ORIGIN OF THE IRON ORES. 79 
phase and does not replace it, it may be inferred that the ore-bearing 
solutions were introduced into the limestone after its temperature had 
fallen below 1,000°. There is nothing to show that the temperature 
of the wall rock may not have been much lower, but the introduction 
of the ore followed so closely that the presumption is that the tem- 
perature may not have been much below 1 ; 000° C. The rock pressure 
above the laccolith, amounting at a maximum to the weight of 4,500 
feet of rock, was sufficient to hold the solution in a liquid state at 
temperatures of less than 365°. The hydrostatic pressure, which 
was probably the only effective one, was sufficient to hold the solution 
in a liquid state below temperatures of 325° C. 
It is concluded tluit the solutions were probably after-effects of 
the andesite laccolith intrusions. 
It is perhaps better to refer the solutions vaguely to "after-action" 
than to attempt more specifically to indicate whether they have 
come from the andesite itself or from some deep reservoir common to 
the andesite and the solutions. The abundant presence of magnetite 
in the andesite — 34 to 4 per cent — as so-called "reaction rims" about 
biotite, and to a less extent about hornblende, suggests a possible 
connection between the andesite and the ore-bearing solutions. The 
occurrence of magnetite in these relations has been held by Washing- 
ton a to indicate the instability of the biotite and hornblende under 
surface conditions and their breaking down into paramorphic mag- 
netite and augite, the latter being less abundant about the biotite 
than about the hornblende, and thus unimportant or altogether lacking 
in the Iron Springs rocks. By others the magnetite rims have been 
explained as due to absorption (solution) by the surrounding magma. 
The abundance of hydrogen gas (see p. 81) suggests that the develop- 
ment of the magnetite has resulted from the breaking down of water. 
But whatever the origin of the rims, it is apparent that the separation 
of the magnetite has occurred late in the cooling of the andesite. It 
would simplify matters greatly if this segregation of magnetite could 
be shown to be definitely related in some way to the ore-bearing solu- 
tions coming out of the andesite, but the writers know of no way to 
do this. 
CONCENTRATION OF ORE BY WEATHERING PRIOR TO ERUPTION 
OF TERTIARY LAVAS. 
When erosion had uncovered the ore deposits, it is reasonable to 
infer that the same processes of concentration that may be observed 
to-day were effective. These are described on pages 82-84. 
a Washington, H. S., The magmatic alteration of hornblende and biotite: Jour. Geol., vol. 4, 18S6, 
pp 257-282. 
