76 IRON ORES OF IRON SPRINGS DISTRICT, UTAH. 
seem to allow of but one general interpretation, and that is that the 
ore-bearing solutions were hot, rising from a deep-seated source 
through fissures in the andesite now filled with ore, at a period closely 
following the crystallization of at least the outer part of the lacco- 
lithic mass. 
That the ore was introduced after the hardening and crystalli- 
zation of the andesite and not before is shown not only by its occur- 
rence in clear-cut fissures in the andesite and by the metamorphism 
of the andesite near the ore, but also by the lack of anything in the 
nature of a basic edge in the andesite, by the lack of irregularity in 
its composition, by the absence of ore for considerable intervals 
along the andesite contact, and by the fact that the intrusion of the 
andesite metamorphosed the limestone in a clearly recognizable 
manner, recrystallizing it, decarbonating it, rendering it more sili- 
ceous, and indurated the Pinto sandstone to a quartzite spotted by 
the segregation of ferrous iron in the form of amphibole — all before 
iron was introduced. 
The ore-bearing fissures within the andesite (see p. 67) are of a 
kind which results from cooling and stretching. They are curved, 
tapered, branching, and in parallel sets, some of them minutely 
parallel and anastomosing (see PL XXI, B). The contraction 
accompanying the crystallization of the lava may also account for 
the ore-bearing fissures and faults following the periphery of the 
andesite, and affords a good explanation for the occurrence of ores 
at the contact in lens-shaped masses with their longer diameters 
parallel to the contact. The crystallization of a laccolith the size 
of the Iron Mountain laccolith from a viscous or glassy condition 
would yield a radial shortening of 200 to 500 feet, depending on the 
depth assigned to the laccolith. The parting of the andesite from 
the limestone during cooling finds its analogue in the parting of a 
casting from the mold. 
The association of the ores with heavy anhydrous silicates, char- 
acteristic of slow-cooling intrusions, eliminates the possibility of 
development from the later effusives, which develop at their con- 
tacts minerals of a different kind and association, listed on page 85. 
It is thought likely that the ore, at least the part within the zone 
of observation, was originally deposited as magnetite rather than 
as sulphide. Had it originally been sulphide, the subsequent altera- 
tion to magnetite would scarcely have left unchanged the closely 
associated silicates. The magnetites are nowhere observed to 
pass down into sulphides, although the ground-water level has not 
been reached. Until it has been reached, statements regarding the 
sulphides must be regarded as tentative. 
Assuming, then, in the absence of negative evidence, that the 
iron was deposited primarily as magnetite, the iron may be supposed 
