88 IRON ORES OF IRON SPRINGS DISTRICT, UTAH. 
izontal dimensions of these laccoliths with those described by Gilbert 
at the Henry Mountains, where the vertical depth also is known, leads 
to the conclusion that the depth of the " bell " of the Iron Mountain lac- 
colith (the one with outlines best determined) may not exceed 2,500 
feet. The other laccoliths are so covered with desert deposits that 
their horizontal dimensions are not known, and hence even this means 
of determining their depth is not available. Such comparison affords 
a very uncertain means of determining the depth of laccoliths in view 
of the fact that their shape and size depend so much on the nature 
and structure of the rocks intruded, as well as on the relative densities 
of the intrusive and intruded rocks. (See pp. 47-48.) 
For the deposits resting on gently-dipping slopes, such as those 
southwest of Iron Mountain, the maximum depth is likely to be found 
in the direction of dip. Vertically they rest directly upon andesite or 
upon the contact phase of the limestone, except where they are above 
ore-filled fissures in the andesite, in which case the deposit is likely 
to narrow considerably in a vertical direction when it reaches the 
fissures. 
It will be noted that on the western side of Iron Mountain the Cre- 
taceous quartzite comes directly into contact with the andesite at an 
elevation considerably higher than the limestone-andesite contact on 
the southern and eastern sides of the laccolith where the ore is exposed 
Ores are found cementing brecciated quartzite, but do not constitute 
important deposits. The lack of important ore deposits along this 
contact may be explained by the fact that replacement is not possible 
in the quartzite and that any fissure veins developed at the contact 
through the contraction of the andesite would be immediately closed 
by gravity, for the quartzite rests against the andesite with a dip of 35°. 
There is nothing in the supposed origin of the ores to preclude the pos 
sibility that ore deposits may be found beneath the quartzite along 
the limestone-andesite contacts, especially farther down on the steeper 
slopes of the andesite. If deposits are there, it may be pointed out 
that their size has not been diminished by erosion. 
The uniform association of ore with andesite laccoliths in this local 
ity, as well as in the district extending southwest to the Bull Valley 
and beyond, outlines the first rule of exploration — that the andesite 
laccoliths be found and their boundaries determined. The effect of 
the laccoliths upon topography is so marked that this may be done at 
a distance of many miles. (See p. 16.) From high points in the Iron 
Springs district it is easy to determine the southwestward extension of 
the laccolithic area, and therefore of the possible ore-bearing area. 
The laccolith determined, its contact with the adjacent sediments 
should be carefully followed. 
The location of iron-ore deposits usually makes itself evident on 
the weathered surface, but not infrequently the mass of debris which 
