No. 544] BOTANICAL SOCIETY OF AMERICA 211 



the resemblances grow less and less and finally we find our- 

 selves in the presence of floras a large percentage of which 

 are without known or clearly recognized living represen- 

 tatives. In describing these and making them available 

 for stratigraphic use it has been necessary to give them 

 generic and specific names, after the analogy of the living 

 floras, so that we may have convenient handles by which 

 to use them. Many of these are confessedly what may 

 be called genera of convenience, such, for example, being 

 many of the genera of the so-called " ferns" of the Paleo- 

 zoic, Some — but especially botanists— unfamiliar with 

 the geological use of fossil plants, have argued that it is 

 unsafe, or even actually unwise to venture to give names, 

 not only to those without living representatives, but even 

 to those obviously belonging to living groups. A reply 

 to this objection seems unnecessary in view of what has 

 been said. 



The practical application of fossil plants as an aid to 

 geology may be briefly mentioned. There have been de- 

 scribed from— let us say— North America, upwards of 

 5,000 species, of which number some 1,200 are confined to 

 the Paleozoic, perhaps 2,000 to the Mesozoic, and 1,500 to 

 the Cenozoic. During the sixty or seventy years that 

 this information has been accumulating it has developed 

 that certain species or other groups enjoy a considerable 

 time range, and therefore are of little value in answering 

 close questions of age, while others are of such limited 

 vertical distribution that their presence may indicate in- 

 stantly a definite horizon. Thus, if hq^nd in association 

 impressions that we have named Sequoia Nordenskioldi, 

 Thuya interrupta, Populus cuneata, etc., it is known in- 

 stantly that we are dealing with the lower Eocene i ort 

 Union formation, since not one of these species, together 

 with several hundred others, has ever been found outside 

 this horizon. Innumerable other concrete examples could 

 of course be given, though hardly necessary, yet it may 

 be instructive to note that within a single geographic 

 province— the Rocky Mountain region— the several plant- 



