GEOLOGIC STRUCTURE. 37 
To illustrate the use of structure contours, we will suppose that the 
depth of the Pittsburg coal is desired at the junction of Daniels and 
Little Daniels runs, in West Bethlehem Township. As can be seen by 
the map, the elevation of the bottom of the valley at this point is about 
890 feet, and the point is very close to the 500-foot structure contour; 
therefore the Pittsburg coal is calculated to be here about 390 feet 
(890-500) below the surface. 
Degree of accuracy. — It should be borne in mind that it is impossible 
to make structure contours strictly accurate in all parts of the field. 
Over large areas there are no mines nor wells by which the exact depth 
of the coal below the surface can be determined. In such instances it is 
necessary to depend on estimated intervals between the coal and beds 
which show in outcrop, and as the intervals are in no case constant 
over any considerable area an error may be introduced which will affect 
the drawing of the structure contours. In this quadrangle, however, 
the inaccuracy is probably nowhere very great. By reference to the 
map, it will be noticed that in certain localities the contours have many 
waves and turns, while in other parts of the quadrangle they run for 
miles in long regular curves. This difference is due partly to the 
greater regularity of the folds in certain regions, but more largely to 
the greater amount of data that could be secured in such mining dis- 
tricts as Ellsworth, Peters Creek, and Chartiers Creek. In these fields 
the location of the contours was determined from mine maps showing 
the elevations of the coal in the areas which have been worked, and in 
such areas their accuracy is good. In regions where a great many well 
records are available, as in the Zollarsville field, the structure is like- 
wise more accurate than in portions of the quadrangle that lack such 
data. 
In making the studies for this map and report all roads in the quad- 
rangle 4 have been traversed and the positions of the outcropping rocks 
noted. These data have been supplemented by the records of as many 
wells and drill holes in the quadrangle as could be obtained. It is 
believed, therefore, that the structure of the territory has been deter- 
mined with a good degree of accuracy. 
Limits of error.— In general, the structure-contour interval in a 
given area is decided by two factors — (1) the steepness of the dip, and 
(2) the accuracy and abundance of the data available. The Survey is 
indebted for much valuable information to all the oil and gas opera- 
tors who have generously furnished well records, and to the coal 
operators, who have allowed the use of their mine levels. In a region 
like this, where the dips are all fairly gentle, only the second factor has 
to be considered. Obviously, it is useless to make the contour inter- 
val less than the "limit of error." For example, if over a given area 
the elevation of the datum horizon was determined to an accuracy of 
within 50 feet, it would be useless to attempt to draw contours with a 
