ROCK-FORMING MINERALS ol 
wise in gneisses. It is met with also as irregular layers (often 
associated with magnetite) among schistose rocks; while 
crystals, large and small, not infrequently appear in talc- and 
chlorite-schists, and in metamorphosed limestones. Again, it 
forms independent veins of large size, associated with gabbro. 
The earthy and concretionary varieties of phosphate of lime 
are known as Phosphorite—and many of these are of organic 
origin. 
Vill, ELEMENTS 
Carbon, in the form of Graphite, is the only element which plays a 
relatively considerable part as a rock-former. Graphite is usually not 
crystallised, but sometimes it appears as flat, six-sided plates. Hardness 
=1; specific gravity=2. It is black, with an almost metallic lustre ; 
has a greasy feel; and yields a black and shining streak. It is not 
affected by acids. It occurs as a constituent (sparingly or abundantly, as 
the case may be) of many schistose rocks and slates, as in graphite-schist, 
graphite-gneiss. Now and again lenticular beds of it appear among 
schists, and not infrequently it occupies veins and other cavities travers- 
ing such rocks. It has been met with also in granite and basalt. Coal 
is sometimes converted into graphite by contact with eruptive rock, as at 
New Cumnock and near Shotts, in Scotland. 
Many minerais and rocks are rendered dark or even black owing to 
the quantity of carbonaceous matter they contain. When the carbon- 
aceous matter is quite amorphous (ze. destitute of crystalline form and 
structure) it 1s readily driven off by heating. (Pure graphite, however, 
burns only with the greatest difficulty before the blowpipe.) The amor- 
phous carbonaceous colouring matter of black marble, etc., is apt to 
become oxidised on exposure to the weather, and changed into carbon- 
dioxide, so that the rock tends to bleach and whiten. 
oe 
