STKATIGRAPHIC CLASSIFICATION STRUCTURAL CRITERIA 463 



description seems unnecessary. We will, therefore, proceed at once to 

 the discussion of the more obscurely manifested instances. 



Every sharp faunal or lithologic break suggests a stratigraphic hiatus 

 and, hence, an unconformity and preceding sea withdrawal.^^ If the 

 zone in which the suggested unconformity must, if present, occur can be 

 narrowed down to a few feet by continuity of faunal evidence carried 

 downward from above and upward from below the final detection of the 

 plane of unconformity is usually accomplished quickly and satisfactorily. 

 But when the fossils are scarce, or of kinds that are not very definite 

 in their age relations, then the problem is more difficult. In such 

 cases all sharp lithological changes in the undetermined interval should 

 be investigated for possible clues. Assuming unfavorable distribution 

 of conclusive fossil evidence, those unconformities which occur between 

 two lithologically similar formations are, for obvious reasons, the most 

 difficult to find. Still, many unquestionable unconformities between 

 two limestone formations have been satisfactorily located. It is to be 

 admitted, however, that in most of these cases abundant fossil evidence 

 assisted materially. Instances of directly superposed unconformable 

 shale formations without intervention of a bed of sandstone or conglom- 

 erate are more rare. When they occur the determination of the hiatus 

 is almost impossible except by proving gradual intercalation of distinct 

 lithologic units. The Woodford-Caney contact in Oklahoma, which 

 shale formations are locally separated by the Sycamore limestone, is 

 a good example. 



When two well marked faunal zones, or two persistent beds easily 

 recognized by lithic peculiarities, are traced from place to place and 

 found to diverge or converge, as the case may be, the fact almost in- 

 variably indicates the presence of an unconformity in the intervening 

 measures. The upper formation obviously must be limited below by 

 overlap structure, and the first step in locating the unconformity is to 

 find the line along which the additional beds are intercalated. All of 

 these intercalated beds may be referable to the base of the overlapping 

 superior formation, but when the lower formation has been truncated 

 by erosion one or more may belong beneath the unconformity. Again, 



52 It Is to be understood that the lithological aspect of fossils rather than their bio- 

 logical relations are referred to here. Viewed as lithologic features of the rocks, their 

 significance Is distinctly stratigraphic and useful in the same way that phosphatic peb- 

 bles, glauconite, and other mineral substances that are distinguished by peculiarities in 

 crystallization, color, hardness, and composition are employed in the identification of 

 certain beds. The stratigrapher can and often does use fossils in this limited way with- 

 out pretense to paleontological training. Of course, with such training fossils assume 

 the additional significance that enables the paleontologist to estimate the relations of 

 Isolated stratigraphic occurrences to the generalized standard time scale. 



