And (c.) Torbanyte, Boghead Cannel, Boghead Mineral (after Torbane Hill, Scotland).-A dark brown variety of cannel coal; yellowish streak, without lustre, and having a subconchoidal fracture.

Torbanyte yields over 60 per cent. of volatile matter, and is used for the production of burning and lubricating oils, paraffin, and illuminating gas.

66

b. Splint Coal, Splent Coal.-A hard, laminated, bituminous coal; not easily broken or kindled, though, when lighted, it affords a clear, lasting fire.

According to Page, the name "Splint" or Splent," is derived from the coal splitting (or splenting) up "in large flaggy or board-like laminæ."

c. Culm, a fissile, flaggy, shaly, flaky, or scaly variety of non-bituminous coal or Anthracyte.

Culm sometimes occurs in independent beds, but more often it is found associated with Anthracyte. The flaky or scaly nature of some culm may possibly be due to the growth and decay of organic matter in layers, but in others undoubtedly it is due to carbonaceous matter having been deposited in

water.

d. Bituminous Shale, and e. Carbonaceous Shale. -Respectively bituminous or non-bituminous shaly aggregates of coal, clay, sand, and such-like; more or less crystalline; colour black, or blackish-brown.

Both the bituminous and carbonaceous shales graduate on the one hand into argillaceous shale, and on the other into coal. The former merges

into Cannel, and the latter into Culm. The subvarieties a and b are bituminous, while c, d, and e are non-bituminous. Some bituminous shales

are impure coals; that is, they are capable of being used as fuel, while the poorer kind will not continue to burn in the fire, but become ash, the "slate" of the coal-merchant. Burning and lubricating oils, illuminating gas, &c., can be procured from many of the Bituminous shales. In general the Carbonaceous shales are not of much value; a few, however, when mixed with anthracyte, make a strong and lasting fire.

Local Terms for Bituminous Shale.

a. Batt and Bass (Staffordshire), (b.) Dauks. Local Terms for Carbonaceous Shale.

c. Kelve (Leinster), (d.) Pindy (Cork), (e.) Slaty Culm (Limerick and Clare).

NOTE. -The other coals, such as Peat, Lignyte, and Black Coal, are classed and described among the SUBAERIAL ROCKS (page 122); they having accumulated on the surface of the earth, prior to having been buried and covered by more recent deposits.

B. LIMESTONE. A more or less crystalline aggregate of calcite, in association with a greater or less quantity of carbonaceous, argillaceous, and arenaceous matter; compact, porous, fissile, amorphous, or oolitic; very variable in colour. Some limestones are made up almost entirely, others only partially, of shells, fragments of shells, corals, madrepores, and such-like; others for the most part are due to the precipitation of lime held in solution by water; while some in a great measure are mechanically formed, preëxisting calcareous

rocks having been denuded away and the detritus deposited in water. Limestones in general, however. appear to be more or less due to a combination of two or more of these modes of formation.

Limestone may contain magnesia, silica, alumina, bitumen, ores of iron, &c., either as mechanical admixures or as chemical deposits, in conjunction with the carbonate of lime; and the presence of such minerals occasions many varieties in colour as well as composition. Of limestone Jukes writes: "Varieties of limestone occur in different localities, both geographical and geological, peculiar forms of it being often confined to particular geological formations over wide areas; so that it is much more frequently possible to say what geological formation a specimen was derived from, by the examination of its lithological characters, in the case of limestone, than in that of any other rock."

NOTE. The colour of many, if not of most, limestones seems due, in a great measure, to the ores of iron chemically or mechanically contained therein. Dark blue or blackish limestones, when followed to a good depth, are nearly always found to be of a greenish colour. Red and variegated limestones in depth are generally grey or dove-colour, while all yellowish limestones, when in their normal state, have a greenish shade, no matter how pale.

a. Compact Limestone.-Homogeneous and finegrained; fracture sometimes uneven, but generally conchoidal, either earthy or smooth; may be argillaceous, siliceous, magnesian, or ferruginous.

b. Crystalline Limestone.-An aggregate of crystals of calcite, fine or coarse, in association with argil, silica, and the like.

Typical Crystalline limestone belongs to the me

tamorphic rock series; nevertheless, most sedimentary limestones are more or less crystalline, some being aggregates of large well-developed crystals. Jukes found the limestone forming the "great barrier coral reef," Australia, "to have a crystalline structure internally."

c. Marble [Lat. marmor].-Any limestone capable of being cut and of taking a fine polish. Marbles may be unicoloured, such as pure black, white, grey, &c.; or they may be particoloured, deriving their colour from contained minerals or animal remains. Homogereous rocks, both as to texture and hardness, cut more evenly and take finer polish than others, and make the better marbles.

The names by which marbles are known are very various ; some are called after the localities in which they are found; others after their colour or the shells they contain, or any other peculiarity that may give a character to a rock.

d. Oolyte, or Oolite [Gr. oon, an egg, and lithos, a stone], Roestone.-Spheroidal concretionary structure, having the appearance of the egg or roe of a fish.

Subvarieties are—(a.) Pisolyte, or Pisolite [Lat. pisum, a pea], Peastone, which has a resemblance to an agglutination of peas; and (b.) Dolomitic Oolyte, there being minute spheroids of dolomite in a limy matrix.

Each concretion is formed of numerous concentric layers, being sometimes hollow at the centre, or it may inclose a minute particle of sand or other mineral substance.

NOTE.-Oolitic rocks are typical of one of the Secondary geological groups, which has been given the name of the Oolitic formation. This structure, however, occurs in other groups, but more especially in the Carboniferous limestone.

e. Chalk [Lat. cala, lime].-Usually a white or whitish fine-grained rock, but sometimes greyish, or even red; often earthy or pulverulent, but sometimes hard and compact. Subvarieties are-(a.) Indurated chalk, called White limestone in Ulster. Jukes thus describes it:

66

Considerably harder and firmer than the friable rock which is commonly known as chalk." (b.) Chalk-rock (Whitaker), a siliceous variety; (c.) Pisolitic chalk, or Maestricht, which has a minutely spheroidal structure; (d.) Glauconitic chalk, greenish from containing glauconite; and (e.) Ferruginous or Red chalk, coloured red by oxide of iron.

f. Calc-sinter, or Calcareous Tufa, a loose and friable variety of calcite deposited from water charged with lime.

Subvarieties are-(a.) Travertine, (b.) Stalactyte, (c.) Stalagmyte, and (d.) Rock-meal. When compact, hard, and semi-crystalline, it is called Travertine; if deposited in long mammillated and pendent masses from the roof of a cave or fissure, Stalactyte [Gr. stalactis, that drops]; while the boss formed on the floor of the cave or fissure from the lime in the water which dropped, is named Stalagma [Gr. stalagma, a drop]. Rock-meal is a white and light calc-sinter that becomes. powder on the slightest pressure.

g. Coral-reef Limestone.-Limestones in course of formation, consisting of corals, shells, and