Part II. Scr. iv. § 2.] CARBONIFEROUS. ; 741 
_ representing the upper part of the typical formation in Central England, 
consists mainly of sandstones, shales, fire-clays, and coal-seams, with a few 
comparatively thin seams of encrinal limestone. The thickest of these 
limestones, known as the Hurlet or Main limestone, is usually about 6 
feet in thickness, but in the north of Ayrshire swells out to 100 feet, 
_ which is the most massive bed of limestone in any part of the Scottish 
Carboniferous system. One of a group of limestone beds at the base 
of the series, it lies upon a seam of coal, and is in some places associated 
with pyritous shales, which have been largely worked as a source of 
alum. This superposition of a bed of marine limestone on a seam of 
coal is of frequent occurrence in Scotland. Above these lower lime- 
stones comes a thick mass of strata containing many valuable coal-seams 
and ironstones (Lower or Edge Coals). Some of these strata are full 
of terrestrial plants (Lepidodendron, Sigillaria, Stigmaria, Sphenopteris, 
Alethopteris) ; others, particularly the ironstones, contain marine shells, 
_ such as Lingula, Discina, Leda, Myalina, Euomphalus. Numerous remains 
of fishes have been obtained, more especially from some of the ironstones 
_and coals (Gyracanthus formosus and other placoid fin-spines, Megalichthys 
Hibberti, Bhizodus Hibberti, with species of Llonichthys, Acanthodes, 
Cienoptychius, &c.). Remains of labyrinthodonts have also been found 
in this group of strata, and have been detected even down in, the Burdie 
House limestone. The highest division of the Scottish Carboniferous 
Limestone series consists of a group of sandstones and shales, with a few 
coal-seams, and three, sometimes more, bands of marine limestone. 
Although these limestones are each seldom more than 3 or 4 feet thick, 
_ they have a wonderful persistence throughout the coal-fields of central 
Scotland. As already mentioned (p. 492), they can be traced ove1 an 
area of at least 1000 square miles, and they probably extended originally 
over a considerably greater region. The Hurlet limestone with its 
underlying coal can also be followed across a similar extent of country. 
Hence it is evident that during certain epochs of the Carboniferous 
period a singular uniformity of conditions prevailed over a large region 
of deposit in the centre of Scotland. 
The difference between the lithological characters of the Carboniferous 
Limestone series, in its typical development, asa great marine formation, 
and in its arenaceous and argillaceous prolongation into the north of 
England and Scotland, has long been a familiar example of the nature and 
application of the evidence furnished by strata as to former geographical 
conditions. It shows that the deeper and clearer water of the Carbon- 
iferous sea spread over the site of Yorkshire, Derbyshire, and Lancashire ; 
_ that the land lay to the north, and that, while the whole area was under- 
_ going subsidence, the maximum movement took place over the area of 
deeper water. The sediment derived from the north during the time of 
the Carboniferous Limestone seems to have sunk to the bottom before it 
could reach the great basin in which foraminifers, corals, crinoids, and 
molluscs were building up the thick calcareous deposit. Yet the thin 
limestone bands, which run so persistently among the Lower Carbon- 
iferous rocks in Scotland, prove that there were occasional episodes 
during which the sediment ceased to arrive, and when the same species 
' of shells, corals, and crinoids spread northwards towards the land, form- 
| ing for a time over the sea-bottom a continuous sheet of calcareous ooze 
like that of the deeper water further south. These intervals of limestone 
~ growth no doubt point to times of more rapid submergence, perhaps also 
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