STRATlGRArHlC CLA!5S1F1CAT10^• DIASTROFHIC CRITERIA 399 



5,000 feet. Those exceeding this amount (Cambrian and Devonian) are 

 thought to comprise more of the sedimentary record in accessible situa- 

 tions than usual, while those of which the known amount falls consider- 

 ably beneath the average suggest longer intersystemic and intrasystemic 

 intervals of inaccessible depositional records. This suggested explanation 

 of tlie difference in the known aggregate thickness of the respective sys- 

 tems is based on the wide extent of the Warsaw, or first Tennessean inva- 

 sion, and the similarly wide extent of the Pottsville transgression, imply- 

 ing in both cases a preceding long period of baseleveling, and it finds 

 support in the well marked differentiation sustained by the southern At- 

 lantic fauna in the time between the Keokuk and the Warsaw and again 

 between the last of the Chester and the first of the Pottsville. In both 

 instances the change is greater than between the latest Devonian and the 

 early Waverlyan facies of the same fauna. Great caution, however, is 

 required in making such faunal comparisons, they being truly significant 

 only when the successive facies are derived from the same oceanic basin. 

 In my opinion a rhythmic relationship connects nearly all diastrophic 

 movements. For a few the meter is very long, for others shorter, and for 

 still others much shorter. The last may be arranged into cycles and these 

 again into grand cycles, the whole arrangement probably corresponding 

 in units to the divisions of an ideal classification of stratified rocks and, 

 so far as these go, of geologic time. The determination of the relative 

 importance of diastrophic phenomena is of course exceedingly difficult. 

 At present their classification is fraught with uncertainties, whose elimi- 

 nation offers a never-ending task to present and future generations of 

 geologists. Although prevailing interpretations vary greatly and progress 

 is much impeded by inherited erroneous opinions, we may yet claim that 

 enough has been established to give a reasonable generalized conception of 

 the order of geologic events. And as to the broader relations of the minor 

 occurrences, these, too, in many instances, are beginning to be understood 

 and used in correlation. As I shall endeavor to show in succeeding parts 

 of the present work and on future occasions, diastrophism affords a true 

 basis for intercontinental correlation of not only the grander cycles but 

 also of their subordinate stages. As yet I am unwilling to acknowledge 

 any limit to the possibilities of stratigraphic correlation. The principle 

 f)f rhythmic periodicity being recognized, it seems to me merely a matter 

 of time and close comparative study of sedimentary records and faunal 

 associations to determine the time relations of interruptions in sedimenta- 

 tion in any one section to similar interruptions in another. 



