238 writers to an indurated clay requiring to be ground and mixed with water before it acquires vlasticity. When clay has been I. cposited intermittently so as to assume a thinly stratified or fissile structure, it receives the encral name of Shale. Under this term are included all laminat and indurated clays which are capable of being split along the lines of deposit into hard leaves. They present almost endless varieties of texture and composition, passing on the one hand into clays, on the other into flagstones and sandstones, or again, through calcareous grada- tions into limestone, or through ferruginous varieties into clay- ironstone, and through bituminous kinds into coal. An important variety, known as Oil-shale, and containing so much bituminous matter that it is now extensively used as a source for the manufac- ture of solid paraffin and mineral oils is described in the next section. Flint;/-slate (Lydicm-sttmc, Hornstmw) is siliceous shale or mud- stone which has been indurated into an exceedingly compact flinty mass, breaking with a conchoidal or splintery fracture, and usually of dark colours, black, brown, and red, more rarely white. Cla.y—slate is a compact close-grained, very hard, fissile argillaceous roek, dull lead-blue, grey, green, red, purple, or black in colour, splitting into thin leaves which are not those of original deposit but those produced by a superinduced cleavage. In this case the rock has been affected by great lateral pressure, whereby its particles have been forced to adjust themselves with their longer axes perpendicular to the direction of pressure. This rearrange- ment has imparted to the rock a fissility wholly independent of original lamination. The possession of this cleavage is the distinc- tive character of a true slate. (c.) Rocks formed of the .Del2rz's of I’lants.—These have sometimes been produced by the decay and entombmcnt of vegetation ;on the spot where it grew, sometimes by the drifting of the plants to a distance and their consolidation there. In the latter case, they may be mingled with inorganic sediment, so as to pass into carbonaceous shale. Peal is vegetable matter, more or less decomposed and chemically altered, found in boggy places and clsewhere where marshy mlants grow and decay. It varies from a pale yellow or brown brous substance, like turf or compressed hay, in which the plant remains are abundant and conspicuous, to a compact dark-brown or black material, resembling black clay when wet and some varieties of lignite when dried. The nature and proportions of the constituent elements of peat, after being dried at 100° C., are illustrated by the analysis of an Irish example which gave—carbon, 60'48 ; hydrogen, 610; oxygen, 32 '55; nitrogen, 0'88; while the ash was 3'30. There is always a large proportion fof water which cannot be driven off even by drying the peat. In the manufacture of com- pressed peat for fuel this constituent, which of course greatly lessens the value of the peat as compared with an equal weight of coal, is driven off to a great cxtent by chopping the peat into fine pieces, and thereby exposing a large surface to evaporation. The ash varies in amount from less than 1'00 to more than 65 per cent., and consists of sand, clay, ferric oxide, sulphuric acid, and minute proportions of lime, soda, potash, and magnesia. Lignitc is compressed and chemically altered vegetable matter, often retaining a lamcllar or ligncous texture, and stems with woody fibre crossinrr each other in all directions. It varies from pale brown or yellow to deep brown or black. Some shade of brown is the usual colour, whence the name brown coal, by which it is often known. It occurs in beds chiefly among the Tertiary strata, under conditions similar to those in which coal is found in older forma- tions. It may be regarded as a stage in the alteration and mineral- ization of vegetable matter intermediate between peat and true coal. Coal, the most completely mineralizcd form of vegetable matter, occurs as a black (sometimes dark-brown), brittle, usually lustrous substance, intercalated in beds between strata of sandstone, shale, fireclay, &c., in geological formations of Palzcozoic, Secondary, and Tertiary age. ’l‘he word coal is rather a popular than a scienific term, as it is indiscriminately ap lied to any mineral substance capable of being used as fuel. Strictly employed it ought only to be used with reference to beds of fossilized vegetation, the result e}i]th§r of the growth of plants on the spot or of the drifting of them t it er. The following analyses show the chemical constituents in some of the principal varieties of coal :— I Caking Coal. Splint Coal. 'canne1 Coal. Anthracite. Carbon ................ .. 86 '75 79'58 66 '4 91 '44 Hydrogen ............ . . 5 '24 5 '50 7 '54 3 '46 Oxygen 6 .61 8 '33 10 '84 2 '58 Nitrogen """"" ' ' 1 ‘I3 1 '36 0 '21 Earthy substances 1 '40 5'46 13'82 2'31 Specific gravity ..... .. 1'28 1-31 1-27 1-39 GEOLOGY [u. cr:oo.'o.<'. Diatom-earth is a siliceous deposit formed chiefly of the frustllh s of diatoms. It is laid down both in salt and in fresh water. Wide tracts of it are now being deposited on the bed of the South Pacific-. In Virginia, United States, an extensive tract occurs-covered with diatom-earth to a depth of 40 feet. It is used as a polishing or tripoli powder. Uil shale (Bra7uiscIu'c_/'cr) is shale containing such a proportion of hydrocarbons as to be capable of yielding mineral oil on slow di.~1il- lation. This substance occurs as ordinary shales do, in lfl_'l'I'.‘~ or beds, interstratified with other aqueous deposits, as in the SL‘0lll'll coal—fields. It is in a geological sense true shale, and owes its peculiarity to the quantity of vegetable (or animal) matter which has been preserved among its inorganic coiistitmnts. It consists of fissile argillaccous layers, highly impregnated with bituminous matter, passing on one side into common shale, on the other into eanncl or parrot coal. The richer varieties yield from 30 to 4! I gallons of crude oil to the ton of shale. They may be di.-tinguishcd fr nu non-bituminous or fcebly bituminous shales (throughout the .slml-- districts of Scotland) by the peculiarity that a thin paring curl» up in front of the knife, and shows a brown lustrous streak. Smut. « I‘ the shales in the Lothians are crowded with the valves of osfracn-1 crustaceans, besides scales, corprolites, &c., of ganoid fishes (I’aIa.-1 u- iscus, A mblyptcrus, Jllrgalichtlzys, &c.); and it is possible that the bituminous matter may in some cases have resulted from animal organisms, though the abundance of plant remains imlicatcs that it is probably in most cases of vegetable origin. l'nder the nann- “pyroschists" Sterry llunt classes the clays or shah‘-s (of all geo- logical ages) which are hydrocarbonaccous, and yield by distillation volatile hydrocarbons, inflammable gas, Sic. ((1.) Racks formed of Animal ]t’cm(u'ns.—'l'hese may be formed on land, as in bone caves, but most abundantly under water, as on the bottom of lakes and of the sea. They may be calcareous, siliceous, or phosphatie. Lz'nwstonc.—l3esides the limcstones resulting from the deposition of chemical precipitates of carbonate of lime, there is another important serics derived from the remains of organisms, either by growth on the spot, or by accumulation as mcclianicul sediment. Limestone so originating has often been so altered that it cannot always be distinguished from that which has been cliclnic-:1ll_' pro- duced, especially when it has been exposed to the action of per- colating acidulated water_. for in that case a crystalline texture i4 gradually superinduced, by which the original organic structures in the mass are wholly or in great part obliterated. Limestone com- posed of the remains of living organisms forms thin layers and massive beds. In some instances, as in that of the I'Ingli.<h and Irish Mountain Limestone, it occurs in masses several thousand feet thick, which extend for hundreds of square miles, and form the rock out of which picturesque valleys, gorges, hills, and table-lands have been excavated. Limestone may be either of fresh ‘'Il.l('l' or of marine origin. Some of the more common and important varicti +- may be here enumerated :— Coral-rock is limestone formed by the continuous growth of coral- building polyps. This substance affords an excellent illustration of the way in which organic structure may be effaced from u limestone entirely formed from the remains of once living animals. Though the skeletons of the reef-building corals remain distinct on the upper surface, those of their predecessors beneath them are gradually obliterated by the passage through them of percolating water dissolving and rcdcpositing carbonate of lime. This same action may be observed among the stalactitcs of a damp vault, in which, though the successive rings of growth are preserved, a crystalline divergent structure is supcrinduccd, which traverses these rings from the centre outward. thus understand hov a mass of crystalline limestone may have been produced from one formed out of organic remains without the action of any subterranean heat, but merely by the permeation of vat.,r from the surface. C'rinoz'dal (Encrimlc) Limestone is a rock com- posed in great part of joints of cncrinites, with Foramimflru, corals, and mollusks. It varies in colour from white or pale grey, through shades of bluish-grey (sometimes yellow or brown, less commonly red) to a dark-grey or even black colour. It is abundant among Paheozoic formations, being especially characteristic of the lower part of the Carboniferous system. C'halk as a litliological term is applied to awhite soft rock, meagre to the touch, soiling the fingers, formed of a fine calcareous flour derived from the remains of Fora- minifcm, echinodcrms, mollusks, and other marine organisms. It occurs in massive beds, and covers a great part of the south-east and east of England. In Ireland and elsewhere it assumes a firmer grain and various colours, so as to pass into some of the numerous varieties of compact white limestone. S7wIl-Mm-I, a soft white, earthy, or crumbly deposit, is formed in lakes and ponds by the accu- mulation of the remains of shells and E'7Ltomostrm:a on the bottom. When such calcareous deposits become solid compact stone they are known as fresh-water (lacustrinc) liwwstone. These are generally of a
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