10 GEOLOGY OF SW. IDAHO AND SE, OKEGON. [bull. 217. 
and the mountain mahogany in certain favored ranges pines and firs 
appear and occasionally form valuable forests. The scarcity of trees 
throughout the arid portions of the two States visited is indicated by 
the fact that along the route traveled pines and firs are only seen at 
a distance. It is almost literally true that a traveler over the route 
referred to, about 600 miles in length, is never beneath the shade of 
a tree, unless it is one which has been planted and irrigated. The 
Lombardy poplar is usually chosen as a shade tree in villages and 
about farm houses and to serve as a wind-break on the borders of 
orchards and gardens. So adverse to tree growth are the climatic 
conditions that in the region covered by the reconnaissance there are 
only two areas to be seen which are dark with forests. 
Castle Rock, a x)rominent crag situated in the northwest portion of 
Malheur County, Oreg. , is on the southern border of an extensive for- 
est of pines, firs, juniper, mountain mahogany, and other trees, and 
from its commanding summit the mountains to the northwest are seen 
to be black with evergreens. In autumn the somber tone of the 
mountains is relieved by dashes of golden yellow, mostly in the gulches 
on the lower mountain slopes, which mark groves of aspens. Again, 
from the hills and mountains near Harney and Burns, in Harney 
County, Oreg., a splendid forest may be seen mantling the gradually 
ascending mountain to the northwest. It is in this extensive forested 
region that Silvies River has its source. One of the most noticeable 
features of this forest, apart of which has recently, and very judiciously, 
been set apart as a forest reserve, is the absence, so far as can be 
judged from distant views, of burned areas within it. On account of 
the remoteness of the forests of eastern Oregon from railroads, they 
have not as yet been encroached upon by lumbermen and still preserve 
their primeval beauty. 
One interesting feature in connection with the distribution of trees 
on the mountains which rise from an arid region is the occurrence in 
certain instances of two "timber lines." There is an upper limit of 
timber growth, the true timber line, the position of which is deter- 
mined principally by the severity of the climate and especially by the 
prolonged cold and icy gales of winter. There is also a lower limit, 
determined principally by lack of moisture, below which trees are 
absent. Each of these borders of the mountain forests is in many 
instances sharply defined. On lofty mountains surrounded by arid 
valleys, when seen from a distance, there is frequently present a dusky 
band appearing not unlike the shadow of a cloud, midway up their 
rugged slopes. The band of shade crosses bold crests and deeply 
sculptured valleys, but on both its upper and lower margins is some- 
what more extended in the gorges and ravines than on the interven- 
ing ridges. The upper limit of tree growth, or the "cold timber line," 
as it may be termed, as is well known, declines in elevation when 
traced from equatorial toward polar regions. 
