PRINCIPLES OF STRATIGRAPHIC CORRELATIONS 537 



di tion is indicated by most of the overlaps found on the flanks of the 

 Cincinnati and Nashville domes. (See pages 305 to 307 and 416 to 419.) 

 Examples of surface irregularity due to solution are seen in the sink- 

 holes in the Ozarkian rocks of southern Missouri. Some of these 

 solution cavities were filled by Sylamore sandstone, others by Osage 

 limestones, and more of them by later Tennessean (Carterville) and 

 Pennsylvanian (Brentwood and Cherokee) deposits. 



(4) Conspicuously irregular contacts in interior areas, between Pale- 

 ozoic formations in particular, denote relatively short periods of emer- 

 gence, because long emergence tends to baseleveling and consequently to 

 smoothed surfaces that are often difficult to distinguish from ordinary 

 bedding planes. Examples mentioned in the preceding paragraph will 

 serve also in illustrating this principle. 



The time value of a stratigraphic hiatus is usually suggested and 

 often clearly indicated by fossil evidence, the local sequence of faunas 

 being compared with the composite standard. However, as this com- 

 posite faunal record is far from complete and ever in need of corrobora- 

 tion and emendation, it is always desirable to procure all possible strati- 

 graphic evidence that may have a bearing on the problem. Most of the 

 recent additions to the time scale have been occasioned by such strati- 

 graphic investigations. 



(5) The local time value of a stratigraphic hiatus is determined pri- 

 marily by the number and respective values of the intercalated beds or 

 formations that are found on tracing the unconformable contact in the 

 direction from whence the overlap proceeds. Obviously the time value 

 of the hiatus increases as the overlap advances and decreases in the 

 opposite direction. In the latter direction, as shown in figure 17 D, on 

 page 450, the hiatus may split up into a number of smaller hiatuses ; or 

 when it is followed in the locally prevailing direction of overlaps — that is, 

 shoreward — two or more planes may converge into a single, correspond- 

 ingly more important, unconformity. 



In the case of formations overlapping two or more preexisting em- 

 bayments, or a folded and subsequently baseleveled area (see figue 17 A, 

 page 450), the time value of the hiatus varies from place to place. In 

 all cases the maximum determinable value of the hiatus, so far as this 

 can be established in the accessible area covered by the overlapping 

 formation, is the aggregate value of the beds comprised between the 

 base of the oldest and the top of the youngest of the intercalated forma- 

 tions. The complete solution of the more important problems generally 

 involves also the next principle. 



XXXVI— Bull. Oeol. Sor. Am., Vol. 22. 1910. 



