FOSSIL PLANTS AS AN AID TO GEOLOGY. 3/1 



never, if we are to judge by the recent trend of attempts at wide- 

 spread correlation, hold the position of importance that correla- 

 tion within circumscribed areas does. The minor subdivisions of 

 the geological time-standard established for Europe, for example, 

 is found to be of only limited application in North America, and 

 attempts to bring them into complete harmony are little short of 

 wasted energy. But with limited or natural areas the case is far 

 different. 



Organic remains are unquestionably of first importance in 

 identifying formations. The study of the mineral composition 

 and lithojogical characteristics of formations must be abandoned 

 as the sole means necessary for their identification. Recourse 

 must be had to the fossils to set the stratigraphist aright, for as 

 Professor J. W. Judd has said,^ "We still regard fossils as the 

 'medals of creation,' and certain types of life we take to be as 

 truly characteristic of definite periods as the coins which bear the 

 image and superscription of a Roman emperor or of a Saxon 

 king." Of the various kinds of such remains fossil plants occupy 

 relatively as important a position as those afforded by most of 

 the other biological groups. 



It is by no means uncommon to find that fossil plants are 

 almost the only organic remains present in a formation, but if 

 they are not, the evidence they afford, when properly interpreted, 

 confirms that obtained from other groups of organic life, as the 

 following examples will show. 



As an illustration of the first mentioned condition, viz. : that 

 in which plants only are present in numbers sufficient to entitle 

 them to exclusive consideration, the Dakota group offers an 

 exceptionally fine example. This formation is four or five hun- 

 dred miles wide, more than a thousand miles long and of consid- 

 erable thickness, yet not a single vertebrate fossil, and hardly ten 

 species of invertebrates have thus far been detected throughout its 

 vast extent. The Dakota flora, however, is one of the most exten- 

 sive and thoroughly known fossil floras. According to Lesquereux^ 



'Nature, Vol. XXXVII., i888, p. 426. 

 ^ Flora of the Dakota Group, p. 14. 



