CHAPTER VII. 
ORIGIN OF THE IRON ORES. 
GENERAL. 
The principal ore deposits, viz, those near the contact of the 
andesite and limestone, are partly replacements of limestone. The 
original bedding of the limestone has been preserved in the ore in a 
number of places, and there is gradation between the ore and the 
limestone. These deposits are also in part fillings of fissures in 
limestone or between limestone and andesite. Where ore occurs 
within the andesite it fills fissures. The source of the iron-bearing 
solutions is the same for the limestone replacements and for the vein 
fillings in the limestone and in the andesite, for' their mineralogical 
and textural characters are the same and in a few cases they are 
actually connected. Several hypotheses as to this source have 
suggested themselves: (1) That the ore-bearing solutions were 
associated with the intrusion of the andesite as ' k igneous after- 
effects;' 7 (2) that they were meteoric waters, cold, or heated by 
contact with the laccolith, acting after the laccolithic intrusions 
and before the eruption of the surface flows; (3) that they were hot 
solutions, magmatic or meteoric or both, connected with the late 
eruptives of the district, deriving the ores from the efTusives or from 
the underlying rocks; (4) that they were cold meteoric waters later 
than the efTusives; (5) that they were due to some combination of 
these sources. The source of the ore is best explained by the first 
hypothesis, but later concentrations of the ore have occurred in the 
order named. 
CONCENTRATION AND ALTERATION. 
DEPOSITION OF ORE FOLLOWING LACCOLITH INTRUSIONS AS 
IGNEOUS AFTER-EFFECTS. 
INTRODUCTION OF THE ORE. 
The general association of the ores with the andesite and their 
specific association with fissures and faults in the andesite and the 
immediately adjacent limestones, the nature of the ores and gangue 
materials, especially the primary association of magnetite with 
garnet, amphibole, pyroxene, mica, apatite, iron sulphide, and glass, 
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