36 IRON ORES OF IRON SPRINGS DISTRICT, UTAH. 
Physical conditions of contact metamorphism. — The andesite in its 
molten state may be supposed to have had a temperature common 
to acidic silicate solutions under similar conditions anywhere from 
500° C. up; probably above 1,000° C. After sufficient heat had 
been communicated to the limestone to raise the temperature to 
550° C, expulsion of carbon dioxide began, and continued during 
the subsequent increase in temperature. For any given temperature 
above 550° C. expelled carbon dioxide has a definite pressure which 
must find relief, if liberation of the carbon dioxide is to continue. 
The nearly complete elimination of carbon dioxide along the andesite 
contact indicates that the pressure was not sufficient to restrain its 
exit; that the pressure was therefore small; and that continuous 
openings for escape to the surface probably existed. 
When the temperature reached 1,100° C. there probably began 
the incipient fusion of the alumina, silica, lime and magnesia, which 
remained in the limestone in about the proportions of a calcareous 
clay. The presence of lime in clay permits fusion at about this 
temperature. The pore space in the limestones, possibly increased 
by the expulsion of carbon dioxide, was eliminated, probably in part 
by pressure, but certainly and completely by fusion. Had the tem- 
perature not reached 1,100° it is possible that a porous texture 
would have resulted, such as that sometimes seen along the lime- 
stone contacts with igneous rocks, 6 which would have favored the 
introduction of new substances in the contact phase to a larger 
extent than is here observed. 
After the limestone reached its maximum temperature, probably 
somewhat below that of the andesite magma, the temperature slowly 
fell, allowing fairly coarse crystallization of both andesite and lime- 
stone. Local chilling, however, developed glass in parts of the 
fused limestone mass, and the glass remains, at least in part, to-day. 
The introduction of the ore-bearing solutions accompanied or fol- 
lowed the crystallization, and still later came the mineral-bearing 
waters from the late lavas. The minerals of these different periods 
are listed on page 85. 
SANDY PHASE OF LIMESTONE. 
The altered contact phases of limestone are often hard to distin- 
guish from a much-fractured quartzite or clayey sandstone which 
is locally exposed below the limestone aad constitutes a part of the 
same formation. The sandstone lies between the ore deposits and 
the andesite, or between the ore and the limestone. It is conspic- 
uous also in the limestone patches faulted into the andesite. In 
a Ries, Heinrich, and Kiimmel, H. B., The clays and clay industry of New Jersey: Final Rept. State 
Geologist New Jersey, vol. 6, 1904, p. 103. 
6 Barren", Joseph, The physical effects of contact metamorphism: Am. Jour. Sci., 4th ser., vol. 13, 
1902, pp. 279-296. 
