536 F. H. KNOWLTON EVOLUTION OF GEOLOGIC CLIMATES 



were numerous species of insects, and from their combined study Heer 

 was able to work out many interesting conclusions, such as the succession 

 of seasons and, above all, of the climatic requirements. Concerning the 

 climate, Heer^^ wrote as follows : 



"In the Upper Miocene (of CEningen) the tropical types constitute only 7 

 per cent of the total number of vascular plants, whilst in the Lower Miocene 

 (Aquitanian) the tropical types are 15 per cent of the whole, which shows 

 that a decrease of temperature must have taken place, although the frequent 

 occurrence of the camphor- and cinnamon-trees, ^and the appearance of feather- 

 and fan-palms, demonstrate that (Eningen still enjoyed a warm climate. 



"If we sum up all the data furnished by the flora, we are led to the con- 

 clusion that the Swiss Lower Miocene district possessed a climate similar to 

 that now prevailing in Louisiana, the Canaries, North Africa, and South 

 China, namely, a climate with a mean annual temperature of 20-21° C. (68'- 

 69.8° F.), and that the Swiss Upper Miocene district had a climate resembling 

 that of Madeira, Malaga, and the south of Sicily, southern Japan, and New 

 Georgia, with an annual temperature of 18-19° C. (64-66.2° F.)." 



Pliocene floras and their climatic requirements. — The Pliocene flora 

 of North America is almost a negligible quantity, and is far too small to 

 be of value in the present connection. It consists of a small fiorule, 

 thought to be of this age, from the Falls of the Columbia Eiver. It in- 

 cludes species in the genera Woodwardia, Sassafras, Sterculia, etcetera, 

 all being extinct, though very closely related to living forms. 



A florule of 20 species, all living today in the Coast Eanges of Cali- 

 fornia, has been described by HannibaP^ from the Santa Clara formation 

 of the Coast Ranges of California. Concerning it the author says : 



"This flora agrees very well with the evidence of the aquatic MoUusca and 

 points to perceptibly colder conditions in central California during Pliocene 

 time, but makes it certain that this cold facies was due not to elevation, but 

 to actual migration of isotherms. Such a condition could not have been a 

 local phenomenon, but was probably widespread on the Pacific coast." 



In western Europe, thanks especially to the persistent labor of Clement 

 and Eleanor M. Reid, knowledge of the Pliocene flora is extensive and on 

 an established observational basis. In England the Lower and Middle 

 Pliocene are marine; hence are without plant deposits. The only beds 

 yielding plants are known as the Cromerian, or Cromer Forest beds, of 

 extreme Upper Pliocene age. Fortunately, older Pliocene beds have been 

 discovered in Dutch Limburg and adjacent Prussia, and made known by 

 the Reids^* in a splendid monograph published in 1915. The oldest of 



" Oswald Heer : The Primseval World of Switzerland, vol. 2, 1876, p. 138. 

 23 Harold Hannibal : Torr. Bot. Club BuU., vol. 38, 1911, pp. 329-342. 

 2* Clement and Eleanor M. Reid : The Pliocene flora of the Dutch-Prussian border. 

 The Hague, 1915. 



