80 IRON ORES OF IRON SPRINGS DISTRICT, UTAH. 
CONCENTRATION UNDER THE INFLUENCE OF TERTIARY LAVAS. 
Inference of the modification of the ores by contact of the late lavas 
is to be drawn from the topographic relations of the ores and lavas. 
The lavas at one time rested directly against the andesitic cores of the 
mountains, upon the eroded edges of limestones and other sediments, 
and upon the ores. The principal ore deposits on the lower slopes of 
the hills are exposed at about the same elevation as the lower part 
of the lavas. Erosion has cut the lavas back, so that they now fringe 
the ore-bearing areas. (See PI. II.) The heated waters associated 
with the lavas flowed along the andesite slopes where the ores occur. 
Under the influence of the lavas and accompanying solutions, limo- 
nite was dehydrated, limonite and hematite were deoxidized (a process 
easily brought about in the laboratory by passing steam over such 
ores) and all of the minerals were coarsely recrystallized, making the 
upper parts of the deposits nearest the lavas consist principally of 
coarsely crystallized magnetite and some hematite. Hot solutions 
from the lavas introduced new minerals into cavities in the ore then 
and previously formed. These minerals consist principally of quartz 
and chalcedony and to a subordinate extent of hematite, magnetite, 
siderite, limonite, chlorite, calcite, barite, galena, and the copper car- 
bonates. The heavy anhydrous minerals of the early deep-seated 
concentration were not developed. 
Immediately beneath the lavas at observed contacts with the 
Claron limestone, there has been introduced, obviously from the lava, 
an abundance of more or less iron-stained chalcedony, with a jaspery 
appearance, identical with that observed in the upper parts of the ore 
deposits. The lavas themselves contain abundant chalcedonic quartz, 
filling large and small cavities and also minutely disseminated 
through the rock so intimately as to suggest hot solutions accompany- 
ing and immediately following the cooling of the lavas. 
To test the conclusion that the chalcedony, taken to be the chief 
contribution of the extrusions, was not really deposited from meteoric 
waters, determination was made of the gas content of the chalcedony 
associated with the ore, for which the writers are indebted to Rollin 
T. Chamberlin of the University of Chicago. This analysis is com- 
pared in the following table with analyses of gas content of the andes- 
ite of the Iron Springs district by Mr. Chamberlin, one of volcanic 
emanations by Fouque, and one of crystalline rocks by Tilden. 
