RECENT WORK IN THE BITUMINOUS COAL FIELD OF 
PENNSYLVANIA. 
By M. R. Campbell. 
INTRODUCTION. 
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During the past three years the United States Geological Survey, in 
cooperation with the State, has been engaged in a geologic survey of 
the bituminous coal field of Pennsvlvania. The region is one in 
which considerable geologic work has already been done, and con- 
sequently particular attention has been paid to those features which 
had received least attention in the previous work. 
In this region the feature of greatest economic importance is coal, 
but petroleum, natural gas, and clay have each come to be recog- 
nized as second in value only to the great coal beds which have made 
this part of the State famous. Inasmuch as the accumulations of 
oil and gas are directly influenced by the geologic structure of the 
region, and since the economical mining of coal and clay also depends 
upon the same element, it was decided to give most attention to the 
working out in detail of the form and dimensions of the folds which 
traverse the strata in the bituminous coal field. As is well known, 
the rocks of the northern end of this field are crumpled into long, nar- 
row folds which traverse t he basin along lines rudely parallel with the 
Allegheny front and gradually decrease in magnitude from east to 
west and also from the point of the basin tow 7 ard its center in the 
southwestern corner of the State. Although these facts have long been 
understood, the exact form and irregularities of these folds have never 
been accurately determined. 
Since the geologic structure is based upon very accurate contour 
maps, in which the vertical element is generally ascertained with a 
spirit level, the determination of the attitude of the beds is a com- 
paratively easy matter. Some particularly prominent bed was selected 
as the reference stratum, and its altitude was determined at a great 
many points. From these collected data contours of equal elevation 
were drawn upon the surface of the stratum so selected, and by this 
means the size and shape of the folds are made manifest. 
Aside from this structure work, the outcrops of the coal were care- 
fully studied and correlated, and they are represented on the geologic 
maps by means of heavy lines, which show the extent of their known 
outcrops and the position which they occupy in the hillsides. 
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