Chat. I.
GEOLOGY.
631
western part of Yorkshire the series is still more varied and complicated. After the coalformation we have the millstone grit, or series of grit-stones, separated by shales, andseveral other flaggy and freestone grits, cherts, thin limestone, ironstone, and several coal-seams, 1000 feet in thickness.
Yoredale rocks are a series of five or more limestones, with many freestones, flagstones,abundance of plates, some ironstone, chert, and several coal seams, 1000 feet in thickness.
Scar I.hnestone , divided by grits and slates, with some beds of coal 800 feet in thickness.
Alternations of Red Sandstone, clay, and limestone, 100 feet red sandstones, and conglo-merates, very limited in their range and of variable thickness.
Pursuing this system in Northumberland, we find the scar limestone interfered with bythe interposition of various grits and abundance of coal. The usual thickness of coal inEngland and Scotland is about 50 or 60 feet, divided into 20 or more beds, alternatingwith from 20 to 50 or 100 times as great a quantity of sandstones and shales. Though insome districts the coal is deposited in beds above one another, with but little earthy matterintervening, in which the different beds are traceable, and possessing various qualities, pro-bably arising from the differences of the vegetable matter that comprise them, and themanner in which the accumulation has taken place.
In the coal tracts of the Tyne and Wear there is very little limestone ; in Yorkshire thetotal thickness of the coal formation is from 1000 to 1500 yards; in Lancashire a greaterthickness, whilst in South Staffordshire it does not exceed 1000 feet. The most variableare the sandstones and shales, the most regular the coal beds and ironstones.
The organic remains in the coal formation consist of many varieties of plants in fine pre-servation ; abundance of zoophytes, mollusca, Crustacea , many fishes, but, as far as has yetbeen ascertained, neither reptiles, birds, nor mammalia. Many of the plants are of terrestrialgrowth, whilst all the zoophytes, nearly all the mollusca, Crustacea , and fishes are of marineorigin. The plants are somewhat similar to existing species, as the large group of ferns,though others are quite dissimilar, as in the furrowed stem of the Sigillaria, &c. &e.
The following may be taken as a brief summary of the plants : —
Cryptogamia vaseulosa - - Equisetacea» - 20 species.
• Silices - - 100
Lycopodiacea? - 60
Phanerogamia monocotyledoniac - - - - 10
Conifera? - - 10
Cachacea? - - 50
Undetermined - 50
300 species.
The remains of these plants usually compose the coal seams, and one cause of the differ-ence among them is the various structural composition of the plants themselves, which aregenerally confined to arenaceous or argillaceous deposits; they abound in the upper part ofthe carboniferous system, and they also occur in the midst of the millstone grit, in the sand-stones and shales, and also among limestones, where coal-beds are found, but they are rare,or almost wholly unknown, in the midst of the undivided limestone, and in the old red sand-stone. Among the Zoophytes are 40 species of the Polyparia, 40 of the Crinoidea, and 3of the Eschenida. The Mollusca consist of 326 species, among which are of the Conchifera40 species of the Plagymyona, 28 of the Mesomyona, 100 of the Brachopoda; Gastero-poda 92 species; Cephalopoda monothalamia 10, and Cephalopoda polythalamia 69, 10species of which are of estuary formation, and about 60 per cent, belong to species ofextinct genera.
The fishes of the carboniferous system are mostly of the Ganoid division, and both theplants and animals are very distinct from existing types.
Carboniferous and Mountain Limestone lies beneath the coal measures, and sometimesalternates with the shales and sandstones of the coal; it is destitute of land plants, andusually abounds with corals of large size, several of which belong to the lamelliferous class,which enter largely into the structure of coral reefs: there are many Crinoides, Echinides ,&c., associated with the Zoophytes. Among the Mollusca are Brachiopoda , several ofwhich are referrible to the Spirifera and Product® ; Univalve and Bivalve shells, such asTurritella, Buccinum, Patella, Isocardia, Nucula and Pecten, abound ; but the Cephalopoda differ widely from living genera.
The Carboniferous limestone has a sub-crystalline texture, and some of the varieties takea fine polish, their surfaces being ornamented by the sections of inclosed Crinoidea, coralsand shells ; the prevailing colour is a bluish grey, the organic remains being of a purewhite, but some varieties have a ground of red, others nearly black, the shells which areembedded being of a deep ochreous colour.
The Derbyshire marbles and those of St. Vincent’s rocks are among the finest examplesof mountain limestone; in Gloucestershire , Somersetshire , Shropshire , Derbyshire, and
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