THE STRUCTURAL CLASSIFICATION 573 



scribed by him were in the Findlay field of northwestern Ohio, where the 

 oil and gas existed in two terraces, separated by a short monocline. The 

 upper terrace yielded dry gas, the lower one yielded oil and water. While 

 structural terraces might be described under Class II, they are more prop- 

 erly a variety of monoclinal structures, and an extreme case of Subclass 

 III (a) or (5). They were named by Orton^^ "arrested anticlines," and 

 the Macksburg field of southern Ohio was cited by him as an example. 

 The terrace structure of the Macksburg field was first recognized and de- 

 scribed by Newhall in the same volume. 



During the past two decades hundreds of structural terraces have been 

 discovered in southeastern Ohio, Kansas, West Virginia, and to some 

 extent in other States, and most of them are available for oil and gas 

 development. Generally, though not always, the structure can be prac- 

 tically determined from the geology of the surface without the need of 

 borings until one is ready to make his test. Other good examples of ter- 

 race structures and relations of oil to them have been described by Gris- 

 wold and Munn in Jefferson County, Ohio,^° and figure 9 is an illustra- 

 tion of this class of structure taken from their report. 



Subclass Ill(d) — Accumulations on monoclines due to thinning out 

 or change in texture of the sand. — ^While. it has been said that texture or 

 dying out of the sand is not responsible as a rule for the exact positions 

 and limits of oil pools on monoclinal dips, there are exceptions to this 

 statement. In the Louisiana fields some of the oil and gas accumulations 

 are contained in lenticular sands, which thin out or grade into shale 

 laterally.*^ This appears to be much more frequently true in Kansas, 

 where the sands diminish in importance northward, than it is in Okla- 

 homa, where they are more persistent. In such cases the relations of oil, 

 gas, and water contained in the sands are commonly similar to their gen- 

 eral relations in any other monocline, except that the outlines of the pools 

 are bounded by the extent of the sands (see figure 6). Similar lenticular 

 sands are abundant in the California fields, where the structural relations 

 are described and illustrated by Arnold. 



Doubtless a great number of cases of this type exist ; but the best known 

 is that of the so-called Clinton sand of Ohio (in reality the Medina sand), 

 which rises from a great depth in the Appalachian basin and gradually 

 thins out as it approaches the surface in central Ohio, so that it never 

 reaches the surface, the feather edge being bounded by shale, furnishing 

 an ideal substitute for an anticline and being a repository of one of the 



39 Geology of Ohio, vol. 6, 1888, p. 94. 

 *oBull. 318, U. S. Geol. Survey, 1907. 

 "■ G. D. Harris : Bull. 429, U. S. Geol. Survey, X910, pp. 128-129. 



