ROCKS 71 
in the same way as clay-ironstone. It is sometimes used for 
making hydraulic cement. Carbonaceous Limestone contains 
a considerable quantity of carbonaceous matter; limestones 
of this kind often emit a fetid odour when rubbed or struck 
with a hammer. Aoften-stone is a siliceous limestone, from 
which calcium-carbonate has been removed in solution, so as 
to leave only the skeleton of the rock. 
Carbonaceous Rocks.—Under this division are grouped 
all accumulations of vegetable débris and the preducts of 
their destructive distillation by natural causes. The least 
mineralised accumulations are included under the head of 
Peat or Turf. ‘This might be described as a yellow, brown, 
or black aggregate of vegetable débris, interwoven, as it were, 
and more or less compressed and decomposed. Impurities 
are common, such as various earthy admixtures, ochre, 
limonite, pyrite, diatomaceous remains, etc., and the amount 
of ash after combustion is very variable. Lzguzte or Brown 
Coal is a compact or earthy mass, brown or black with a 
brown streak, and very inflammable. It differs from common 
coal in containing a greater proportion of bitumen, or the 
elements which combine with carbon to form bitumen. The 
proportion of carbon in lignite ranges from 55 to 75 per cent. 
Common Coal is a black, compact carbonaceous mass, which 
on a fresh fracture has usually a resinous lustre. It has a 
black streak; is commonly friable; is not so inflammable as 
lignite; and contains 75 to 85 per cent. of carbon. The per- 
centage of oxygen, hydrogen, and nitrogen (the elements 
which with carbon form bitumen), is lower than in lignite, but 
higher than in anthracite. 
Varieties of Common Coal.—These probably owe their distinctive 
characters to the nature of the plants or portions of plants of which they 
are composed. The chief kinds are Caking-coal, Cherry-coal, Splint- 
(Hard-, or Steam-) coal, and Cannel-coal. Caking-coal has a short, uneven 
fracture, while Cherry-coal/ breaks with a clear shaly fracture: the former 
fuses or runs together when burnt, the latter does not. These are the 
so-called “bituminous coals.” S//z7¢-coal has a cubical fracture, is not so 
shattery or friable as the bituminous coals, and is more difficult to ignite 
than these, but has greater heating-power. Cazmel-coal is smoothly com- 
pact, breaks with a conchoidal fracture, does not soil the fingers, burns 
with a clear flame like a candle, and crackles or chatters when burnt— 
hence the common name Parvot-coal. Anthracite (Stone- or Blind-coal) 
is the most highly mineralised form of coal, consisting almost entirely of 
