
" Parr Il. §vi] FRAGMENTAL ROCKS—ORGANIC. 171 
all directions. It varies from pale brown or yellow to deep brown or 
black. Some shade of brown is the usual colour, whence the name 
brown coal, by which it is often known. It contains from 55 to 75 
per cent. of carbon, has a specific gravity of 0°5 to 1°5, burns’ easily 
to a light ash with a sooty flame andastrong burnt smell. It occurs 
in beds chiefly among the Tertiary strata, under conditions similar to 
those in which coal is found in older formations. It may be regarded 
as a stage in the alteration and mineralization of vegetable matter 
intermediate between peat and true coal. 
Coal.—A compact usually britiie velvet- black to pitch-black, iron- 
black, or dull, sometimes brownish rock, with a greyish black or brown 
streak, and in some varieties a distinctly cubical cleavage, in others a 
conchoidal fracture. It contains from 75 to 85 per cent. of carbon, 

~ Wie. 29.—Microscoric Structure oF DALKEITH COAL, SHEWING LYCOPODIACEOUS 
SPORANGIA (MAGNIFIED 200 DIAMETERS). 
has a specific gravity of 1-2—1:35, burns with comparative readiness, 
giving a clear flame, a strong aromatic or bituminous smell, some 
varieties fusing and caking into cinder, others burning away to a 
mere white or red ash. 
In coal, though it consists of compressed vegetation, no trace 
of organic structure is usually apparent. An attentive examination, 
however, will often disclose portions of stems, leaves, &c., or at least of 
carbonized woody fibre. Some kinds are almost wholly made up of 
the spore-cases of lycopodiaceous plants. ‘There is reason to believe 
that different varieties of coal may have arisen from original diversities 
in the nature cf the vegetation out of which they were formed. 
Coal occurs in seams or beds intercalated between strata of 
sandstone, shale, fireclay, &c., in geological formations of Paleeozoic, 
Secondary, and Tertiary age. It should be remembered that the 
word coal is rather. a popular than a scientific term, being indis- 
eriminately applied to any mineral substance capable of being used 
as fuel. Strictly employed, it ought only to be used with reference to 
beds of fossilized vegetation, the result either of the growth of plants 
on the spot or of the drifting of them thither. 
