Chap. XVII.] goxdite series : coxclusioxs. 
365 
age, the metamorphism of these sediments ha\Tng taken 
place towards the end of the Dharwar period. 
2. A portion of the ores has been formed directly by the compres- 
sion of the purest of the original manganese-oxide sediments. 
3. Another portion of the ores has been formed by the subsequent 
alteration of the manganese-silicates produced by the above- 
mentioned metamorphism. 
The points that are to be considered as more doubtful are put forth 
below. Even if they do not express the whole truth, I believe that they 
form an approximation to it ; anyway, together with the points given 
above, they form a working hypothesis that will suggest to a future 
geologist the e^"idence to look for in trying to put the theory as to the 
origin of these manganese-ore deposits on a surer footing. 
4. The ores formed by the alteration of the rocks of the gondite series 
were formed by a combined decomposition and replacement 
of the gondite, spessartite-rock, or rhodonite-rock, as the case 
may be. 
5. The alteration of manganese silicates to manganese-ores took place 
at the close of the Dharwar period of folding, and henot in 
Archaean times. 
6. The alteration took place at considerable depths, so that, taken 
in conjunction with the supposition that a portion of the ores 
are merely compressed manganese- oxide sediments, work- 
able manganese-ores may be expected to extend in some 
places to as grtat a depth as the rocks of the gondite series. 
7. The alteration was due to the attack on the manganese-sUicate- 
rocks by waters containing either carbon dioxide or sulphuric 
acid, more probably the former ; and also oxygen. 
8. The carbon dioxide may have been a portion of that liberated 
in the conversion of original impure calcareous se^liments 
into the quartz-pyroxene-gneisses of this area ; and the 
oxygen a portion of that Ub era ted in the conversion of 
original impure manganiferous sediments into the manganese- 
silicate-rocks. 
9. A sma^J proportion of softish and more or less porous ore has been 
formed since the rocks of the gondite series were exposed at 
the surface, and is probably still being formed. 
