760 FLORAL ZONES OF THE POTTSVILLE FORMATION. 



the d«^liinitation of the fields in their entirety, as well as in complicat- 

 ing the minor structure of the basins. The general geolog'ic features 

 of the region are shown in the large geologic map of the State, pub- 

 lished in 18l>3, and in the county maps accompanying the descriptive 

 reports of the second geological survey of the State. The relation of 

 this region to the other anthracite regions is illustrated in the new Gen- 

 eral Map of the Anthracite Region, revised to 189(>, published sepa- 

 rately by the survey in reduced form, as one of the miscellaneous maps 

 in Pt. I of the Atlas of the Southern Anthracite Field. The geology 

 of the Southern field in particular has been worked out at great pains 

 and expense by the late geological survev of Pennsjdvania. The six 

 parts or volumes, with two supplements, comprising the Atlas of 

 this field, in which are presented in great detail the mine workings and 

 the areal geology, on a scale of 800 feet to the inch; numerous 

 cross sections, mostly on a scale of 400 feet to the inch, and a great 

 numl)er of columnar sections, chiefly at a unit of 40 feet to the inch, 

 represent the latest and most experienced work of the survey. Not- 

 withstanding the fact that the field methods were of necessit}' devel- 

 oped in the course of the work in order to meet existing conditions, 

 and the presence of many errors in correlation, the Atlas of the South- 

 ern Anthracite Field in Pennsylvania represents the most minuteh' 

 detailed and most excellent economic work on sedimentary depos- 

 its that has j'et been accomplished over an extensive area in this 

 country. To the abundant mine maps, profiles, and sections therein 

 contained reference will frequently be made. Wherever the work 

 here reported results in additions or corrections to the State mine 

 maps, they will be so described that the changes or additional matter 

 can in most cases l)e readily applied. The flexures of the held, which 

 ofl'er a most interesting study, were described at considerable length 

 by H. D. Rogers in the Geologj^ of Pennsylvania. The geographic 

 features of the ))asin are incompletely represented in Pi. CLXXX of 

 this report, which is here presented as an index to the State maps, as 

 well as for the purpose of indicating the localities at which fossils 

 have })een collected. 



On examining the maps referred to above, it will be noted that 

 in general the folding of the strata is closer toward the southern 

 border of the fleld and more opcMi to tln^ north. Thus the southern 

 limit of the l)asin, along Sharp Mountain, is somewhat overturned 

 throughout the greater part of its length, while in the widest part of 

 the V)asin, near the northern ])order, the undulation of the beds is com- 

 paratively gentle. To this is possibly due the variable and interesting- 

 topography of the district; for where the strata are more steeply 

 inclined the ridges caused by the (n-osion of the soft shales on either 

 side of a hard formation are naj-i-ow and sharp, while the hard ter- 

 ranes, when but slightly flexed and nearly horizontal, form the resist- 



