DISTRIBUTION OF NORTH AMERICAN FORESTS. 523 



impression on the flora, are all prairie, with scattered groves of oak 

 ( Quercus gargana), and this of but one species. The Abies Donglasii, 

 Pinus ponderosa, and Thuga gigantea, are samples of this flora, fori 

 have never seen any of them growing on any land that was not made 

 from the disintegration of basaltic rocks. 



The Nevadian province is composed of nearly all the geological 

 formations common to North America, and, in accordance with our 

 views, it has a flora of corresponding variety. Nearly all the genera 

 of Eastern forests are represented, though different species are com- 

 mon. It has some very local trees, no doubt confined to geological 

 formations of a peculiar character. Of these are the two Sequoice 

 the redwood and the " big tree." The former is confined to a narrow 

 strip along the coast, the latter to the tops of the high mountains in 

 isolated groves. Exact data are wanting, but it appears from the 

 geological maps at the writer's command that the redwood is con- 

 fined to the cretaceous formation which extends from about latitude 

 34 to 40. As this is about the range of the redwood-groves, it will 

 probably be proved, on close investigation, that this tree is confined 

 to the above formation. 



The Sequoice have a peculiar interest for the students of natural 

 history, being the only living representatives of a once large and widely- 

 distributed genus now found in the tertiary beds from British Columbia 

 to California, and east to Nebraska. It appears to have been nearly 

 exterminated about the glacial epoch, and is now confined to small 

 localities that appear not to have been covered by the ice at that time. 



In the foregoing pages I have made use only of trees to illustrate 

 the affinity of plants for certain geological strata, but, should I have 

 taken the general flora, the argument would appear still more convinc- 

 ing. To do this, however, it would have been necessary to divide the 

 country into smaller regions, and to have given the geological charac- 

 ters more in detail than is at present practicable. 



Were other proofs wanting to demonstrate the intimate relations 

 existing between geological formations and the geographical distribu- 

 tion of the flora, they are close at hand in the writings of our eminent 

 botanists. Sir Joseph D. Hooker, in a lecture on the distribution of 

 the North American flora,* treats the subject upon the theory that all 

 plants originated from small centers of creation and spread by slow 

 encroachment upon the adjacent territory as fast as this was in a con- 

 dition to receive them, and that climatic influences alone limit their 

 extension. He makes four general floral regions : 



" 1. The great Eastern forest-region, extending over half the conti- 

 nent, and consisting of mixed deciduous and evergreen trees, reaches 

 from the Atlantic to beyond the Mississippi, dwindling away as it 

 ascends the western feeders of that river on the prairies. It is note- 

 worthy for the number of kinds, especially of deciduous trees and 



* "American Naturalist," xiii, 155. 



