162 THE OLD COAL-MEASURES. 



trace the sands of open and exposed shores ;* in the shales 

 and fire-clays and ironstones, the muddy deposits of deeper 

 waters; and in the limestones, which also vary much in 

 composition and character, the shell-beds, the coral-growths, 

 and zoophyte drifts, both of the brackish estuary and of the 

 outer ocean. Of course, among sediments so varied we 

 may expect to find every degree of admixture sandstones 

 pure, quartzose, and compact; sandstones flaggy, laminated, 

 and clayey ; and sandstones calcareous and coaly. Shales 

 so purely argillaceous as to be termed fire-clays, shales cal- 

 careous, shales bituminous, and shales so sandy as to pass 

 into flaggy sandstones. So also it is with the limestones ; 

 some so pure as to contain scarcely a trace of earthy matters, 

 and others so mingled with earthy impurities as to be alto- 

 gether unfit for economical purposes. The ironstones, too, 

 which were merely the ferruginous muds of the Carbonifer- 

 ous sea (chemically aggregated by the union of the carbonic 

 acid given off by decaying vegetation, and the iron held in 

 solution in the waters), appear as " clay-bands" or clay- 

 carbonates, as " black-bands " mingled more or less with 

 coaly matter, or as stony impregnations too poor to be 

 worked to advantage. 



Respecting the coals, which we separate from the strictly 

 sedimentary beds, there are also many varieties both as to 

 composition and structural peculiarity. "Where the vege- 

 table mass has evidently accumulated on the spot, as peat- 

 moss, swamp-growth, and forest-growth, the seam is generally 



* Several of the thick-bedded sandstones of the British coal-fields have 

 evidently arisen, in the first instance, from JMian or wind-blown sands, 

 like those that form the " links " and " dunes " of the present day. 

 Their whole internal arrangement points to this mode of aggregation, 

 though they have, of course, been subsequently submerged and planed 

 down on their upper surfaces by the action of water. Illustrative ex- 

 amples may be seen along the eastern shores of Fife, and especially 

 between Crail and St Andrews. 



