Conceiitratio7i. by WcatJicriug. — Kimball. 163 
tions, have yielded to the stronger base. While chemical 
action is here only in a limited sense superficial, progression 
is from without inward. Replacement of calcareous material, 
like limestones, marbles and coralline rock, with ferrous salts, 
is then effected, whence spontaneous development of ferric 
products on receding surfaces. Among the several examples 
of advanced or advancing replacement of this kind which 1 
have had opportunities to study, are several where it is com- 
plete, and others again where it is only partial. In the latter 
class the mode of occurrence is always the more obvious. 
In every case the degree of purity of the ultimate ferriferous 
products depends, of course, on relative degree of siliceous 
admixtures in the parent material. 
The common association of the higher ferric oxides in 
more or less concrete form, as on Texada and Vancouver isl- 
ands, with epidotic products from epigenesis of hornblendic 
aggregates, in which products the iron base is present as ferric 
oxide, points to ultimate or residual concentration of the same 
oxide from the secondary product by progressive epigenesis, 
and perhaps also in some ratio of original distribution of fer- 
rous oxide in the parent material. Beyond some ratio limit 
of this kind still further development of ferriferous products 
points again to extraneous or diffuse sources of ferrous solu- 
tions, and likewise to circulation of alkaline carbonates 
evolved from silicates. 
The phenomena above described, in common with others 
of the same class previously illustrated,* notwithstanding wide- 
ly varying paragenesis and environment, are, in a word, all 
incidental to fixation of stable iron oxides from penetrating 
solutions of ferrous salts by progressive oxidation, following 
as a final result from primary double decomposition with 
alkaline carbonates by processt-s more or less regenerative. 
In the memoir first cited below the cyclus referred to has 
been fully discussed from a chemical point of view. 
*Am. Jour. Sci., XLII, 1891, 231: XXVIII, 1884, 4i6. 
Am. Geologist, VIII, 1891, 352: XIX. July, 1897. 
Trans. Am. Inst. Min. Eng., XIII. 1884, 613. 
