difficulty in forming a conception of the estuaries in which the sandstones, shales, shell limestones, and coal of the carboniferous era were deposited.

198. The igneous rocks associated with the carboniferous system are all of the trap family-greenstones, clinkstones, basalts, trap-tuffs, &c. They are principally composed of felspar and hornblende, with admixtures of clay, augite, and occasionally hypersthene. The greenstones (vulgarly, whinstones) occur in large indeterminate or tabular masses, and are often hypersthenic; the clinkstones differ little from the greenstones in mineral composition, but are more compact, split up into thin schistose-like fragments, and yield a metallic sound when struck by the hammer; the basalts are easily known by their columnar structure, their dark and compact aspect, and from their containing little spherical crystals of a greenish mineral called olivine; and the trap-tuffs are of all varieties, from a soft scoriaceous-looking mass to a confused conglomerate of fragments of basalt, greenstone, sedimentary rocks, &c. The trap rocks of the carboniferous era are easily distinguishable from those of any other, partly by their darker colour, and from the fact of their yielding more or less bitumen by distillation; and partly from the prevalence of basalts, and of trap-tuffs containing fragments of limestone, sandstone, and shale. Among the traps of the old red sandstone, felspar, porphyries, and amygdaloids prevail, but are rarely to be met with among those of the coal measures; while, on the other hand, the traps associated with the tertiary strata assume a lighter colour, and a decidedly scoriaceous and lava-like aspect.

199. The positions of the carboniferous trap rocks are either disrupting, overlying, or interstratified. They disrupt and elevate, as in the mountain limestone hills, and in the rounded heights and isolated irregular cones of the coal measures. Basalt, or greenstone, sometimes overlie, as if poured in a state of liquid lava over the subjacent strata; and trap-tuffs also overlie, from their evidently having been strown abroad in the form of volcanic dust and ashes. The trap rocks of this era more frequently assume the interstratified form than those of any other formation; apparently from the fact, that volcanic discharges took place in the seas and estuaries in which the coal measures were being depositedthese discharges, whether in the form of lava or ashes, being overlaid by subsequent deposits of sedimentary matter.

200. The structure and texture of these igneous rocks differ as widely from the granitic series beneath as from the volcanic above. They are generally close-grained, and less

distinctly crystalline than the former, and more compact and less vesicular than the latter. The structure of the tuffs and porphyries is massive and indeterminate; of the greenstones sometimes massive, but generally tabular or cuboidal; and of the basalts always columnar. This difference in the structure and texture of these rocks seems to have arisen not so much from any difference in their mineral composition, as from the circumstances attending their cooling. This has been satisfactorily proved by the experiments of Sir James Hall and Mr Gregory Watt, who, by fusing various kinds of trap, produced, by different modes of cooling, not only columnar basalt, but spherical greenstone and vesicular tufa. same substance which, when suddenly cooled, forms a black glass or obsidian, will, by a slower process of refrigeration, form basalt, or, by a still slower, pass into earthy tufa. By gradually cooling the fused mass, columnar basalt may be formed; but if, at a certain stage of the process, it be rapidly cooled, spherical masses will be produced, which, when exposed to the weather, exfoliate, or decompose coating after coating. By these experiments, it was also proved that the primary form or crystal into which volcanic rock arranges itself when cooled, is spherical; and by these spheres pressing

The

upon each other, are produced tabular, cuboidal, and columnar forms. This fact can be readily illustrated by putting a number of spherical pellets (of putty or any other yielding

material) into a vessel, and then gently pressing upon them, when they will be seen to arrange themselves in five and sixsided columns, precisely similar to the five and six-sided columns of Staffa, or the Giant's Causeway.

201. The effect of igneous forces upon the coal measures has been to throw them into troughs and basins, to elevate and depress them in a very extraordinary manner. The following engraving represents a section of the South Gloucestershire coal-field (omitting faults), and may be taken as the type of the trough or basin. Here a is the elevatory axis of the

e

m

с

Trough or Basin Form of Coal-fields.

Mendip hills, overlaid by the old red sandstone d; b b are strata of carboniferous limestone resting at a high inclination upon the slopes of the hills, and reappearing at f; c c and the other darkened layers are beds of coal; e e e denote the new red sandstone lying unconformably upon the coal measures; gg detached outliers of lias; and h is a detached outlier of inferior oolite, which are respectively continued in i and k; I is the upper oolite, and m m are beds of Oxford clay situated to the north of the town of Malmesbury. The upthrows and downthrows of the coal strata, which have taken place in consequence of irregular upheavings and sinkings among the

masses on which they rest, are represented in the a subjoined figure. Here the faults or slips d d d, and the dyke e, are respectively accompanied not only by an upthrow or down

b

throw, but also by a different inclination of the strata. This mode of fracture is of frequent occurrence in every known coal-field, showing in a striking manner the nature of the agitations which have taken place below. In the vale of the Esk, in Mid-Lothian, which does not measure more than ten

miles each way, the coal-field shows 120 ascertained dislocations, one of which, at Sheriff hall, throws down the strata 500 feet. In the Newcastle coal-field, there is a famous slip called the ninety-fathom hitch, the deviation from the line of stratification being no less than 450 feet. The coal-fields of Fife and Clackmannan abound in such dislocations, several of them throwing the strata from 400 to 1200 feet up or down, as the case may be, from the general position.

It pre

202. The extent of country occupied by the carboniferous system is not great, the deposit being found only in limited and detached areas. It occurs largely in the British islands, and to this circumstance is mainly owing our greatness as a nation-the formation being rich in coal, iron, and lime-three of the most essential minerals to civilised existence. sents itself in the Lowlands of Scotland, in the northern and middle districts of England, in Wales, and in Ireland; it occurs also in some districts of Spain, in central France, Germany, and in Middle Europe; in Hindostan, in Australia, and New Zealand; in the island of Batavia, and on the eastern coast of China; in Melville island; in Nova Scotia ; in the States of North America; and on the coasts of Chili.

203. The physical aspect of carboniferous districts is rather tame and unprepossessing. The hills connected with the mountain limestones sometimes present considerable variety of scenery, owing to the bold escarpments of that rock, its extensive fissures and caverns, and the irregular undulations of the trap. These features are also aided by the general verdure and fertility of limestone districts, which present a freshness and luxuriance peculiar to themselves. The coal districts are almost always tame and unattractive, relieved by few elevations or depressions of picturesque beauty, and being, in general, bleak and unfertile from the cold and retentive nature of the soil. Occasionally, a basaltic crag or isolated trap-hill relieves the monotony; but this is the exception to the general rule.

204. The economical value of the carboniferous system fully compensates for any deficiency in the fertility of its soil, or in the picturesque beauty of its geographical features. Buildingstone of the finest quality (of which that of Craigleith, near Edinburgh, and of Calelo in Fife, are good examples) are obtained from the sandstones beneath the mountain limestone; while the millstone grit and flaggy beds of the coal measures yield other valuable freestones. Of all the limestones in the crust of the earth, the Mountain is that which is most valuable and abundant; and from it are principally derived those stores of lime so indispensable to the purposes of the builder,

x

on go workable mineral veins are seldom found in England

GEOLOGY.

in strata of recent than the carboneftrous limestone: S. Ifod o in younger bids upheaved on the flanks of some of the continental chairs. agriculturist, iron-founder, &c. Where the encrinal beds are sufficiently hard and crystalline, they furnish a very prettilymarked marble; the joints, stalks, plates, and star-like tubes of the corals shining out from the darker matrix in which they are imbedded. Many ornamental spars (Derbyshire spar) are found in the veins of the mountain limestone, which are also the principal sources of lead ore in the British islands; Derbyshire, Alston Moor in Cumberland, and Lead Hills in Lanarkshire, being well-known lead-mining districts. Silver and gold are both more or less associated with the ores of lead, especially the former; but they are seldom sought after, unless in connexion with lead. Fire-clay is dug up from the coal measures for the making of fire-bricks, furnace linings, &c.; ochre (hydrated oxide of iron), which occurs in several coal-fields, is extensively used as a pigment; and alum (sulphate of alumina) is obtained from many of the pyritous shales of Germany. Ironstone, which is found in the coalshales either in bands, nodules, or septaria, is one of the most valuable products of the carboniferous system. It is obtained in great abundance, and being easily reduced to a metallic form by the application of coal and lime, in which the system abounds, it may be said to form one of the prime elements of our country's mechanical and commercial greatness. About two millions of tons are annually manufactured in Britain for the fabrication of the innumerable machines, utensils, and implements to which cast-iron, malleable-iron, and steel, are respectively applied. But notwithstanding the great value of these rocks and metals, they do not equal in importance those strata of coal, which form the main distinguishing feature of the system. The varieties principally used in Britain are caking, splint, cubic, and cannel coal; anthracite is that mainly employed in the United States; lignite and brown coal in Germany; and jet wherever it can be obtained, for the manufacture of ornaments. Coal, of which we have historical notice so early as the beginning of the twelfth century, is justly regarded as the main support of the whole system of British production; it fuses the metals, produces steam which sets machinery in motion, yields gas for light, heats our apartments, prepares our food, and, in short, renders all the resources of nature fit for civilised use. The annual consumption of coal in the British Islands is estimated at about thirty millions of tons; and the export at from three to four millions. At this rate of consumption, fears have been entertained that our coal-fields would speedily be exhausted; and these fears, considering that the deposit is limited, are not altogether In 1806, Humboldt estimated the total annual produce of gold & silver in Europe Asia America at £10, 755,000. Diela Becheug report un Cornwall estimated the total rotus of all the metals raised derived in Great Britain ar -£10, 597,000, of which £50,000

from