Other subvarieties of slate are-(e.) Calcareous, (f.) Carbonaceous (these often afford an even roofing-slate, but are more or less soft and affected by nodules and seams (Bulls'-eyes and Rucks) of pyrite), (g.) Arenaceous, (h.) Tuffose, (i.) Ribaned, and (k.) Conglomeritic. The last has, in a slaty matrix, fragments, often of quartz, rarely exceeding the size of a large pea or small marble, but usually about the size of shot. This rock is sometimes erroneously called Porphyry.

Local Names for Shale and Slate.

Rock-bind, Stone-bind, Fekes (Scotland), and Bluerock (Leinster), arenaceous shales. Bind, Killas (Cornish) and Slig or Sliggeen (Irish), either shale or slate. Bury (Kilkenny) and Flucan (Cornish), soft shale or clay. Metal, compact flinty shale. Plate, flaggy shale-bed. Shingle and Gravel, names used in Ireland for shale or slate that breaks up into small angular pieces; usually they cannot be blasted, but must be worked by the pick, bar, or wedge. C. FAULT-ROCK.—Fault-rock is usually a mechanically formed rock, but in many instances it is also in part chemically formed. It may be conglomeritic, brecciated, arenaceous, argillous, shaly, or schistose (metamorphosed). Some of the metamorphosed fault-rocks are peculiar, especially the brecciated, calcareous, and dolomitic rocks.

NOTE.-Fault-rocks, properly speaking, are not subaqueous; nevertheless, it seems more appropriate to mention them here than elsewhere.

B. Rocks for the most part chemically formed.—

Such rocks, when typical, are nearly solely due

to chemical action; they are more common in veins, nodules, and patches, than in beds or interstratified masses. Chemically-formed rocks are always insignificant, when compared with the associated mechanically-formed rocks, and often the former graduate into the latter. A. HALYTE, ROCK-SALT [Gr. hals, salt; lithos, stone]. -A crystalline, granular, translucent or transparent rock, consisting almost entirely of chloride of sodium; colourless, or of grey, yellow, reddish, occasionally bluish, or greenish colour; soluble in water; taste saline. Of Halyte, Cotta writes: "In Nature it almost always contains sulphate of lime, chloride of calcium, chloride of magnesium, and other salts; frequently admixtures of bitumen, clay, or boracite."

Varieties are-a. Granular, b. Sparry, and c, Fibrous, or with a wood-like aspect.

Halyte always merges into impure varieties, the impurities having a more or less mechanical origin. B. GYPSUM [Gr. ge, earth; epso, I boil].-A hydrated sulphate of lime; usually crystalline, but sometimes compact or fibrous; soft; generally white or whitish.

a. Alabaster [after Alabastron, a town in Egypt]. b. Granular Gypsum, always almost white, some

what translucent; granular and finely crystalline; sometimes it is fissile, each bed being composed of many layers of little crystals, slightly differing in colour and

texture.

c. Compact gypsum, rare.

d. Fibrous, or having a wood-like structure.

H

e. Selenyte, Spathic or Sparry gypsum, transparent varieties.

f. Tripestone, "a variety both of texture and composition. It is formed of thin layers of pure white gypsum, alternating with grey argillaceous gypsum, the whole twisted or crumpled to resemble a ruff." - Cotta. Gypsum merges into impure, more or less mechanically formed varieties.

C. ANHYDRYTE, or ANHYDRITE.-"A granular or compact aggregate of anhydrous sulphate of lime; harder than gypsum; white, grey, or blue."-Cotta.

The varieties of anhydryte are very analogous to those of gypsum, the more marked being-a. Granular, b. Compact, and c. Fibrous.

D. DOLOMYTE, or DOLOMITE [after M. Dolomieu].—A granular crystalline aggregate of dolomite associated with some calcite; more or less saccharoid; does not effervesce, or only slightly with dilute acid. Dolomyte weathers into a ferruginous sand, Dolomitic sand.

NOTE.-Some dolomytes are of purely chemical origin, while most dolomytes are partly chemically and partly mechanically, or even perhaps organically, formed. Only the first belongs to this group (B), the others are described in group (C), page 120. E. QUARTZ.--Pure or nearly pure silica.

Varieties of Quartz are-a. Chert [Celtic cairt or chairt, rough bark; as chert forms a rough bark or coat to the beds of limestone], b. Flint, c. Menilyte, or Menilite. Hard, compact, containing more or less carbonaceous matter; breaks with from a conchoidal to a semiconchoidal fracture; colour white, grey, yellowish, brown, black, and sometimes blue.

Chert, Flint, and Menilyte occur in layers of nodules, thin beds, and concretions; sometimes, however, as is especially the case with chert, they form massive strata. Menilyte is found in the Tertiary limestone. Flint is characteristic of some chalk, while chert is mostly found in the Carboniferous and older limestone, and in some shales. Jukes writes of these rocks, "Almost all large masses of limestone have their flints or siliceous concretions. Pure siliceous concretions occur even in the fresh-water limestones and gypsum beds of Montmartre."

d. Jasper and e. Agate (particoloured varieties). f. Basa nyte, or Basanite, Touchstone, or Lapis Lydius (Pliny), a velvet-black cherty variety, used on account of its hardness and black colour for trying the purity of the precious metals.

F. LIMONITE ROCK.-A fissile, porous, compact or fibrous, earthy or arenaceous hydrated oxide of iron, bedded or in veins; reddish, yellowish, or blackish in colour. Varieties are- -a. Limonitic Shale; b. Oolitic Brown Ore; c. Reniform Iron Ore; d. Pea-iron Ore ; e. Bog-iron Ore the latter having subvarieties, (a.) Black Bog-iron Ore ; (b.) Yellow Bog-iron Ore, or Yellow Ochre; and (c.) Sparry Bog-iron Ore.

Limonite Rock sometimes occurs interstratified among rocks of Secondary age,* but other vari

*In co. Clare, Ireland, it occurs in beds among Cambrosilurian rocks, while in Limerick and Queen's counties it has been found associated with black shales near the junction of the Carboniferous limestone and the lower shales of the Coalmeasures. In Ulster it occurs interbedded with Whinstone of Tertiary age.

eties, such as c, d, and e, usually occur associated with surface accumulations, such as drift or peat. Cotta writes of Oolitic Brown Ore, that it occurs in the form of layers in many formations." Bogiron Ore, and its sub-varieties, are due to the decomposition of iron ores (often pyrite or marcasite), or minerals, or rocks containing a large percentage of iron. Black Bog-iron Ore in general is rich in manganese, while the yellow and sparry subvarieties are respectively argillaceous and siliceous.

G. HEMATITE ROCK.-A granular or compact aggregate of hematite, with arenaceous or argillaceous matter; usually reddish in colour, but sometimes blackish or rich brown.

Varieties are—a. Red-iron Mould, or Red Ochre ; b. Reddle or Raddle; c. Sparry Red-iron stone; d. Specular Iron.

Of these varieties, the principal are a, b, and c, while d seems to be rather rarely a rock-mass. e. Topanhoacanga, or Moorshead Rock. This Brazil variety is found among the surface deposits, and consists of fragments of specular iron, micaceous iron, and magnetite in a ferruginous matrix. In this rock there are various accessories; even, on rare occasions, grains of native gold.

H. SPHEROSIDERYTE, or SPHEROSIDERITE, Clay-iron Stone. A crystalline aggregate of siderite combined with argillaceous and carbonaceous matter. Impure varieties are arenaceous.

Spherosideryte, or Clay-iron Stone, occurs in thin beds, layers, and nodules, in beds of clay, clunch, and shale. It is typical of the coal-measures, but