Class II.-SUBAËRIAL ROCKS.

[Lat. sub, under; aer, lower air].-Rocks that were formed on the surface of the earth. Most of these, however, subsequently were covered by, or buried under, newer-formed strata, which may be either subaqueous, subaërial, or igneous.

Rocks.

D. Mechanically, chemically, and organically-formed Subaërial rocks are due to mechanical, chemical, or organic action, or two or more of these combined. The major part, however, including the principal rocks of the group, are more or less due to the growth and decay of organic matter, united with chemical action.

A. COAL.-This geologically includes and signifies

all rocks, recent or otherwise, that can be ignited and kept burning without the addition of any other substance.

NOTE. The burning properties of some coals can be improved by the addition of other substances, as of water to anthracyte; nevertheless, all rocks that are classed as coal should be capable of burning by themselves.

a. PEAT, Turf, Bog [Celtic ? peait or puit, a soft miry substance].-An aggregate of organic matter more or less solidified, or indurated and mineralized; colour whitish-brown, yellowish-brown, blackish or yellowish white. Subvarieties are-(a.) White Turf, (b.) Brown Turf, (c.) Black, or Stone Turf, (d.) Gas or Candle Turf.

Peat, or Bog, varies in appearance and texture, according to the position in which it is situated,

and the rate at which the accumulation was made. On low, flat land it grows much more rapidly than on hills or hill-slopes; consequently on the low land the peat is quite different from that on the hills, and in Ireland they are classed as Low-bog, or Bawn, and Mountain-boy.

The Low-bog gives (a.) White Turf, (b.) Brown Turf, and (c.) Black, or Stone Turf while the Mountain-bog only gives (b.) Brown, and (c.) Black Turf. On both there is an unprofitable surface or "clearing" (more or less living organic matter), which varies in thickness; on the former ranging from three to six feet in depth, while on the latter it rarely exceeds twelve inches in thickness.

Peat may be fissile, but more generally it is felt-like, the vegetable remains being interwoven, and forming a tough mass. The fissile texture of subaerial peat is due to vegetable growth;† each annual growth forming a separate layer. This is more common in some bogs than in others.

White turf is a nearly pure, if not an entirely pure, organic substance, and when burnt has little or no ash. Brown turf is always more or less

*Bawn (Anglicè white), so called from the white appearance the dead moss, grass, &c., have during the major part of the year, compared with the colour of the grass land and tillage in the vicinity.

There are fissile peats in which the texture is caused by lamination, peat masses being denuded and subsequently deposited. Such peats, however, are not Subaërial, but are Subaqueous Rocks, and they are often interstratified, or mixed with clay, sand, or the like. Extreme heat or frost will break up peat, and leave it ready to be denuded and carried away by wind, or rain and rivers," to be deposited in hollows, lakes, and such other reservoirs.

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mineralized; while Black, or Stone turf, is a chemico-organic rock, often containing such minerals as pyrite, marcasite, and the like; when burnt, it leaves a considerable residue or ash; sometimes it is semi-crystalline, and often it is scarcely distinguishable from Lignyte.

NOTE.—The residue or ash of peat attains its maximum in the deepest portions of the bog; thus Stone turf will have more ash than the overlying Brown turf, and the latter than the White turf; while the subordinate parts of each have respectively more ash than the portions above them. The plants growing on the surface collect their inorganic food from the atmosphere, and after their decay, the mineral substances are being continually carried downwards by the water that percolates through the mass, the lower portions thereby becoming the most impregnated: some impurities also come up in water from springs.

Gas peat, or Candle turf, is not very common. It was found in a mountain bog on the island of Valencia, Ireland; was of a dirty yellowish-white colour; had the consistency of soap; when dried was very inflammable, and burned with a clear, bright, steady flame.

Peat accumulations are usually surface rocks, that is, rocks still growing at the surface, and being added to at the present day; it has, however, been found under drift and other surface deposits. Eolian drift may be blown over it; on a sunken coast-line it often is found under Marine drift; while in some places it has been found under Boulder-clay drift.

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NOTE. Cotta mentions peat in Germany diluvial loam." Oldham records peat "under a considerable depth of drift," near Nenagh, co. Tipperary, Ireland. Other Irish localities are the Boleyneendorrish valley, near Gort, co. Galway, under 25 feet of boulder-clay drift; and Newtown, Queen's co., under a thickness of 96 feet. The latter is three

feet thick, compact, solid, semi-crystalline, very bituminous, and blazing like a candle when lighted. It might almost be classed as lignyte.

b. Lignyte, or Lignite, Brown Coal [Lat. lignum, wood]. Fissile or compact; woody or earthy; usually brown or black; streak brown; very inflammable, burning with much smoke and smell. A non-caking coal.

a. Woody Lignyte, (b.) Compact Lignyte.

Fremy has divided lignyte into two kinds; 1st, Lignyte that still displays woody structure, and 2nd, Lignyte exhibiting the aspect and compactness of soft coal.

c. Jet [Jayet, Gagates, after Gagas, a place in Lydia, Asia Minor].-"A black variety of brown coal, compact in texture, and taking a good polish.”—Dana.

NOTE.-Jukes considers jet as a subvariety of cannel coal, while Page supposes it "is rather a species of amber than coal."

c. Black Coal, Steinkohle, Common Coal, Pit Coal. -A black, brownish-black, or greyish-black, compact or semi-compact mass, occasionally iridescent; lustre dull to brilliant, and either earthy, resinous, or submetallic; opaque; fracture conchoidal to uneven.

Subvarieties are-(a.) Caking Coal, (b.) Noncaking Coal, (c.) Cherry or soft Coal.

Mineral coal, according to Dana, should be "compact, massive, without crystalline structure or cleavage; sometimes breaking with a degree of regularity, but from a jointed rather than a cleavage

structure." Black coal may be a caking or a noncaking coal; the latter is sometimes called Cherry or soft coal. Caking coal softens and becomes pasty or semi-viscid in the fire, also it is inclined to form clinkers or to become welded together. Cherry or soft coal burns freely without softening, or any appearance of incipient fusion.

d. Anthracyte, or Anthracite, Stone Coal [Gr. anthrax, carbon].-Non-bituminous coal; lustre bright, often submetallic, and frequently iridescent; conchoidal, sharp-edged, shining fracture, or breaking readily into small cubical lumps.

Anthracyte appears not to be a normal rock, but to be an altered bituminous coal, from which the bituminous qualities have been extracted or expelled; sometimes it is an associate of Metamorphic rocks, but it also is found interstratified and associated with unaltered rock.

NOTE. In Munster, Leinster, and Connaught, Ireland, anthracyte is interstratified with fossiliferous grits, shales, clunch, and fire-clay, the ordinary associates of bituminous coal, and these strata are not altered, except perhaps that they are more siliceous and harder than the ordinary coal-measure rocks in other countries. In places they are also cleaved.

A rock allied to Anthracyte is Native coke. It is undoubtedly an altered rock, due to the irruption of igneous rocks into beds of coal. Native coke is harder and more solid than artificial coke.

B. SURFACE DEPOSITS AND ACCUMULATIONS.-These may be peaty, clayey, sandy, gravelly, shingly, or a combination of two or more.

a. Boulder-clay Drift, a clayey or sandy-clayey matrix, sometimes calcareous, inclosing