832 FLORAL ZONES OF THE POTTSVILLE FORMATION. 



field, as well as in the other anthracite coal fields, the contact of the 

 Pottsville formation with the Mauch Chunk is very much more sharply 

 defined. 



As perhaps the less of two evils, I have adopted the topmost bed of 

 normal red shale in each section as the lower boundary of the Potts- 

 ville formation, thus applying- the method used under more favora))le 

 circumstances b_v the geoloo-ists who have Avorked in the bituiuinous 

 regions of Penns3"lvania. Though arbitrary and variable, since the 

 topmost beds are possibly in certain cases mere washes or redepositions 

 of the true Mauch Chunk, such a boundary line possesses at least the 

 merit that when once seen it may usually be readily recognized by 

 subsequent visitors to the locality. 



The difl'erences in the estimates of thickness, which are largely due 

 to the lack of uniformity in selecting a basal boundary for the Potts- 

 ville, will be illustrated in the discussion of the thickness of the 

 formation. 



THE POTTSVILIiE FORMATION IX THE 13AITPHIK BASIX. 



Dauphin Basin and Schuylkill-Dauphin Basin are terms applied to 

 the entire south prong of the '''fish-tail" in the western portion of the 

 Southern Anthracite field. It is a long, narrow trough extending 

 about 80 miles west from Lorberr^- Gap to within 1^ miles of the Sus- 

 quehanna River at Dauphin. From a width of about 1^ miles near 

 the eastern end and of nearh' a mile at Rattling Run, over half way 

 toward the western end, it tapers to a narrow, relatively acute apex. 

 Structurally the basin is essentially a simple close fold. The hard 

 conglomerates of the Pottsville formation, which constitute the floor 

 of the field, rise as rim walls on either side of the basin, forming the 

 axes of Stony Mountain, the northern limb of the syncline, and Sharp 

 Mountain, the southern limb. The profound erosion of the thick forma- 

 tion of soft red shales on either side of the basin causes the coal field 

 to stand out topographically as an elevated trough. The beds of the 

 north limb, Stony Mountani, whose crest dips southeast about 70", are 

 not so steep as those of Sharp Mountain, which, from a nearly A'ertical 

 attitude in the region of Black Spring Gap, become overturned at 

 Lorberry Gap to a dip of 78'^ N. 



AVestward from Black Spring Gap the Pottsville wall (l(>clines with 

 gradually lessening dip as Ave approach the apex of the field. Stony 

 Mountain presents a regular and unbroken crest. Sharp Mountain, on 

 the other hand, is cut by six V-shaped gaps to the west of Lorberry 

 Gn]). The softer interior Coal Measures have been gently eroded 

 to form a generall}- shallow, rounded interior valley in the trough, 

 which is locally more deeply cut, but still broadly rounded, by the 

 work of the small streams whicii escape through the jaws of Pottsville 

 conglomerate in the gaps. At Big Flats, over 8 miles from the west- 



