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THEORY AND PRACTICE OF ENGINEERING.

Book II

clays with green or blue by the protoxide; and these colours are supposed to be the resultof volcanic influence on the particles which formed the first deposits at the bottom of theocean. The limestone is found of various densities, arising from the degree of consolidationto which its particles have been subjected. Some claim a corallaginous origin, and havetheir particles in different degrees of aggregation. The globular concretions of others maybe owing to the submarine springs yielding carbonate of lime, mingled with magnesia,which after deposition were consolidated.

The origin of the gypsum and rock salt have been variously accounted for, and from theabsence of all marine exuvia», it does not seem to have been deposited by the sea underordinary circumstances.

The fossils belonging to the saliferous or Poikilitic system are in Britain confined to about50 species, the great proportion of which belong to the magnesian limestone, and containfishes, zoophytes, and mollusca; in the lower red sandstone are many plants, and in themarls marine shells.

Carboniferous System . — Arenaceous, argillaceous, and calcareous rocks, associated withbeds of chert, ironstone, and coal, are the six substances which compose it: it is of greatthickness, and exhibits throughout proofs of slow and successive deposits.

Coal Measures, or the upper part of the carboniferous system, consists of alternations ofargillaceous and arenaceous strata, with beds of coal and ironstone, the latter yielding SOper cent, of iron ; the sandstones are micaceous, and both coarse and fine-grained, alternatingwith shale. The coal is often found in 50 seams or beds, varying in thickness from 1 inchto 6 feet or more. The rocks with which it is associated are of various colours, and thereis almost every possible gradation between the sandstones and argillaceous deposits;the latter being laminated are called plate or bass; when the lamination is less perfect,shale; and when quite unobservable it takes the name of clunch, bend, or some otherpeculiar to the locality; they are all more or less bituminous, and of a dark colour.

The limestones are compact, oolitic, or granularly crystallised, and mostly formed ofpure carbonate of lime, except the granular varieties, which contain magnesia. Thewhole are of marine origin, though there are some exceptions. All are, however, the resultof aqueous deposition, as lamination occurs throughout the whole of the six substanceswhich compose this system.

Coal is of vegetable origin, which has been most satisfactorily proved by a microscopicexamination of many varieties, after cutting them into thin plates. Impressions of plantsand trunks of trees are frequently met with in the sandstone and shale; leaves, branches,and fruits occur in the nodules of clay ironstone, the vegetable itself having served asthe nucleus round which the carbonate of iron has concreted.

The latest of the carboniferous deposits is said to be that in the neighbourhood ofShrewsbury, •which was of freshwater origin : it consists of shales and sandstones, about150 feet in thickness, with coal and traces of plants, including a bed of limestone of from2 to 9 feet in thickness, of a cellular texture. The fossils contained in it are smallbivalves, having the form of Cyclas, a small Cypris, and the Microconchus of an extinctgenus.

In the lower coal measures of Colebrook Dale the strata often change materially withinvery short distances ; beds of sandstone passing horizontally into clay, and clay into sand-stone ; the strata are from 700 to 800 feet in thickness, and more than 50 terrestrial plantshave been found, besides various fishes and Trilobites, as •well as 40 species of Mollusca ,among which are two or three of the freshwater genus Unio, and others of marine forms, asthe Nautilus, Orthoceras, Sperifer, and Productus, from whence it is inferred that thesedeposits were made in a bay of the sea or estuary, into which some large river poured itsflood.

In the South of England the carboniferous strata consists of three formations, viz.—

Coal formation, a mass of more than 1000 yards in thickness, composed of alternations ofshales and sandstones of various kinds, with about 50 feet of coal in beds, a portion of iron-stone, and sometimes thin layers of limestone.

Mountain Limestone, formed of calcareous rocks, with few partings of argillaceous matter,and with few grits, no coal, some chert nodules, and occasionally layers of red oxide ofiron ; the whole from 500 to 1500 feet in thickness.

Old Red Sandstone, composed of arenaceous rocks, from 500 to 10,000 feet in thick-ness, contains some conglomerates of extreme coarseness, and sandstones of many kinds :among the argillaceous beds is concretionary limestone irregularly developed. In theNorth of England this triple system is somewhat modified, and consists of the millstonegrit group, composed of quartzose and felspathic gritstones, and some bad coal, with shalesand sandstones.

Limestone shale formed of a series of laminated shales or plates, mostly bituminous, withsome ironstone, and thin black limestone, but no'coal; the thickness is 1000 feet or more.

Mountain Limestone formation, where the old red sandstone is almost absent; some mill-stone grit and limestone shale are found about the South Wales coal-field. In the north-