862 FLORAL ZONES OF THE POTTSVILLE FORMATION. 



margin of the Southern Anthracite tield. the Pottsville formation 

 decreases to about 850 feet at Shamokin Gap, toward the west end of the 

 Western Middle Anthracite tield, and to 830 feet, more or less, at the 

 ]Mahanoy tunnel at the eastern end of the same field. Here the upper 

 conolomerates often contain pel)bles of the size of an egg, while the 

 lowest beds are interlarded with red shale, as in the Southern Anthra- 

 cite field. A very rapid change is to be observed in the basins of the 

 Eastern Middle field, where the contact with the red shale becomes 

 distinct. Thus in the Silver Brook Basin, on the southern border of 

 that field, the formation is but 500 feet thick, while in the Upper Lehigh, 

 on the north, it is said to l)e not over 200 feet in thicknes.s. The meas- 

 urements of the formation in the Northern Anthracite field varv, the 

 average being about 225 feet. It is undoubtedly much less than this 

 at points, such as the well-known fossil plant and insect locality at 

 Campbells Ledge, near Pittston, where, if Dr. I. C. White is correct 

 in the recognition of the equivalent of the Mauch Chunk formation, 

 the Pottsville, assigned a thickness of but 54 feet by him, can hardly 

 exceed 100 feet at most, as limited according to the standard employed 

 in the bituminous basins. 



The diiriinition of the formation from 1,100 feet at Rattling Hun, 

 in the Dauphin Basin, to lOO feet in the Broad Top field is perhaps 

 less remarkable than the decrease in passing from the Southern field 

 to Upper Lehigh, which is but 18 miles from Tamaqua and 14 miles 

 from Nesquehoning. Both of the thinner sections may be considered 

 as offshore stations, as compared with the thick sections farther to the 

 southeast. It is, however, difficult to form an estimate of the relative 

 remoteness of any of these points from the original coast of the inte- 

 rior Carboniferous sea. 



Li the Bernice Basin, Sullivan County, the Pottsville does not appear 

 to exceed 125 feet in thickness, and a similar measurement is reported 

 where the formation touches the New York State line. Throughout 

 most of the ])ituminous l)asins in southei-n and W(>stern Pennsylvania, 

 including the northern margin of the coal field, near the Ohio line, the 

 formation averages about 250 feet, more or less, in thickness. South- 

 west of Broad Top. on the Potomac River, the section is somewhat 

 thicker, and from that point the PottsviUe shows a generally, though 

 not invarialily. increasing thickness until we reach the Kentucky- 

 Virginia border, wiiere it })ro))ably exceeds 2.500 feet. 



VARIATION IN THE CONSTITUENT TERRANES OF THE 

 FORMATION. 



It necnls ]>ut a (•onq)aris()n of the carefully measured, detailed colum- 

 nar' sections of diamond-drill bore holes and of tunnels, published in 

 Pts. IV and IV B of the Atlas of the Southern Anthracite Field, to 

 demonstrate not only the varialMlity in the thickness and composition 



