PENNELL—SCROPHULARIACEAE OF THE ROCKY MOUNTAINS. 315 
geologic faults. It is also more evident westward that the streams, 
and temporary washes as well, have cut for themselves deep and 
precipitous chasms. The greatest of these are along the Colorado 
River, but in Utah such canyons occur along most of the lowland 
streams, The flora of the buttes and canyon walls brings down to 
the plateaus many species of the hills. Low, flat or slightly sloping 
table-topped ridges, whose steeper slopes are conspicuously covered 
by junipers, occur over much of this area, and many of the species 
which geographically are ascribed to certain valley plains grow only 
upon these ‘‘mesas.”’ 
From central Colorado, or in fact from central northern New 
Mexico, northward and westward, the basal plateau is broken by 
many mountain chains. All these are more or less isolated. Some 
are low ridges scarcely to be distinguished from the mesas except 
by their sharper contour, but some are among the highest mountains 
of the continent, having about their bases masses of foothills which 
themselves resemble mountains. From the viewpoint of plant dis- 
tribution these mountains and the highlands about them may be 
grouped in three associations: 
1. Northern Rockies.—These enter our region from the northwest 
and include all ranges north of the great ‘‘saddle”’ valley plateau of 
southern Wyoming. They include the ranges about Yellowstone 
Park, the Teton and Wind River ranges, and the outlying Bighorn 
Mountains, 
2. Southern or Ooclorada Rockies.—These include all the intricate 
chains of mountains of Colorado, extending northward into the Med- 
icine Bow Range of southeastern Wyoming, and including the out- 
lying La Sal and Abajo mountains of southeastern Utah. 
3. Wasatch or Utah Rockies.—These extend from the Bear River 
Range of southeastern Idaho southward across central Utah to the 
southwestern extremity of that State. Adjoining or somewhat out- 
lying these to the east are the Uinta Mountains, extending to the 
Colorado line across northeastern Utah, and, farther south, the 
Henry Mountains. Dr. Rydberg includes the Wasatch in the South- 
ern Rockies, but at least the Penstemon flora of each group is quite 
distinct. 
While each of these mountain groups has its peculiar species, the 
parallelism of the vegetation upon each is most striking. This 
parallelism is dependent upon altitude, and much has been written 
concerning the zones of vegetation or life zones which may be 
recognized. From the sagebrush semidesert at the base one 
ascends to a scattered low growth of junipers and pinyons; then to 
a ‘‘chaparral’’ of shrubby, gnarled oaks; then to an open belt of 
conifers, largely of Pinus scopulorum; then to a denser forest of other 
pines and Pseudotsuga, with open groves of Populus tremuloides, the 
