16 BULLETIN UNITED STATES GEOLOGICAL SUEVEY. [Yol.YI, 



form a feature where streams issue from the mountains, and are the 

 principal available shade-trees in places artificially irrigated, while their 

 soft white wood is of some account in the absence of better. The Pop- 

 lars of this kind, or the Cotton woods of the region, are: 



Populus monilifera, the Eastern Cottonwood, which reaches the eastern 

 slope of the Eocky Mountains, but probably does not cross them. 



Populus Fremontij a Californian species, a doubtful variety of which, 

 (or perhaps P. Mexicana) is the prevalent Cottonwood of the southern 

 part of the interior district. 



Populus trichocarpa, a kind of Balsam Poplar, which ranges from: 

 British Columbia to Southern California, and reaches Western Nevada. 



Populus halsamifera and its broad-leaved variety, candicans, North- 

 eastern Poplars, which reach and more or less cross the Eocky Mount- 

 ains ; and the related — 



Populus angustifolia, the common Balsam Poplar of the middle part 

 of the whole region under consideration. 



Populus tremuloides, the American Aspen, is jDcrhaps the most widely 

 distributed of North 'American trees, and economically one of the most 

 insignificant, except that the soft wood is used of late for paper pulp, 

 and in Utah it is said to be employed in turnery and for flooring. It 

 ranges from the Arctic coast to all the cooler parts of the Atlantic States, 

 through the Eocky Mountains to New Mexico and Arizona, and on the 

 western side of the continent to the middle of California. It is always 

 a small tree, fond of moist bottoms and slopes, but on the higher mount- 

 ains southward it takes to the higher ridges, and forms thick copses 

 toward the upper limit of tree growth. 



Betula occidentalis is a sparing but somewhat noteworthy element of 

 the Eocky Mountain forest along its northern border in British Columbia, 

 and is found down to Colorado and New Mexico, yet only as a shrub ; 

 also along the Sierra Nevada, where, at its southern known limit, above 

 Owen's Valley, and in a dry region bordering the Great Basin, " it is 

 reported to be abundant, and often the main reliance of the settlers for 

 timber for fencing and'other purposes." (Bot. Calif., ii, 79.) 



Prom the whole region Oaks are conspicuously absent as trees, though 

 Quercus undulata and the forms referred to it are prominent as shrubs 

 southward on the eastern slopes of the Colorado Eocky Mountains, and 

 around them into New Mexico and Arizona, and although one or two 

 Mexican types, such as Q. hypoleuca, Q. Mnoryi, and Q. reticulata, form 

 small trees in the southern portions of Arizona. 



The shrubby vegetation might be taken into account in connection 

 with the forest growth. But in this regidh, where almost everything 

 that is perennial becomes more or less lignescent, and where a predom- 

 inant part of the vegetation of the woodless districts is sufi'ruticose, the 

 herbs and shrubs may as well go together. 



Without entering here into a comparison of the Eocky Mountain forest 

 with any other, it may be noted that the species are peculiar to the 



