Spongy limestone, usually called Agaric mineral, stone marrow, &c., belongs to this kind of formation. It has a very white colour, a very fine grain, is soft to the touch, very tender, and light enough to float for an instant on water. It occurs in rather thin layers, in the crevices of calcareous rocks, and is so common in Switzerland as to be employed for whitening houses.
3. Compact limestone, is of a grain more or less fine, does not polish, nor afford large blocks free from fissures, has a conchoidal, or uneven scaly fracture. Colours very various. Its varieties are; a, The sub-lamellar, compact, with some appearance of a foliated texture. b, Compact fine-grained limestone, the zechstein of the Germans, to which M. Brongniart refers the lithographic stone in his classification of rocks (Dictionnaire des Sciences Naturelles,) but the English geologists place the locality of the famous lithographic quarry of Solenhofen much higher in the plane of secondary superposition. Its fracture is conchoidal; colour from gray to whitish; c, Compact common limestone. Grain of middle size; earthy aspect; uneven fracture; perfectly opaque; colour, whitish to pale gray, yellow, or reddish. The limestones of the Jura formation are referred to this head, as well as most of those interspersed among the coal strata. d, The coarse compact, or Cornbrash; texture somewhat open, earthy aspect, rough to the touch, ragged fracture, colour yellow, gray, or dirty red. e, Compact cellular, the Rauchekalk and Holekalk of the Germans, on account of the numerous holes or caverns distributed through it.
4. Oolite or roe-stone.—It consists of spherical grains of various size, from a millet seed, to a pea, or even an egg; texture compact; fracture even; colours, whitish, yellow, gray, reddish, brownish. The larger balls have almost always a foreign body for their centre or nucleus.
5. Chalk; texture earthy; grains fine, tender, friable; colours white, grayish, or pale yellowish.
6. Coarse-grained limestone; an earthy texture, in large particles, often loose; fracture foliated, uneven; colour pale and dirty yellow. Coarse lias has has been referred to this head.
7. Marly limestone; lake and fresh-water limestone formation; texture fine-grained, more or less dense; apt to crumble down in the air; colour white or pale yellow; fracture rough-grained, sometimes conchoidal; somewhat tenacious. Texture occasionally cavernous; with cylindrical winding cavities. This true limestone must not be confounded with the lime-marl, composed of calcareous matter and clay.
8. Siliceous limestone; of a compact texture; scratching steel, and scratched by it; leaves a siliceous residuum after the action of muriatic acid.
9. Calp; texture compact; fine-grained; schistose structure; hard, as the preceding; not burning into quicklime, affording to dilute muriatic acid a copious residuum of clay and silica; colour blackish; found in beds in the transition district near Dublin.
10. Lucullite or stinkstone; texture compact or sub-lamellar, colour grayish; emits the smell of sulphuretted hydrogen by friction or a blow. It occurs at Assynt, in Sutherlandshire; in Derbyshire; counties of Kilkenny, Cork, and Galway.
11. Bituminous limestone; black or blackish colour; diffusing by the action of fire a bituminous odour, and becoming white.