SYNOPSIS OF PART II. Ixxxi 
have been some calcareous ones, deposited chemically. Sometimes 
these were pure and sometimes they were rendered impure by ad- 
mixture, at the time of deposition, with mechanical sediments, such as 
sand and clay. When this series of sediments was metamorphosed the 
pure calcareous sediments were converted direct into crystalline 
limestones, whilst the impure ones, owing to interactions between the 
calcareous matter and the sand and clay, were converted into gneisses 
with the liberation of carbon dioxide. These gneisses are the ones I 
have called quartz-pyroxene- gneisses. On the amelioration of the condi- 
tions of pressure and temperature under which these gneisses were 
formed, the carbon dioyide set free during their formation was 
able to attack the gneisses and bring about their more or less 
complete conversion into crystalline limestones, studded with accessory 
minerals, representing the constituents of the gneisses that were able to 
escape or resist this attack by carbon dioxide. In places amongst the 
original calcareous sediments, manganese oxides were chemically 
deposited, sometimes pure and at other times admixed with calcareous 
and other impurities. When these sediments were metamorphosed 
they were, if sufficiently pure, converted direct into crystalHne lime- 
stones containing manganese-ore or piedmontite, according to whether 
the manganese oxides were deposited pure or admixed with calcareous 
matter. But where the whole mass of the sediment was impure, owing 
to admixture with sand, clay, etc., the resultant rock seems to have 
been a piedmontite-gneiss. This has been subsequently attacked with 
formation of piedmontite-limestone in the same way as the quartz- 
pyroxene-gneiss has been converted into limestone without piedmontite. 
It should be noticed that for the sediments to yield a gneiss on 
metamorphism they must have contained some felspathic material. 
It is doubtful if any considerable proportion of the manganese-ores 
in crystalline limestones has been formed by the subsequent chemical 
alteration of the manganese silicate, piedmontite ; it seems more 
probable that in the majority of cases the manganese-ore was formed 
direct, by the compression of the manganese-oxide sediment originally 
deposited. Where, however, the limestones contain spessartite and 
rhodonite, as well as piedmontite, some ores seem to have been formed 
by secondary alteration ; but such ores do not seem to be of any 
commercial importance. 
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