PALEOBOTANY AND GEOLOGY KNOWLTON. 355 



botany. For example, the collections (vf the United States National 

 Museum embrace over 100,000 spcciuKMis of the impressions of 

 Paleozoic plants, whereas of those shoeing internal structure there 

 is hardly a half dozen unit trays full. In the Mesozoic and C^enozoic 

 collections belonging to the same institution there are thousands 

 upon thousands of specimens from hundreds of locaKties and horizons, 

 while of those retaining their internal structure there are so few that 

 they can ahnost be numbered in tens. 



There is another and an excellent practical reason why the impres- 

 sions of plants are, and will always rema^in, of more value to geology 

 than those exhibiting internal structure, no matter how well this 

 structure may be preserved. As soon as a plant impression is ex- 

 humed it is instantly ready for study and may be interrogated at 

 once as to the stratigraphic story it has to tell, whereas the plant 

 with the structure preserved usually shows Httle or nothing on a 

 superficial examination, and requires laborious, expensive prepara- 

 tion before it can be identified. For example — to make a personal 

 application — for the past five years I have annually studied and 

 reported on from 500 to 700 collections, each of which embraced from 

 one to hundreds of individuals, and with them have helped the geolo- 

 gists to fix perhaps 50 horizons in a dozen States. If it had been 

 necessary to cut sections of these specimens before the geologist 

 could have had his answer, it is safe to say that very little would 

 have been accomplished. 



All fossil plants must be interpreted, by and through the Living 

 flora. In the more recent geological horizons the plants are naturally 

 found to be most closely related to those iiow living, but as we proceed 

 backward in time the resemblances grow less and less, and finally we 

 fuid ourselves in the presence of floras n large percentage of which 

 are without known or clearly recognized living representatives. In 

 describing these and making them available for stratigrapliic 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 Paleozoic. 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 representa- 

 tives, 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 fossU plants as an aid to geology may 

 be briefly mentioned. There have been described from — let us say — 

 North America, ui)war(ls of 5,000 species, of which number some 



