244 DISCUSSION OF THE DESICCATION QUESTION. 



seems indicated by the oaks of the Mexican type, and by species of Ficus ; 

 but this is counterbalanced by species of Betula, Fagus, Ulinus, etc., whose 

 range of distribution goes much farther north, and scarcely descends below 

 the 30th parallel. Hence, a climate like that of the Gulf shores, the zone 

 of the Live Oak, is about the same as that represented by the fossil plants 

 described from Nevada County." * 



In considering, ftirther, the evidence of changes of temperature through the 

 Cordilleras of North America, to the east of the Sierra Nevada, Ave have still 

 chiefly to depend on the researches of the same eminent fossil botanist, from 

 the report of whose examination of the Californian Pliocene flora an extract 

 has just been given. On the whole, the facts indicate decidedly a gradual 

 diminution of temperature as well as of moisture during the whole Tertiary 

 period, although there are anomalies, for which, in the present stage of our 

 knowledge of the nature of the causes influencing the distribution and range 

 of both animal and vegetable life, we have no satislactory explanation to 

 offer, but which we may reasonably encounter without surprise. In an elab- 

 orate review of the fossil flora of North America, Mr. Lesquereux thus 

 expres.ses himself in regard to the point in question : '■■ In considering the 

 distribution of the plants in the whole Tertiary of the Rocky Mountains, 

 there is evidence of a slow upheaval of the land, and a comparative dimi- 

 nution of atmospheric humidity, and consequently of a lowering of the 

 temperature." t 



Considering the distribution and relations of the Tertiary fossil plants 

 found in diiferent localities in the Rocky Mountains, Mr. Lesquei-eux divides 

 them into four groups. The lowest is the so-called Lower Lignitic, charac- 

 terized by the presence of coal beds of considerable economical importance, 

 and by some designated as of Upper Cretaceous age, by others as Eocene. In 

 this formation palms predominate, whose remains, especially those of the 

 genus Sabal, exist in profusi(m. As indicated by some of their trunks and 

 leaves, these palms were often of great size. Associated with them were 

 genera related to Soiithern rather than to Northern types, such as Ficus, 

 Cinnamomum, Magnolia, Mgrica, etc. In regard to the climate of this epoch 

 Mr. Lesquereux remarks, that the atmosphere was evidently charged with 

 a high degree of humidity, the conditions recalling those of the Carboniferous 



* 1. c, pp. 53, 54. 



t A Review of the Fossil Flora of North America. By Leo Lesquereux, Bull, of the Geol. and Geog. Surv. of 

 the Territories. Second Series, No. V. p. 13. 



