STRATtGRAPHIC CLASSIFICATION DlASTROPHlC CRITERIA 411 



done by correlating negative displacements of the strandline on corre- 

 sponding parts of the several continents rather than faunas and floras. 



Regarding the slow movements that induced positive displacements of 

 the strandline, except on the polar sides of the continents, their records 

 are to a large extent clearly preserved in the accessible deposits of the 

 continental seas. Detailed stratigraphic studies in the Appalachian re- 

 gion and on the flanks of the Adirondack, Ozark, Cincinnati, Nashville, 

 and Wisconsin domes have brought out much interesting information 

 respecting their operation. 



These studies show that shifting of the strandline, which, from the 

 viewpoint of the systematic stratigrapher, is the most important of the 

 criteria of diastrophism, is recorded, organically, (1) by abrupt changes 

 in the aspect of faunas, and (2) by the sudden appearance or reappear- 

 ance of species and genera in deposits of continental seas (see examples 

 noted on pages 298, 302, and 514) ; physically it is recorded (1) by more 

 or less obvious breaks and hiatuses in the stratigraphic column indicating 

 sea withdrawals, (2) by changes in the character of deposits, especially 

 when this involves abrupt transition from biochemical and organic sedi- 

 mentation to elastics, or a change from land to marine deposition, and 

 (3) by overlaps of marine sediments. The relative elevation and subsi- 

 dence of the continents, each taken as a whole or considering only parts 

 of each, also the varying climatic and other superficial conditions, are 

 recorded by local or more or less general changes in character and volume 

 of marine and land deposits. 



The rate of transgression of stratigraphic overlaps is dependent on the 

 relative relief of the land in course of submergence. It is most rapid 

 and widest in extent when baseleveling of the lands, whether due solely 

 to subaerial erosion or assisted by general subsidence and continental tilt- 

 ing, has been most nearly accomplished; and it is partly for the same 

 reasons that the introductory submergence of a period or epoch, some- 

 times spreads farther and more uniformly than its succeeding stages 

 (see page 338). The rate and extent of the submergence, especially of 

 the eastern, western, and southern parts of northern continents, there- 

 fore affords a means of estimating the duration of the emergent inter- 

 vals. The character and volume of clastic sediments following a 

 stratigraphic hiatus and the rate of change from clastic to non-clastic 

 limestone deposition may also be used to the same end. 



Minor and local tilting — Examples. — There were many relatively local 

 changes in the strandline of continental seas that seem to have been 

 caused by correspondingly local differential vertical movements of the 



