USE OF PLANTS IN EVALUATING HIATUSES 529 



above (Eaton, Denver, Dawson, Lance, etcetera) have a flora of over 700 

 species. About 90 per cent of the Cretaceous flora was wiped out by the 

 diastrophic activities believed to have occurred at the close of the Creta- 

 ceous, yet the flora in the beds above the unconformity consists of the 

 same general types, many of the genera being identical. This shows that 

 the Tertiary flora did not come in "ready made," but is presumed to have 

 been evolved on the ground from the remnants of its Cretaceous ancestors, 

 and this is interpreted as a confirmation of the value of the unconformity, 

 for it must have taken considerable time to have accomplished the evolu- 

 tion indicated. 



Method of Correlation by fossil Plants 



Having set forth above a number of general principles that may be 

 said to form the basis of stratigraphic correlation by plants, we may now 

 detail the actual practical steps that may be taken in establishing such 

 correlation. The first step to be taken in correlation by fossil plants is 

 the matching of species and genera, and, since it has been shown that 

 recurrent floras are not known, this is believed to be essentially reliable. 

 The statement made by Ulrich 2 regarding correlation by faunas is wholly 

 applicable to correlation by floras. On this point he says : 



"The degree of similarity exhibited by geographically separated faunas is 

 usually proportionate to their respective ages. If great, then the evidence is 

 provisionally accepted as indicative of essential contemporaneity." 



When a collection of fossil plants whose stratigraphic position is not 

 known is found to contain a very considerable proportion of forms iden- 

 tical with those from a known horizon, the presumption is that the two 

 are essentially identical in age. Thus, when it is disclosed that over 47 

 per cent of the flora of the Knoxville formation is also present in known 

 Jurassic beds in various parts of the world, the conclusion is justified 

 that the part of the Knoxville containing this flora is also of Jurassic age. 

 This is further emphasized by the fact that in the remainder of this flora 

 there is nothing that is incompatible with its being of Jurassic age. When 

 40 per cent of the described Denver flora is found represented in the 

 Eaton flora, it is believed to indicate essential contemporaneity. 



As a part of the process of matching fossils mention may be made of 

 so-called guide or key fossils — that is, single species that experience has 

 shown to be confined to a particular horizon or other limited stratigraphic 

 unit. Such a fossil, or preferably two or three of similar import, is of 



2 E. O. Ulrich : Bull. Geol. Soc. Am., vol. 22, 1911, p. 508. 



