22 
SURVEY OF THE COLORADO OF THE WEST. 
In the sections along the Grand Canon, we are able to study three 
great periods of oceanic sedimentation, three great periods of atmos- 
pheric erosion, and three great periods of eruptive activity. 
LITHOLOGY. 
The lithology of the country has also been studied, and collections 
have been made of hand-specimens of the metamorphic crystalline 
schists and of the intrusive granites. Others have been made of the 
traps found in the dikes and intrusive beds of Silurian and Devonian 
age. 
Extensive suites of the eruptive rocks of later age, traehites, rhyolites, 
and basalts have been collected. The geological limits and relations of 
these beds have been studied carefully, and when the hand-specimens 
are carefully studied it is believed that important facts will be revealed 
from all the data that are collected. 
EROSION. 
Erosion has been a subject of much study. The region of country 
drained by the Colorado River and its tributaries is more than eight 
hundred miles in length, and from two hundred and fifty to five hun- 
dred miles in breadth. There are two distinct portions embraced in 
this great drainage basin. The lower third of the valley of the Colo- 
rado lies but little above the level of the sea, except that here and there 
ranges composed of older rocks and eruptive matter stand across the 
country, whose peaks reach an altitude of from three thousand to six 
thousand feet above the sea. 
The upper two-thirds lies from 4,000 to 8,000 feet above the level of 
the sea. The line between these two regions is often well defined : 
sometimes it is a bold escarpment, in other places it is complicated by 
folds and groups of eruptive mountains. All of the region embraced in 
the survey is in this upper portion of the valley. 
On the east, Avest, and north the rim of the basin is set with snow-clad 
mountains, reaching an altitude of from 10,000 to 14,000 feet above the 
level of the sea. The plains, benches, mesas, plateaus, and groups of 
Ioav mountains within the basin recehe but a scant supply of water from 
the annual discharge of the clouds, but all winter long snow falls on the 
mountain-crested rim, filling the gorges, half burying the forests, and 
^covering the crags and peaks Avitli a mantle of shoav. When the sum- 
mer sun comes this snow melts and tumbles down the mountain sides in 
innumerable cascades. Millions of these brooks, interrupted by cas- 
cades, unite to form thousands of creeks that run in torrents, and these 
unite to form a hundred rivers beset with cataracts, and these all unite 
to form the Colorado that rolls a mad stream into the Gulf of California. 
Let us look at the action of one of these rivers, its source in the mount- 
ains, its way through the arid plateaus and plains. If at the river’s 
