536 E. O. ULRICH — REVISION OF THE PALEOZOIC SYSTEMS 



ment of these principles is confined to those which are more or less 

 clearly suggested by criteria and examples discussed in foregoing parts 

 of this work. Though making no pretense to exhaustive enumeration it 

 is yet believed that adequate treatment has been given to all such 

 criteria that have been shown to have a practical value in modernized 

 field studies of stratigraphy. To avoid repetition the several principles 

 are illustrated so far as practicable by reference to descriptions of 

 suitable examples on preceding pages. 



The principles are divisible into two groups : (a) those based on mani- 

 fest stratigraphic relations and (&) those inferred through deductive 

 reasoning. 



Principles based on manifest stratigraphic relation — (1—8) Correla- 

 tion by unconformities, overlaps, and determinable values of hiatuses. — 

 (1) A stratigraphic unconformity indicates an interruption of the pro- 

 cess of sedimentation. Usually, too, it implies emergence and sub aerial 

 erosion during the period of such suspension. 



(2) A stratigraphic overlap similarly denotes a preceding cessation of 

 deposition and as a rule also submergence or resubmergence of the area 

 thereby transgressed. 



(3) Neither the relative discordance in dip nor the degree of ir- 

 regularity of an unconformable contact is proportional to the time value 

 of the stratigraphic hiatus. The discordance of the unconformity is 

 small or greater according to varying local or regional conditions, hence 

 all unconformities vary more or less decidedly in this respect when fol- 

 lowed from place to place. The discordance may be almost imperceptible 

 and always is smaller in the broad interior areas of flat-lying formations 

 than in areas lying within or adjacent to the inland migrating submar- 

 ginal belt of active folding (see pages 435 to 442 and figure 17, A and B, 

 page 450). The irregularity of the contact, on the other hand, depends 

 largely on the solubility of the surface rock and on such other factors 

 as surface contour and climatic conditions, all of which affect the rate 

 and method of rock decomposition and erosion. Under favorable cir- 

 cumstances a land surface may become very irregular in a geologically 

 brief time, while under less favorable conditions the efficiency of ero- 

 sional agents is correspondingly reduced. The first condition is illus- 

 trated by rapid local erosion of the Girardeau limestone at Thebes, 

 Illinois, where wave-action — an erosional agent but seldom clearly sug- 

 gested in Paleozoic unconformities — seems to have formed a cliff of this 

 limestone before its fauna was replaced by that found in the limestone 

 which usually succeeds the Girardeau in this vicinity. The second con- 



