18 IRON ORES OF IRON SPRINGS DISTRICT, UTAH. 
principally by pink and bright-red colors. A basal conglomerate 
separates it from the Cretaceous sediments below. The dip of the 
Cretaceous averages about the same as that of the Tertiary, but 
there is a marked erosion unconformity between them. 
Miocene lavas and tuffs rest nearly horizontally upon the eroded 
edge of the Eocene sediments and to a less extent upon the Cretaceous. 
In the northwestern part of the area, near Antelope Springs, a 
Pliocene or Pleistocene fluviatile deposit occupies an embayment in 
the lavas. It is principally a conglomerate containing fragments of 
the lavas, of the earlier andesite laccolith, and of the sediments. 
Pleistocene and Recent lake, stream, and outwash deposits, con- 
sisting of gravels, sands, and clays derived from the erosion of all of 
the rocks of the district, occupy nearly half of the area and mask the 
rock formations of the lower ground. As a whole this material is a 
result of disintegration to a larger extent than of decomposition. It 
is accumulating with extreme rapidity. The hillsides are covered 
with it and during heavy rain storms a great mass of debris, 
including fragments many feet in diameter, creeps down the slopes. 
The finer material is carried down by the torrents and distributed in 
broad, low fans on the deficient slopes below. The apparently flat 
" desert" is made up principally of overlapping fans. An hour's 
heavy rain brings about a conspicuous modification of alluvial fans 
or other deposits at these places. The extreme rapidity of erosion 
and transportation in an arid region has often been described, but 
the amount of material moved in a few hours can scarcely be realized 
without direct observation. 
FAULTS, JOINTS, AXD FISSURES. 
That faulting complicates the elementary relations above sketched 
is apparent from an inspection of the general map (PL II, pocket). 
Fault scarps are common. Streams or canyons follow faults and 
joints, especially the former, so prevailingly that in the mapping 
faults were looked for whenever a canyon was encountered. Faults 
have been mapped only where they could be actually proved to 
exist by the relations of the rock formations, otherwise they are not 
shown on the map, even where their absence or their abrupt termina- 
tion looks structurally improbable. It is certain that many have 
been missed. 
The larger and more conspicuous faults have a prevailing north- 
south trend, but many have other directions, as summarized in fig. 3. 
At several localities, especially near the ore deposits, a tendency in 
the faults to follow about the periphery of the andesite laccoliths is 
observable. 
The fault planes are vertical, or nearly so. The displacements 
vary from vertical to horizontal— principally the former in the faults 
