THE STRUCTURAL CLASSIFICATION 569 



Ivrotwithstancling the old definition, however, a new term — homocline— 

 \\as introduced by Daly/^ to apply to monoclinal folds (those having a 

 single obliqne inclination). This term lias apparently been tacitly 

 adopted by some j^etrolemn geologists; but since the term monocline, 

 meaning the same Ibing, appeared in our original classification, and has 

 been generally applied before and since, no change is being made her^. 

 Ttiis term monocline was proposed by \Y. B. and H. D. Eogers in 1812, 

 and since oil men have, by the geologists' persistent efforts, finally been 

 educated to its use, the term homocline, while valuable in a scientific 

 geologic sense, would in this classification accomplish no practical results. 

 Monocline is used by Anderson and Pack in a recent bulletin,^* where it 

 is defined, as "in conformity with general usage, to mean a successioji of 

 beds dipping in one direction.'' 



A monoclinal dip is seldom, if ever, perfectly uniform for many niiles 

 continuously, and it commonly has many changes of dip in short dis- 

 tances. Figures 8 and 9 are examples of monoclinal structure in Ohio, 

 where one of the best known monoclines exists, the "lay" of the produ(^tive 

 sands being represented by structure-contour lines. The rate of 'dip 

 ranges from 20 to 200 feet j^er mile, according to locality. The steeper 

 dips are generally confined within small areas, while the gentler ones are 

 frequently uniformly continuous for many miles. 



Judging by our detailed surveys, the evidence seems conclusive that, the 

 oil has been widely disseminated in the porous strata, and ultimately 

 accumulated at fa^()rable positions where the regularity of the dip is in- 

 terrupted locally. Gas, in such cases, has collected on the up-dip side, 

 where the sand is interrupted by pinching out, according to Subclass 

 II (ri), or by local flattening, according to Subclass 11(c). Oil has I col- 

 lected on the do'\\m-dip side, generally where the change in rate of dip is 

 most pronounced. In the Bremen pools of Ohio the most productive oil 

 wells are situated at the points of greatest change in the rate of dip. 

 Since the sand in those pools is perfectly dry, the accumulations are pre- 

 sumably due to catchment of the descending oil by these interruptions 

 during the process of lowering of the original water level in the sand. 



Cause of monoclinal accumulations. — In searching for the cause of the 

 accumulations of oil such as the Bremen, Straitville, Junction City, 

 Mingo, Cadiz, and other pools in strata of monoclinal dip in Ohio, and 

 in similar but less familiar fields in Kansas and Oklahoma, the first step 

 •was the collection of numerous well records and data from which to de- 

 termine whether the porous character of the productive sands was limited 



..''SR. W.-Daly: Canada. Dept. of Mines, Geol. SuFvey Memoir 68, p. 53^. - ' _. .•^.. 

 "'Robert Andevsou and Roberl W. Pack : Bull. 60:^, U. S. GpqI. Sru-vey, 1915, p. 109, 



