Page:Popular Science Monthly Volume 23.djvu/539

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DISTRIBUTION OF NORTH AMERICAN FORESTS.

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 common. It has some very local trees, no doubt confined to geological formations of a peculiar character. Of these are the two Sequoiæ—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 confined 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 Sequoiæ 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 convincing. To do this, however, it would have been necessary to divide the country into smaller regions, and to have given the geological characters more in detail than is at present practicable.

Were other proofs wanting to demonstrate the intimate relations existing between geological formations and the geographical distribution 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,[1] 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 condition 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 continent, 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 noteworthy for the number of kinds, especially of deciduous trees and

  1. "American Naturalist," xiii, 155.