Chap. XIII.] kodurite series : alteration. 
263 
As rhodonite does not occur to any considerable extent in the deposits 
where economically valuable ore-bodies occur, it need not be taken 
into account. Further, the fact that the composition of the other 
manganese-pyroxenes has been left undetermined does not matter ; 
for these constituents, although more abundant than rhodonite, are not 
sufficiently abundant in the majority of deposits to have given rise to 
any considerable proportion of the ore. 
The changes that have affected the other minerals, as judged by the 
products of alteration left in the deposits, are, however, well worth 
consideration. 
The quartz seems to a large extent to have escaped alteration, for it 
The replacement of t)e detected as visible grains in much of the 
quartz. lithomarge. But it may to a certain extent have 
been dissolved and re-deposited as chert by the carbonated alkaline 
waters that decomposed the felspars (see below). It has also often 
been metasomatically replaced by manganese-ore (see fig. 4, Plate 1.3). 
Almost everywhere the felspar seems to have been converted into 
The alteration of the lithomarges. These are often stained yellow, brown, 
felspar. lavender, etc., owing to impregnation with oxides 
of iron ; but they are sometimes pure white. One such white lithomarge 
was analysed by Mr. J. C. Brown (see page 1060), and, except for a little 
mechanically mixed quartz, was found to have the composition of pure 
kaolin ; and probably all these lithomarges would be round on analysis 
to approach closely to this mineral, any deviations from the composition 
of kaolin being explicable as due to the presence of oxides of iron and 
manganese and of particles of quartz. The alteration of the felspar 
may be supposed to have been produced by the action of carbonated 
waters thus : — 
K2O . AI2O3. f)Si02 ^ CO2 + 2H2O = 2H2O . AI0O3. 2Si02 + K2CO3 + 4Si02- 
Orthoi-lase. Kaolin. 
If these carbonated waters were not originally alkaline then they speedily 
became so by taking into solution the potash of the felspar. As is well 
known carbonated waters exert a powerful solvent action on silica, 
especially when it is in the gelatinous form in which it is so often liberated 
on the decomposition of silicates ; these waters hence doubtless took intu 
solution the silica released from the felspar in accordance with the above 
The formation of equation. In spots where the conditions were 
chert and opal. favourable this silica was often deposited in the 
cryptocrystalline or colloidal condition as chert or opal respectively ; 
