STRATIGRAPHIC CLASSIFICATION STRUCTURAL CRITERIA 449 



The following remarks, therefore, will be confined so far as practicable 

 to the criteria themselves. 



Less definite in their bearing, and of use only in broader correlations, 

 are those phenomena which indicate periods of rejuvenated orogenic 

 activity. Having devoted considerable space in preceding chapters to 

 the discussion of the earth movements which occasioned them, this phase 

 of the subject may be passed without further comment. Moreover, since 

 these periodic activities are chiefly indicated by consequent quickening of 

 erosional processes, and as matter pertaining also to this aspect of the 

 subject is presented in the introductory chapters comprising Part I, it is 

 thought advisable to defer additional consideration of such criteria to a 

 more appropriate succeeding chapter on degradational and lithological 

 criteria. 



PROGRESSIVE OVERLAP OF MARINE DEPOSITS 



Rate of progress. — The criteria of progressive overlap are primarily 

 such stratigraphic phenomena as indicate landward spreading of succes- 

 sive zones of deposition in an advancing sea. Obviously the rate of prog- 

 ress depends on the relative steepness or gentleness of the slope of the land 

 in course of submergence. If steep the successive stages of the overlap 

 as registered by the landward progression of each layer beyond the next 

 preceding is correspondingly slow and small in horizontal dimensions ; if 

 very gentle the younger layer spreads with proportionate rapidity and in 

 extreme cases may extend great distances beyond the limits of the next 

 older stratum. Obviously again in the case of long and relatively narrow 

 troughs the longitudinal progress of the overlap is proportionately much 

 more rapid than the transverse advance. Figures A and C of the accom- 

 panying sketches illustrate relatively rapid transgression of overlapping 

 formations, while in D and E, more especially the latter, the overlaps 

 progress much more slowly. The Boone at the top of D is a widely 

 transgressing formation; hence relatively rapid in overlap. 



Most of the Cambrian, Ozarkian, Silurian, and early Devonian forma- 

 tions in the several depositional troughs of the Appalachian Valley con- 

 stitute excellent examples of rapid longitudinal and slow transverse ad- 

 vance of overlapping deposits. Some of these formations, notably a 

 reddish middle Cambrian shale, the Copper Eidge chert, and the typical 

 Clinton, extend northwardly from Alabama and Tennessee to or beyond 

 southern Pennsylvania, while their present distribution in an east and 

 west direction is commonly confined to strips less than 75 miles in 

 width. The Arctic invasions of the relatively flat median areas of the 



