The amounts of iron in fresh rocks and their altered equivalents have been compared by recalculating many published analyses with a view to determining whether or not the iron already present in the rock is sufficient to account for the pyrite in the altered zone.Although there are numerous instances of addition of very significant quantities of iron during alteration, as well as other examples of partial removal (at least from certain sub-zones) it would appear that in the majority of cases the change indicated by analyses was within the presumed limits of error in sampling.As might be expected, the proportion of the iron determined as ferric decreases from the fresh rock to the altered zone. Except where iron is added, the alteration results in a decrease in density.Presently available data for most districts, however, are inadequate to afford satisfactorily quantitative conclusions as to the change in the amount of iron or of any other constituent during alteration. Carefully planned sampling as carried out in Butte, Yellowknife and a very few other districts is needed elsewhere before trustworthy generalizations regarding the chemistry of rock alteration are possible.

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