916 



Subsurface Geologic Methods 



by solid isopachous contours. Obviously, the 900 feet of convergence 

 over the map area will have a profound effect on the form of the structure 

 at the base of the Pennsylvanian or the top of the Mississippian. The pro- 

 cedure for reducing the Pennsylvanian structure to the Mississippian is as 

 follows. 



Point a in figure 485 is the intersection of the 1,500-foot structure 

 contour and the 500-foot isopach contour. At this point the top of the 

 Mississippian is 500 feet below the Pennsylvanian datum or at an eleva- 

 tion of 1,000 feet. At point h the Mississippian is 700 feet below the 



Figure 485. Isopach data of Pennsylvanian superimposed on structural map (fig. 484) 



structure datum, and the elevation is, therefore, 800 feet. All intersec- 

 tions are reduced to Mississippian elevations in this manner, and these 

 values are then contoured, as shown in the Mississippian structure map 

 in figure 486. Now, if it is desired to determine the structure on the top 

 of the Devonian, the Mississippian isopach is superimposed on the Mis- 

 sissippian structure, as in figure 487, and the process just described is 

 repeated. The result of this step is shown in figure 488, the Devonian 

 structure map. This figure shows the original Pennsylvanian structure 

 map superimposed on the underlying Devonian structure map in order 

 that the two may be compared directly. It should be pointed out that the 

 closed Pennsylvanian structure is a northeasterly-plunging nose at the 

 Devonian horizon. The south-plunging nose on the east side of the area 



