THE FORESTS. 9 



found in tree form), 3 species of alder, 4 of birch, 3 of cherry, 1 of 

 plum, 4 of hawthorns, 1 of mountain mahogany, 1 of maple, 1 of 

 mountain ash, 1 of dogwood in tree form, and 1 of cascara. The 

 Rocky Mountain forests, however, are more noted for things which 

 they lack than for things which they have. Genera which are con- 

 spicuous east or west are either wholly lacking or at least very spar- 

 ingly represented here. There is no native oak, hickory, elm, walnut, 

 chestnut, sycamore, or locust. Of the many and vigorous kinds of 

 birch, alder, maple, plum, cherry, and hawthorn of the eastern and 

 western forests, there are only the few and inferior species mentioned. 



The northern Rockies have a composite flora, not only with refer- 

 ence to forests but also in the lesser vegetation. That is, few species 

 of trees seem really to be indigenous, and these are mingled with the 

 majority which have migrated in from other sources during long 

 periods of time. To get a better idea of what this means, it is neces- 

 sary to review some of the earlier conditions. 



All vegetation of the earth has been subject to periodic change 

 and influence in relation to geology. Long ages ago, 1 in the early 

 Cretaceous period, a continuous forest of conifers and cycads cov- 

 ered the North American Continent. In the late Cretaceous the 

 land became divided by an arm of the sea which extended north 

 from the Gulf to the Arctic through what is now Texas and Mon- 

 tana. During this period was the beginning of the marked differ- 

 entiation of the eastern flora from that of the West. In the East 

 are the elm, hickory, chestnut, persimmon, sassafras, tulip, Osage 

 orange, and magnolia, which are not found on the Pacific coast; in 

 the West are the Sequoias, cypress, and giant cactus, which do not 

 occur east of the Rocky Mountains. Oaks are on both coasts, but 

 not the same species of oaks; the same to a large extent is true of 

 maples, alders, pines, firs, and many others. Toward the close of 

 the Cretaceous there entered many hardwood types of modern form. 

 The eastern slope of the Rockies was more humid in the Cretaceous 

 and Tertiary than now, as one may judge by the petrified forests 

 scattered from the Yellowstone Park to Arizona, and the remains of 

 holly, oaks, elms, chestnuts, Sequoias, and other trees in the Floris- 

 sant shales of Colorado. 



In the Tertiary age the East and West were again united by land. 

 But this was a period of successive uplifts and depressions. Areas 

 became submerged and their forests destroyed. Other areas were 

 subjected to severe changes locally. Erosion and other disturbances 

 must have greatly modified the character, proportion, and distri- 

 bution of the species. 



With the advent of the glacial period great fields of ice pushed 

 southward, carrying destruction to the forests before them. Smaller 



1 Harshborgor, John W. Phytogoographic Survey of North America. Leipzig, 1911. 



