50 IRON ORES OF IRON SPRINGS DISTRICT, UTAH. 
EFFUSIVES (LATE MIOCENE). 
DISTRIBUTION AND STRUCTURE. 
The effusives occupy two main areas in the district — the Antelope 
Range area and the Swett Hills, Eightmile Hills, and Harmony 
Mountains area. There are small exposures at Upper Point, in the 
northeastern quarter, and northwest of Iron Mountain. 
There is a succession of 9 flows in the following order: 
Succession of lava flows in Iron Springs district. 
Feet. 
9. Biotite-hornblende-pyroxene andesite 200 
8. Late tuffaceous rhyolite (Antelope Range) 400 
7. Biotite daeite 300 
6. Pyroxene andesite agglomerate and breccia 100 
5. Latest trachyte 150-300 
4. Hornblende andesite breccia and agglomerate 150 
3. Later trachyte 50 
2. Early tuffaceous rhyolite 300-400 
1. Early trachyte 50-600 
Of these Nos. 1, 2, 5, and 7, extend throughout the area and for a 
number of miles to the south and west, while the rest are present only 
in parts of the district. There is little evidence of erosion between the 
successive flows, unless the absence of certain beds in different places 
be taken as such. The oldest formation forms the border of the lava 
series and inside of this successively younger flows outcrop, the latest 
in the center. Local faulting and sheets of outwash deposits have 
somewhat obscured these relations in places. 
The Antelope Range lavas occupy a broad syncline pitching to the 
northeast, while those of the Swett Hills and Harmony Mountains 
have a general eastward dip and are not folded. 
. PETROGRAPHY. 
Early trachyte. — The rocks of the early trachyte bed differ consider- 
ably in texture in different parts of the district as well as in different 
parts of the series. The color and mineralogical character, however, 
remain the same throughout the district. In general the rocks are 
dark and dense with few phenocrysts, principally feldspar, subor- 
dinate^ biotite. 
In the Antelope Range, where it is typically exposed, the main part 
of the formation is a dense dark-red or purple porphyritic trachyte 
with very few phenocrysts. The phenocrysts are mainly orthoclase, 
and to a less extent plagioclase, of the variety andesine-labradorite. 
Both feldspars show Carlsbad twinning. They are altered along 
cracks and on the borders to quartz and calcite. The groundmass is 
dense, and in some layers contains amygdules, which are partly or 
