34 
IKON ORES OF IRON SPRINGS DISTRICT, UTAH, 
of silica. " Sample N is a contact phase of the limestone in which 
there has been conspicuous introduction of soda, silica, water, and 
iron and loss of calcite and potassa. K is an intermediate stage 
between the fresh limestone and N. 
General result of alteration. — It is concluded that in general there 
has been large loss of material from the limestones at the contact 
without conspicuous introduction of new material except soda, but 
that exceptional phases show clearly the introduction of silica and 
iron. Both normal and exceptional phases of the contact rocks are 
cut by later veins of calcite and iron ore, with associated minerals 
similar to those of the limestone contact. (See p. 85.) 
Changes in density and volume during contact metamorphism. — 
Specific-gravity determinations of the fresh and contact phases of 
the Homestake limestone are as follows : 
Specific gravities of fresh and contact phases of Homestake limestone. 
Fresh lime- 
stone. 
Contact phase. 
Average 
of fresh 
lime- 
stones. 
A. 
B. 
H. 
I. 
J. 
Average of 
5 specimens 
from con- 
tact, in- 
cluding H, 
I, J, and L. 
Average 
of all 
contact 
phases. 
Specific gravity determined from 
2.739 
2.705 
2.775 
2.706 
2.74 
2.22 
2.20 
2.93 
2.59 
2.92 
10.2 
2.73 
2.57 
2.68 
4.1 
2.76 
2.70 
2.86 
Specific gravity determined from 
hand specimens 
2.56 
2.56 
Specific gravity determined from 
2.60 
Pore space calculated from specific 
gravities of hand specimens and 
2.2 
7.1 
The difference between the gravity determined in the hand specimen 
and that in the crushed specimen measures the pore space. The 
fresh limestone when powdered is slightly soluble in water, vitiating 
gravity determinations. In this case the difference between calcu- 
lated and measured gravities measures the pore space, for there is 
little chance for error in the computed gravity. The only altered 
rock not having pore space, H, is made up largely of glass. Taking 
the glass into account, there has been little average increase of pore 
space in the contact rocks as a group, if the analyses represent proper 
proportions of contact phases. Leaving this out of consideration, 
the pore space at the contact has increased slightly, from 4 per cent 
to 10 per cent. 
The calculated specific gravities correspond fairly well with those 
determined from the powdered specimens, affording a check on the 
computation of the mineral composition from the chemical analyses. 
Sample H is again an exception because the contained glass has a 
specific gravity considerably lower than that computed from the pos- 
sible mineralogical composition based on chemical analysis. By 
