218 PROCEEDINGS OF SECTION OC. 
Segregations of Chrome Iron Ore in Peridotite and 
Serpentine. 
It is well known that the principal occurrences of chrome 
iron ore are connected with serpentine. Formerly all these 
deposits were believed to have been produced during the 
serpentinisation of the peridotite. It is probable that some 
of the chrome iron ore which is enclosed in serpentine has 
originated in this way, because we know that certain original 
constituents of the peridotites, especially chrome-diopsid and 
chrome-spinel (picotite), contain chromium in considerable 
quantities. Chromite may be formed by the decomposition 
of chrome-spinel and silicates containing chromium in the 
same way that magnetite is formed by the alteration of 
ferro-magnesian silicates into serpentine. Still, only a 
portion of the chromite can have been originated in this 
way, for comparatively recently considerable bodies of 
chromite have been found by Vogt in quite fresh peridotite, 
and he proves most convincingly that the greater part of 
the chromite has been formed as a segregation in the 
original magma. 
Almost everything that has been said as to the mode of 
occurrence of the titanic iron ores in gabbro may also be 
said of the deposits of chromic iron ores in peridotites. Step 
by step the ‘transition may be followed from the normal 
peridotite, containing about 1 per cent. of chromite, into 
chromite peridotites containing 5, 10, 25, and 50 per cent. 
chromite, and further, into purer chromite segregations 
containing 75, 90, 95, and even 99 to 100 per cent. chro- 
mite. Moreover, microscopic examination of the transitional 
types proves that the chromite is of earlier crystallisation 
than the accompanying magnesian silicates, olivine, and 
enstatite, which proves that the chromite was formed during 
the period of cooling of the peridotite magma. 
A large deposit of chrome iron ore occurs in serpentine 
at Beaconsfield, Tasmania. It has not, however, been 
exhaustively examined from a genetic point of view.* 
Segregations of Nickeliferous Pyrrhotite in Gabbro. 
The presence of small sporadic particles of pyrites in 
volcanic and plutonic rocks is well known. Iron pyrites is 
one of the most EEN -distributed oy minerals of 
si ges the ‘ahowe was written lisse dbs have fon cine 
by Mr. W. H. Twelvetrees, F.G.S., Government Geologist, and 
his report thereon will shortly be issued by the Mines Department 
of Tasmania. 
