FOSSIL PLANTS AS AN AID TO GEOLOGY. 367 



expressed by Professor Ward.^ It is that " Great types of 

 vegetation are characteristic of great epochs of geology, and it 

 is impossible for the types of one epoch to occur in another." 

 For example, the presence of a dicotyledonous leaf, no matter 

 how fragmentary, is proof positive that the stratum containing it 

 is Mesozoic or younger. It can not possibly be older. Again, 

 the presence of a single scar of Lepidodendron or Sigillaria, 

 when not in redeposited strata, is just as strong evidence that 

 they came from a Paleozoic horizon, since not a single specimen 

 has ever been found later than the Permian. 



The application of this principle is often of the greatest aid 

 in geology, for, as frequently happens, the strata of a region 

 have been much displaced and distorted, and it is no uncommon 

 thing to find Paleozoic rocks occupying the positions that should 

 seemingly, normally be taken by Cretaceous or Tertiary strata. 

 The stratigraphy may be so exceedingly complicated as to render 

 it quite impossible to distinguish Paleozoic from Mesozoic strata. 

 Nor can petrography be always depended upon to supply dis- 

 tinguishing marks. In such cases, which are by no means purely 

 hypothetical, a single fossil plant may serve to set at rest all 

 possibility of dispute. 



An example of this kind is furnished by the well-known case 

 of the beds of Chardonet in France, " studied by Elie de Beau- 

 mont in 1828 and positively referred to the Mesozoic, but in 

 which fossil plants of the genera Calamites, Sigillaria and Lepido- 

 dendron were identified by Brongniart."^ At that time the 

 principle under discussion had not been recognized and Bron- 

 gniart was "inclined to admit" that these genera might have 

 occurred in the Mesozoic, although long before his death he 

 recognized it and realized that the genera indicated beyond 

 question a Paleozoic age. 



Another important principle, bearing upon the limitations of 

 paleobotany, is what has been called the law of homotaxis. 



' Principles and Methods of Geologic Correlation by means of Fossil Plants. Am. 

 Geol., Vol. IX., 1892, p. 36. 



^ Ward, I.e. 



