Chap. XIII.] 
KODURITE SERIES : ALTERATION. 
267 
gradually grow by the continual segregation from the circulating mangan- 
iferous solutions of fresh quantities of manganese oxide ; this being by 
preference deposited round the edges of, or in, the patches of manganese- 
ore already formed, rather than in parts of the rock in which manganese 
oxide has not yet been deposited. As a result of this irregular replace- 
ment many exposures are seen showing more or less kaolinized white 
quartz-felspar-rock or felspar-rock, with patches often many feet across 
that are black in colour due to a network of black manganese oxide, 
deposited along the boundaries of the grains of quartz and felspar, and 
in part replacing them. Here and there the replacement is complete, 
so that these black areas often contain irregularly scattered nodules 
and patches of merchantable manganese-ore, usually pyrolusite, but 
sometimes psilomelane. But the quantity of ore so formed is not usually 
sufficient to make it worth while to extract it, unless it is necessary to 
quarry the mass of rock in which it occurs as a part of the dead-w(,rk 
of the mine. Examples of this irregular replacement of the felspathic 
rocks are given in figures 85 and 86 on pages 1085 and 1086 ; and in 
fig. 4, Plate 13. 
It is difficult to explain this and many other cases of replacement 
Minerals replaced by One mineral by another in terms of chemical 
manganese-ore. equations. But the examination of many hundreds 
of microscope-sections, hand-specimens, and exposures on outcrops and 
in mines, has convinced me that it is possible for a solution containing 
manganese salts to replace with manganese oxide almost any mineral. 
Amongst the minerals I have found thus replaced are iron-ores (hematite 
and hmonite), quartz, calcite, felspar (orthoclase, microcline, and plagio- 
clase), various micas, garnets, pyroxenes, and amphiboles, and apatite. 
In the particular case of the replacement of orthoclase by manganese 
oxide noticed above, it is possible, assuming the manganese to be in 
solution as bicarbonctte — as it would be, in accordance with what has been 
written on page 266, had it suffered no alteration in the meantime — , 
to explain the interchange by the greater affinity of carbon dioxide for 
potassium oxide than for manganese oxide. This interchange might then 
be expressed by the following equation : — 
MnH2(C03)2 + 2(K20.Al203.6Si02) + 02 = 2K2C03 + 2AI2O3+ 12Si02 + H2Mn03. 
Orthoclase. 
In the presence of oxygen, probably present in solution, either in the 
manganiferous waters, or in water occupying the interspaces between 
the grains of mineral in the rock attacked, or less probably mechanically 
II c 3 
