556 Part IV. Chapter 3. 
ranges with the yellow pine with which it is usually found associated some- 
times one preponderating; sometimes the other. It grows well at lower altitudes 
(6,000 feet) and the highest altitude at which it was noted was between 
10,500 and 11,000 feet. On the lower altitudes and along canyons and gulches 
the red fir is found mixed with blue spruce as well as yellow pine and in its 
upper limits it is often scattered among Pieca Engelmanni and Pinus Mur- 
rayana upon which is a parasite Arceuthobium Douglasit. 
Picea Engelmanni white spruce, grows as almost pure forest but commonly 
some Pinus Murrayana, P. aristata, Abies subalpina are found mixed with it. 
ÖOften extending down cool northern mountain slopes and following cold 
canyons and gulches in small numbers to 6,000 or 7,000 feet altitude, it is 
most abundant and seems most at home. It reaches its best development at 
an elevation between 10,000 and 11,000 feet covering the tops of mountains 
under timber line and forming a belt around the highest often up to 11,500 
to 11,700 feet and extending in more or less dwarfed or stunted form accord- 
ing to exposure to the highest limit reached by trees. 
Picea Parryana (= P. pungens), Colorado blue spruce, is uneven in its 
distribution and is confined to the lower altitudes where it is found along 
rivers and creeks or where the average amount of soil moisture is greater. 
It commonly occurs over the same territory occupied by Pinus ponderosa and 
P’sendotsuga which usually grow on the slopes while blue spruce more closely 
follows the water courses. As a rule it seems to range between 6,000 and 
9,000 feet, where it never occurs as pure forest, generally being scattered 
among other species. 
The Picea-Pinus Facies is typically represented in Colorado. It consists 
of two trees which constitute the facies, viz: Picea Engelmanni and Pinus 
arıstata. Pinus aristata is usually found on ridges, rocky ledges from about 
8,000 feet altitude to timber line (11,500 feet), but in favorable situations it 
may extend in twisted and dwarf specimens to ı2 ‚ooo feet. It is often the © 
chief tree on the upper parts of southern slopes of many mountains, the 
upper northern slopes being chiefly occupied by Picea Engelmanni. 
Pinus aristata is not always present as an element, according to Young, for it is absent 
almost entirely in the mountains of Boulder County, Colorado. Thuja plicata (=T. gigantea), 
the giant arbor vitae, 'a tree found generally west of the Cascade Mountains, in the Flathead 
Valley in the midst of the Rocky Mountains of Montana forms sev ige; isolated places a 
stand of trees. It is usually found in moist places (Arbor Vitae Fa 
b) Broad leaved Tree Formations'). 
Populus tremuloides Formation. The tree which constitutes the almost 
exclusive growth of this formation is found from levels of 19,99 to 
je t pages 200 and 242, the ancient presence of deciduous leaved trees in the Rocky 
Mountains has been considered, but see several recent papers viz.: COoCKERELL, T. D. A., er 
fossil Fauna and Flora of the Florissant Shales, University of Colorado, Sal III (1906): 
157—176; Some Results of the Florissant Expedition of 1908, The American Naturalist t XL 
(1908): 569—581; The Miocene Trees of the Rocky Mountains, do. XLIV (1909): 31—47- 
