pottsville deposits in this 

 prong, which do not show the coarseness characterizing them farther 

 east, so that in the whole section the chief mass consists of argillaceous 



* David White : Succession of the fossil floras of the Pottsville formation in the Southern Anthra- 

 cite coal field of Pennsylvania. Twentieth Ann. Rep. of U. S. Geol. Survey, 1900, p. 835 et seq. 



j The Anthracite fields were studied for the Second Geological Survey of Pennsylvania by Charles 

 A. Ashburner, F. A. Hill, and A. I) W. Smith. The reports by these geologists are given in volumes 

 AA and in the annual reports for 1885 and 1886 of that Survey. Mr Smith prepared the summary 

 discussion in the final report published in 1895. The work of these observers is so interwoven that 

 it is difficult to assign to each the credit which belongs to him. The writer desires to make ac- 

 knowledgment here of indebtedness to them for all information not acknowledged to others in the 

 pages which follow. 



