
720 STRATIGRAPHICAL GEOLOGY. — mare 
running through the limestones. In the latter cases it is evident 
that the limestone has been changed into dolomite along lines of 
joint. (p. 805); in the former, the dolomite may be due to contem- 
poraneous alteration of the original calcareous deposit by the mag- 
nesian salts of sea-water in the manner already suggested (p. 305). 
Traced to a distance the limestones are often found to grow thinner, 
and to be separated by increasing thicknesses of shale, or to become 
more and more argillaceous and to pass eventually into shale. The 
shales, too, are often largely calcareous, and charged with fossils ; but 
in some places, assume dark colours, become more thoroughly argil- 
laceous, and contain, besides carbonaceous matter, an impregnation 
of pyrites or marcasite. Where the marine Carboniferous type dies 
out, the shales may become largely bituminous, passing even into coal, 
and being then associated with sandstones, clays, and ironstones. 
The second type of sedimentation points to deposit in shallow 
lagoons, which at first were replenished from the sea, but afterwards 
appear to have been brackish and then fresh. Its most abundant strata 
are sandstones, which, presenting every gradation of fineness of grain 
up to pebbly grits, and even (near former shore-lines) conglomerates, 
are commonly yellow, grey, or white in colour, well-bedded, sometimes ~ 
micaceousand fissile, sometimes compact ; often full of streaks or layers 
of coaly matter. Next in abundance are the shales, commonly black 
and carbonaceous, frequently largely charged with pyritous impreg- 
nations, sometimes crowded with concretions of clay-ironstone. Coal 
occurs among these strata in seams varying from less than an inch 
up to several feet or yards in thickness, but swelling out in some 
rare examples to 100 feet or more. A coal-seam may consist 
entirely of one kind of coal. Frequently, however, it contains one 
or more thin layers or “ partings” of shale, the nature or quality of 
the seam being alike or different on the two sides of the parting. 
The same seam may be a cannel-coal at one part of a mineral-field, 
an ordinary soft coal at a second, and an ironstone at a third. 
Moreover, each coal-seam is usually underlaid by a bed of fire-clay 
or shale, through which rootlets branch freely in all directions. 
These fire-clays, as their name denotes, are used for pottery or brick- 
making. They are the soil on which the plants of the coal grew, 
and it was doubtless the growth of the vegetation that deprived them 
of their alkalies and iron, and thus made them industrially valuable. 
Clay-ironstone occurs abundantly in some coal-fields both in the 
form of concretions (spheerosiderite) and also in distinct layers from 
less than an inch to eighteen inches or more in thickness. The 
nodules have generally been formed round some organic object such 
as a shell, seed-cone, fern-frond, &c. Many of the ironstone beds 
likewise abound in organic remains, some of them, like the “ mussell- 
band” ironstone of Scotland, consisting almost wholly of valves of 
Anthracosia or other shell converted into carbonate of iron. 
The mode of origin of coal cannot be closely paralleled by any 
modern formation. ‘lhe nearest analogy is probably furnished by the 
