528 F. H. KNOWLTON FOSSIL PLANTS IN GEOLOGIC CORRELATION 



lantic coast from Halifax to Cape May, a distance of over 700 miles, in 

 less than 50 years, whence Ulrich concludes that "it might at the same 

 rate encircle the globe no less than 46 times in the time-equivalent of a 

 single Paleozoic correlation unit," and he adds : "If Paleozoic inverte- 

 brates traveled only one-fiftieth or one-hundredth as fast as this living 

 shell, then we may reasonably assert essential contemporaneity for strati- 

 graphic correlation extending across the continent." 



Additional examples could be given, but it is thought that enough has 

 been presented to establish the principle that identical floras in separate 

 regions may be taken as establishing "essential contemporaneity for strati- 

 graphic correlation." Geologic time was sufficiently long to permit such 

 migration within the limits of any single correctable unit, and the migra- 

 tion movement was sufficiently fast to have accomplished this movement 

 many times over within such limit of time. For all reasonable correla- 

 tion purposes, therefore, the colonizing of identical floras in more or less 

 widely separated' areas is practically simultaneous and their stratigraphic 

 position essentially contemporaneous. 



Use of Plants in evaluating geologic Hiatuses 



It may be — doubtless is— true that the sedimentary deposition of rocks 

 has been going on continuously in some part of the globe from the time 

 of the earliest deposits we have knowledge of to the present ; but the fact 

 remains that a large part of the work of # the geologist is devoted to detect- 

 ing, describing, and properly evaluating the numerous instances in which 

 the continuity of deposition within an area was obviously interrupted. 

 Plants may be profitably interrogated as to the value of such time inter- 

 vals. Then, if the plants in the beds above an unconformity are all, or 

 even in major part, of wholly different types from those in the beds under- 

 lying the unconformity, it may mean that they existed contemporaneously 

 in a near-by area and were admitted suddenly, "ready made," as it were, 

 by the elimination of a barrier, and hence the time interval may have 

 been of short duration. If, on the other hand, the flora in the overlying 

 beds is of the same general type as that in the lower beds, differing only 

 or largely in specific character, it is interpreted as meaning that the time 

 must have been long enough for the one to have been evolved from the 

 other and within the immediate area under consideration. 



The floras in the beds just below and just above what is taken to be the 

 line separating the Cretaceous from the Tertiary in the Eocky Mountain 

 area offer a case in point. The beds below the unconformity (Montana, 

 Laramie, etcetera) contain a flora of about 350 species, while the beds 



