356 ANNUAL REPORT SMITTTSONIAN INSTITUTION, 1912. 



1,200 are confined to the Paleozoic, perhaps 2,000 to the Mesozoic, 

 and 1,500 to the Cenozoic. During the 60 or 70 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 mdi- 

 cate instantly a definite horizon. Thus, if he find in association 

 impressions that we have named Sequoia Nordensldoldi, Thuya irder- 

 rupta, Pojmlus cuneata, etc., it is known instantly that we are dealing 

 with the lower Eocene Fort 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 withm a smgle geographic province — the Eocky Moun- 

 tain region — the several plant-bearmg formations present are charac- 

 terized as follows: The Kootenai by 120 species, the Colorado by 

 perhaps 50 species, the Dakota by 460 species, the Montana by 150 

 species, the Laramie by 140 species, the Arapahoe by 30 species, the 

 Denver by more than 140 species, the Fort Union by from 500 to 700 

 species, etc. This shows that, as Prof. J. W. Judd once said, ''We 

 stni 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." 



Just a word may be said on the economic application of strati- 

 gi-aphic paleontology. It is perhaps safe to say that never in the 

 history of American geology has there been so close an interrelation 

 and dependence of geology on paleontology as at present, and of this 

 confidence paleobotany may justly claim its full share. Thus, of the 

 even dozen of paleontologists in the employ of the United States 

 Geological Survey and covering all branches of the subject, four are 

 paleobotanists. 



Among the many subsidiary problems connected with the applica- 

 tion of paleobotany to geology, the use of fossil plants as indices of 

 past climate occupies a most important place. As the majority of 

 plants are attached to the substratum and hence are unable to mi- 

 grate like most animals when the temperature of their habitat becomes 

 unfavorable, they must either give way or adapt themselves gradu- 

 ally to the changed conditions of thek environment. Therefore, 

 fossil plants have always been accorded first place as indices of past 

 climates. "They are," as Dr. Asa Gray has said, "the thermom- 

 eters of the ages, by which climatic extremes and climate m general 

 through long periods are best measured." 



To those who have not given especial consideration to the subject, 

 the idea appeal's to obtain that climatic variations, such as now 



