20 
SURVEY OF THE COLORADO OF THE WEST. 
FAULTS AND FOLDS. 
An extensive system of faults of great magnitude has been discov- 
ered, usually having a northerly and southerly direction. In many 
places these faults change either abruptly or gradually into monoclinal 
folds, and these displacements are often scores or hundreds of miles in 
length, and the throw varies from two or three hundred feet to two or 
three thousand feet. 
Some of the most wonderful and important topographic features of 
the country are due to these displacements. The materials have been 
collected for the preparation of a series of diagrams to represent the 
ever-changing throw of these faults and folds, to be accompanied by 
transverse sections exhibiting the varying characteristics which are 
revealed in tracing these long lines of displacements. 
ERUPTIVE ROOKS. 
After the sea had left this region the last time, and during the prog- 
ress of folding, eruptive matter has been poured out, trachytes and 
rhyolites at first, then basalts. In some regions this eruptive matter 
constitutes an important part of the mass of the groups of mountains 
which stand on the plateaus; this is especially the case in the group of 
mountains standing on the Mar-ka-gunt and Aquarius Plateaus. The 
Henry Mountains are composed of beds of eruptive matter overlying 
sedimentary beds. 
The U-in-kar-et group is composed of three principal mountain-masses, 
Mount Trumbull, Mount Logan, and Mount Emma, and 118 volcanic 
cones, which are distributed in part over the mountains themselves and 
scattered about the adjacent country for a distance of fifteen or twenty 
miles. The general surface of the U-in-kar-et Plateau, on which these 
mountains and cones stand, is composed of the upper strata of the Car- 
boniferous age, so that all the elevation above the general level of the 
plateau is due to the occurrence of other rocks. An examination of 
the three great mountains reveals the fact that the first floods of basalt 
were poured out here, before the beds of Triassic age had been swupt 
away. These mountains are composed very largely of variegated shales 
and sandstones of Triassic age, and the summits are great sheets of 
basalt. Then we fiud a succession of lava-floods coming on with the 
progress of erosion, always breaking out at later times and at lower 
altitudes, so that the flanks of the mountains are covered with beds of 
basalt of later age. 
It is thus seen that the eruptive matter has protected certain districts 
of sedimentary rocks from erosion, while the extension of these same 
beds over the adjacent country has been degraded by atmgspheric causes, 
and these mountains remain as evidences of a wider geographic distri- 
bution of great formations that can now only be studied within limited 
areas. There would have been no mountains had there been no floods 
