20 IRON OPES OF IRON SPRINGS DISTRICT, UTAH. 
The origin of the faults and other fractures is believed to have been 
principally due to the successive extension, shortening, and settling 
accompanying the intrusion and the cooling of the laccoliths — the 
origin cited by Spurr for some of the faults of the Tonopah district. 
The faults are tension phenomena, and are so numerous and so intri- 
cate in their intersections that it is easier to explain them by some- 
what local strains accompanying cooling than by strain on the dis- 
trict as a whole. The faults outlining the periphery of the andesite 
and the stretch fractures within the andesite may be ascribed defi- 
nitely to this cause. A rough calculation of the cubic shrinkage of a 
mass of andesite the size of the Iron Mountain laccolith in crystal- 
lizing from a glass to andesite indicates a horizontal radial shortening 
of between 200 and 500 feet, depending upon the depth assumed for 
the mass — in any case a large enough factor to be important in the 
development of fractures. 
So far as the faulting was due to the intrusion and cooling of the 
andesite, it followed the intrusion closely and is of Tertiary age. 
Other faults cut the lava flows and are therefore considerably later 
than the laccolith intrusions. These faults are, from their nature, 
probably in part due to the cooling of the lavas. 
Certain of the larger faults, and especially those having north-south 
directions, showing great extent and continuity across both igneous 
and sedimentary formations, may be caused otherwise than by cool- 
ing either of laccoliths or of lavas. They seem to belong more to the 
order of deformation producing the Hurricane fault to the east than 
to the deformation associated with local igneous action. The whole 
Iron Springs district represents a downthrow on the west side of the 
Hurricane fault. The stresses have affected large areas. There can 
be little doubt that these great faults are essentially tensional in na- 
ture, but the cause of the tensional stresses is not clear from the facts 
available in this area, nor, judging from the literature, from the facts 
available elsewhere in the Great Basin. King a first emphasized the 
parallelism of faults with folds in the Great Basin region, and Gilbert b 
suggested that the Great Basin faults are but the surface expressions 
of folds similar to those of the Appalachian Mountains. The paral- 
lelism of the faults with axial planes of folds is shown also by the work 
of Huntington and Goldthwait c on the folded and faulted district 
separating the Great Basin from the high plateaus southeast of the 
Iron Springs district, where the fault planes and displacements are 
nearly vertical and the phenomena in general are those of tension. 
The correlation of the tensile nature of the strike faults with the 
shortening of the lithosphere shown by the folds seems possible only 
« King, Clarence, Systematic geology: U. S. Geol. Explor. 40th Par., vol. 1, 1878, p. 735. 
b Gilbert, G. K., Report on the geology of portions of Nevada, Utah, California, and Arizona, ex- 
amined in the years 1871 and 1872: U. S. Geog. Surv. W. 100th Mer., vol. 3, Geology, pt. 1, 1875, p. 62. 
c Huntington, Ellsworth, and Goldthwait, J. W., The Hurricane fault in the Toquerville district, 
Utah; Bull. Mus. Comp. Zool., Harvard Coll., No. 42 (Geol. Ser., vol. 6), 1904, pp. 199-259. 
