222 J. J. STEVENSON CARBONIFEROUS OF APPALACHIAN BASIN 



interval shows only alternating shales and sandstones. In the eastern 

 Middle sandstone, conglomerate, and comparatively little slate are at the 

 south and east, but in the northerly and northwesterly basins shale ap- 

 pears abundantly in many sections, at times predominating. 



Above the Mammoth there is much greater variation. In the eastern 

 part of the Southern field conglomerate occurs only in thin beds, though 

 one section shows 37 and another 78 feet of conglomerate near the 

 Holmes. In some sections shales predominate for a long distance above 

 the Mammoth and coarse beds appear only in the higher portions. In the 

 eastern Middle, which is north from the eastern end of the Southern, 

 sandstone prevails above the Mammoth, but even in the Southern basin, 

 Beaver meadow, the proportion of slate is very large. At the north, in 

 the Big Black creek basin, the coarsest rocks are midway in the basin and 

 slates prevail on the western side. In the central part of the Southern 

 field, near Pottsville, slates predominate to the Little Tracy at 1,450 feet 

 above the Buck Mountain, the beds being from 25 to 100 feet thick, 

 while the total of conglomerate is not more than 45 feet. Southeast from 

 Tremont the interval, Mammoth to Orchard, shows fully 200 feet of 

 slate in beds 10 to 33 feet thick, but fine conglomerate is associated with 

 the Holmes, Primrose, and Orchard coal beds. The coals frequently 

 succeed or precede a bed of conglomerate. In the sections beyond Potts- 

 ville, westward, there is little, aside from slate, to the Primrose, though 

 occasionally a section shows more sandstone than slate, and even some 

 thin streaks of conglomerate. Near the origin of the northerly prong 

 sandstone is the rock for 60 feet above the Mammoth, beyond which there 

 is mostly shale to the Orchard; but above that bed for 150 feet there is 

 little aside from sandstone. Farther west, at Lykens, sandstone prevails 

 to 400 feet above the Mammoth. In the western Middle, west from the 

 line of Mahanoy City, fine sediments predominate above the Mammoth, 

 there being alternating shales and sandstones; but one section near 

 Shamokin differs from others in that region, as in 316 feet below the 

 Holmes it shows but 60 feet of slate. 



On the whole, materials are markedly coarser in the eastern parts of 

 the fields ; but even there the sections prove erroneous the opinion so long 

 prevalent that shales are lacking, while in the greater part of the western 

 areas shales are present in large proportion. The presence of so much 

 coarse material in the northern prong has interest for the geographer. 



NORTHERN FIELD 



Tn the northern part of this field the Coal Measures column is but 400 

 feet, but the length increases southwardly until, in the deep basin between 



