ORIGIN OF THE IRON ORES. 77 
to have been carried as a ferrous compound of some kind, possibly 
chloride or carbonate or sulphate, and its deposition as magnetite 
required partial oxidation. At the time of the intrusion • the cov- 
ering of the laccolith was probably about 4,000 feet thick, which 
would give little opportunity for the entrance of oxygen from the 
air. The expulsion of the carbon dioxide from the limestone which 
preceded the deposition of the ore may also have aided in keeping 
out oxygen. The probable alternative source of the oxygen is that 
derived by the breaking up of the water. Ferrous chloride reacts 
with water at temperatures above 500° C, with the simultaneous 
development of magnetite, hydrochloric acid, and free hydrogen, 
as follows : a 
3FeCl 2 + 4H 2 = Fe 3 4 + 6HC1 + II,, + 77Cal. 8 
As magnetite is soluble in hydrochloric acid, there could obviously 
be no precipitation as long as this acid remained, but it is neu- 
tralized by the limestones which the solutions meet and the ore 
precipitated. Magnetite has abundantly replaced limestone in this 
district, as shown by the retention of the bedding structure of the 
limestone. Interchange of the ferrous iron with the lime would 
also precipitate the iron as iron carbonate, which is found associated 
with the magnetite. Had the iron been carried in ferric solution, 
limestone would have precipitated it as hematite. Possibly the 
hematite found in small amount in the ore may be so explained. 
Magnetite would also be precipitated by the evaporation and dis- 
persion of the solution, which would result from the lowering tem- 
perature and pressure as the solutions left the andesite. Hydro- 
chloric acid is easily volatilized with water, making it difficult to 
dehydrate a chloride without losing chlorine. So far as hydro- 
chloric acid was lost, magnetite would be deposited. Kahlenberg 6 
regards this process as entirely adequate to accomplish abundant 
precipitation of magnetite under the stated conditions. The exist- 
ence of magnetite in dikes in the andesite or as cement in quartzite 
breccias, where limestone, as an alternative precipitating agent, is 
not present, is perhaps to be explained by this method of precipita- 
tion. 
One of the conspicuous features of the contact metamorphism of 
the limestone and the metamorphism of the andesite adjacent to 
the ores is the introduction of soda. The ore of the first concentra- 
tion itself shows a dominance of soda over potassa. These facts 
suggest that soda has been introduced not only into the limestone 
during its metamorphism but also later with the iron. It would 
be easy to explain the transportation of this salt in the chloride 
nMoissan, Henri: Traitc de chimie mine>ale, vol. 4, 1905, p. 330. 
b Personal communication, Louis Kahlenberg, Univ. Wisconsin, 1907. 
