SYNOPSIS OF PART II. 
Ixxvii 
[CHAPTER XIII.] 
The Kodurite Series— concluded. 
The alteration of the series and. the formation of the manganese-ores. 
In this section evidence is giten to show that the manganese-ores 
were formed subsequent to the eruption of the kodurite series by the 
action of aqueous solutions percolating through the masses of rock. 
The various changes brought about are supposed to have been effected 
by solutions containing dissolved carbon dioxide and alkaline 
carbonates. The key to the changes that have taken place lies in the 
following equation : — 
K20.Al203.6Si02 + C02 + 2H20 = 2H20.Al203.2Si02 + K2C03 + 4Si02. 
Orthoclase. Kaolin. 
By this reaction the felspar gets converted into lithomarge or kaolin, 
and a solution of colloidal silica in water containing alkaline carbonates 
and carbon dioxide results. 
In many cases, probably owing to attack by these carbonated 
alkaline waters, manganese has been taken into solution from the 
garnets and pyroxenes, doubtless as bicarbonate, and carried to another 
part of the rock-mass. Here, probably owing to the saturation of the 
solution with manganese bicarbonate, the manganese has been deposited 
so as to replace all the minerals of the rock, except those containing 
manganese, which have in some cases remained fresh and unaltered, and 
in others have also been broken up. Thus the manganese-ore resulting 
from the replacement of kodurite consists sometimes of compact psilo- 
melane studded with bright red or orange spandite -garnets, and at other 
times entirely of manganese-ore. In cases where the mineral replaced 
by the manganese-ore is felspar the reaction can be explained as 
follows : — 
MnH2(C03)2 + 2(K20.Al203.6Si02) + 0 
Orthoclase. 
:=2K2C03 +2Al203+12Si02 -l-H2Mn03. 
The result of the actions represented in both the equations given 
above is the passage into solution of silica in the colloidal form. This 
has often been subsequently deposited in the form of either chert or opal , 
with a complete replacement of the rock where the former is deposited 
and a partial replacement where the latter is deposited. 
