STRATIGRAPHIC CLASSIFICATION DIASTROPHIC CRITERIA 401 



That the lands about the Arctic Ocean have not been folded like those 

 bordering the Atlantic and Pacific oceans and those flanking the east- 

 west Gulf of Mexico and Mediterranean depressions, of course implies 

 decided differences in diastrophic history. Evidently, tensional stresses, 

 with crustal flow toward the equator, predominated in the circumpolar 

 regions, while compression, resulting in part from equatorial flow of 

 lands and seas, and in part from suboceanic spreading acting from the 

 water side and continental creep from the other, prevailed as a rule in 

 the folded tracts. But these differences in history are not prohibitive 

 to exact correlation of geologic events transpiring in the several regions. 

 It is highly improbable that mountain making took place at any one 

 time on all the continents; and it is certainly not true that the folding 

 observed in any mountainous tract dates all from a single period of dias- 

 trophism. To postulate a period of major diastrophic activity, we need 

 but to establish that strong folding actually occurred at such time in cer- 

 tain regions, and this proof should be sufficient warrant for the begin- 

 ning of a new geologic period. What occurred elsewhere at about the 

 same time becomes merely a matter of interesting detail — doubtless im- 

 portant in so far as it shows the extent and relative taxonomic impor- 

 tance of the deformative movements. However, this detailed informa- 

 tion is very desirable and to a large extent essential to properly define 

 and discriminate the respective periods in regions of varying dynamic 

 history. 



And is the evidence wholly satisfactory on which is based the postu- 

 late of marked diastrophism in the Atlantic provinces at the close of the 

 Paleozoic and comparative tranquillity around the Pacific at the same 

 time? Or for the supposition that quiet had supervened about the 

 Atlantic in the middle Mesozoic when the Pacific provinces were greatly 

 disturbed? Since the areas in which decided folding occurred are largely 

 confined to the marginal parts of the continents, may it not be possible 

 that tracts bearing the physical record of important diastrophic move- 

 ments are now submerged or buried beneatli more recent undisturbed 

 sediments? Finally, is the evidence on which it is inferred that the 

 folding of the Appalachian Valley was accomplished and marks the close 

 of the Paleozoic quite incontrovertible ? As will appear from later state- 

 ments (see pages 468 to 477), I think not. 



Displacement of strandline chiefly relied on in proving periodicity of 

 deformative movements. — The only thing that moves apparently in the 

 same way and degree, and which, therefore, offers the most reliable cri- 

 teria in determining the periodicity and contemporaneity of diastrophic 



