570 F. G. CLAPP CLASSIFICATION OP PETROLEUM AND GAS FIELDS 



to the productive areas, whether the internal character changed in any 

 way so as to make it incapable of holding oil, whether the sand disap- 

 peared on the borders of the pools, or whether the areas of productivity 

 were independent of these features. 



During the first field-work done in regions of monoclinal dip, many 

 years ago, data were obtained which subsequent evidence in many fields 

 has not refuted^that is, data indicating that the gas, oil, water, and drv 



Figure 9. — Map illustrating the Occurrence of Petroleum on structural Terraces, in 

 southeastern Ohio, according to Subclass III (c) 



After Griswold and Munn. For explanations see figures 3 and 8. Contour interval, 



10 feet 



areas in a persistent sand horizon are, as a rule, independent in theii' 

 general distribution of the character or thickness of the sand. Some dry 

 holes are found, even in the center of the best pools, due to the thinning 

 out or hardening of the sand locally, and holes having sand of these char- 

 acteristics exist also in extensive dry areas; but among the hundreds of 

 dry holes drilled to the Clinton sand east of the main gas belt in Ohio 

 the great majority contain sand to all appearance as suitable in texture 

 and thickness as most of the sand in which oil and gas are found. This 



