REVISED PRINCIPLES OF CORRELATION 469 



But there were many deformative movements — they include, indeed, 

 most of those which affected the continental parts of the lithosphere — in 

 which the effect on the strand-line could not have been the same, either as 

 regards the several continents or different parts of the same continent. 

 If the surface of a continent was warped or tilted in any way, the dis- 

 placement of the strand-line necessarily differed in direction on different 

 parts of the continent, and on the other continents at such times either 

 advance or retreat of the coastline may have occurred. Though exact 

 correlation of these variously manifested and relatively local differential 

 movements of and within one or another of the "positive" parts of the 

 lithosphere is often exceedingly difficult, the fact yet remains that all of 

 them occasioned some displacement of the strand-line, and with this clue 

 the difficulties are never insuperable. 



Having accepted the periodic "displacement of the strand-line" as the 

 dominant criterion in determining the natural divisions of geologic his- 

 tory, we are ready to formulate the guiding principle, namely, a geologic 

 age is regarded as having closed when the marine waters are largely or 

 wholly withdrawn from one or more of the epicontinental basins, the suc- 

 ceeding new age as having opened when the sea again began to advance 

 in the same or in other basins. In the practical application of the prin- 

 ciple the local stratigraphic sequence is divided at the first plane beneath 

 any well marked faunal change that exhibits evidence of diastrophic 

 movements and consequent displacement of the strand-line. Commonly 

 the bounding plane is merely uneven, but in many instances the bedding 

 planes on either side of it are more or less distinctly discordant. Such 

 boundaries always indicate a stratigraphic hiatus or "unconformity." 

 The time value of the hiatus is usually indicated, though as a rule not 

 completely, by sediments laid down and preserved in other areas. 



The scientific and practical advantages of the diastrophic methods are 

 becoming more and more apparent as the tight grip of the intolerant 

 paleozoologist is being loosened and the physical criteria indicating trans- 

 gressions of the strand-line are given a fair trial. With the adoption of 

 the diastrophic criteria in place of the purely fossil and lithologic methods 

 of determining boundaries between deposits of distinct ages, relative cer- 

 tainty and defmiteness of delimitation supersede uncertainty and indefi- 

 niteness. We may now put our fingers on the exact boundary that for- 

 merly remained undetected and was in fact supposed to be, if not wholly, 

 non-existent, then at ]east an undiscoverable plane in a sequence of 

 transition deposits. In many places the limits of formations, groups, and 

 even series and systems, were arbitrarily drawn. I trust we have passed 

 that stage in the science of stratigraphy. 



