254 AGI EM OV NGS ATES ITA O TOS 
Sawatch Range on the east and the plateau of the Colorado basin on 
the west. It is a highly picturesque and mountainous region, and, 
like the San Juan Mountains to the south, has a more abundant precip- 
itation and is more Alpine in its character than other parts of the 
Rocky Mountains. 
The northern half of the eastern or Crested Butte tract is occupied 
by the southern portion of the Elk Mountains proper, whose culminat- 
ing points have an elevation of over 13,000 feet; the southeastern 
portion of that tract includes the distinct and less elevated Cement Moun- 
tain uplift. The rest of this area and the whole of the Anthracite tract 
is occupied by more or less isolated mountain peaks— Crested Butte, 
Gothic Mountain, Mount Wheatstone, etc.—-and by one prominent 
north-and-south ridge, the Ruby Range, whose higher summits rise 
between 12,000 and 13,000 feet above sea level. 
The drainage of all this area finds its way through the Gunnison 
River into the Colorado, and the greater part is carried to the latter 
stream through the southward-flowing Slate River and its tributaries. 
The towns of Crested Butte (gooo feet) and Baldwin (8750 feet), 
which are near active coal mines, are reached by branches of the Den- 
ver and Rio Grande and the Denver and South Park railroads respec- 
tively. Other towns higher in the mountains, which were founded by 
silver miners, are Gothic, Pittsburg, and Irwin. Owing to its great 
altitude and abundant precipitation, this region is more or less snow- 
bound during eight months of the year, and mining is thereby rendered 
difficult and costly. 
Geological structure — The most striking feature in the geology of 
the region is the great development of eruptive rocks which occur as 
irregular bodies cutting across disturbed and upturned strata; as 
laccolitic bodies doming up the nearly horizontal strata above a given 
horizon; as vertical and comparatively narrow dikes; to a limited 
extent as surface flows; and as a bedded series of breccias, tuffs and 
conglomerates. 
Eruptive activity was most energetic and widespread during the 
Eocene Tertiary ; it continued, however, sporadically, during later 
periods, the most recent outpourings of lava being probably of Pleis- 
tocene age. The principal rock types represented are: in the irregular 
cross-cutting masses, granite and diorite, and at a later period and in 
limited areas, rhyolite; the laccolites are mostly of porphyrite, among 
dike rocks are found diorite, porphyritic diorite, porphyrite, and quartz 
