Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/246

Rh 232 2. Compound Rocks (composed of two or more mine- rals), including (a) Massive series, embracing the various igneous rocks, as granite and lava, and (b) Schistose series, including all the crystalline schists and most of the so-called metamorphic rocks. II. Fragmental or Clastic Roclrs, including (a) Gravel an(l Sand Rocks, (b) Clay Rocks, (c) Plant-formed Rocks, ((1) Rocks formed from animal remains, (e) Volcanic Agglomerates and Tutfs. I. Cr;/stalline and Glassy or ll'on-Fragmental Rocks. The great majority of these rocks are original formations; that is, they have not been palpably derived from the destruction of pre-existing rocks, asin the case of the frag- mental series. They include all chemical precipitates, whether these possess a distinctly crystalline or a dull granular texture, all rocks which have consolidated from igneous fusion, and all the schistose and metamorphic rocks which, whatever may have been their original character, now possess a crystalline or foliated structure. 1. Simple Ii’ocl's. Limestone is a mass of carbonate of lime, either nearly pure 01' mixed with clay or other impurity. Few rocks vary more in texture and composition. It may be a hard flinty close-grained mass, breaking with a s lintery or conchoidal fracture; or a crystal- line rock built up of ne crystals of calcite and resembling loaf sugar in colour and texture (ﬁg. 5); ora dull earthy friable chalk-like deposit; or a compact massive ﬁnely-granular rock resembling a close-grained sandstone or freestone. The colours, too, vary exten- sively, the most common being shades of blue-grey and cream- colour passing into white. Some limestones are highly siliceous, the calcareous matter having been accompanied with silica in the act of deposition; others are argillaceous, sandy, ferruginous, dolo- mitic, or bituminous. To some of these varieties particular names have been assigned:—0olite, a granular limestone built up of small roe-like grains, each of which consists of concentric coats of lime; Pisolitc, an oolitie or pisolitic limestone where the grains are as large as peas ; Travcrtiiw (calcareous tufa), tl1e material deposited by calcareous springs, usually white or yellowish, varying in texture from a soft chalk-like substance or marl to a compact building-stone; Stalactitc, the calcareous pendant deposit formed on the roofs of caverns, vaults, bridges, &c. The water from which the hanging lime-icicles are derived drips to the floor, and on further evaporation there gives rise to the crust-like deposit known as stalagmitc. Ilytlraulic limestone contains sullicicnt silica (and usually alumina) that, when it is burnt and subsequently mixed with water, a compound containing silicate of calcium is formed, which has the property of “setting” or hardening under water. Limestones containing perhaps as much as 25 er cent. of silica, alumina, iron, &e., which in themselves woul be unsuitable for many of the ordinary u sea for which limestones are used, can be used for making liy raulic mortar. These limestones occur sometimes in beds like those in the Lias of Lyme Regis, sometimes in nodules like those of Sheppey, from which Roman cement is made. Cement-stone is the name given to many pale dull ferruginous lime- stones, which contain an admixture of clay, and some of which can be proﬁtably used for making hydraulic mortar or cement. Fctirl limestone (stinlcslrzin, swincstone) gives off a fetid smell, like that of sulphuretted hydrogen gas, when struck with a hammer. In some cases, as in that at North Berwick, the rock seems to have been de sited by volcanic springs containing decomposable sulphides as we 1 as lime. In other instances the odour may be connected with the decomposition of organic matter. In some uarrics in the Car- boniferous Limestone of Ireland, as mentione by Mr Jukes, the freshly broken rock may be smelt at a distance of a hundred yards when the men are at work, and occasionally the stench becomes so strong that the workmen are sickened by it and require to leave off work for a time. Comstonc is an arcnaceous or siliceous limestone particularly characteristic of some of the Palaeozoic red sandstone for- mations. Izottcnstom is a decomposed siliceous limestone from which most or all of the lime has been removed, leaving a siliceous skeleton of the rock. A similar decomposition takes place in some ferrugin- ous limestone with the result of leaving a yellow skeleton of ochre. Jllarblc is limestone which has ac uircd a granular crystalline structure. Ordinary statuary marble IS a familiar example of this rock. It is white, ﬁne-grained, composed of minute crystalline granules of calcite, and resembles loaf-sugar, whence the term “ saecl1aroid” often applied to it (ﬁg. 5). Fine silvery scales of mica or talc may often be noticed even in the purest marble. Some lime- stones associated with gnciss and schist are peculiarly rich in GEOLOGY [1I. GEOGNOSY. minerals,—trcmolite, actinolite, anthophyllite, zoisite, and many other species occun'in there, often in great abundance. M-any varieties of colour am texture occur among these limestones, as may be seen in the numerous kinds of ornamental marble. Dolomite (fllagncsian Limestone) is a massive formation of the car- bonates of lime and magnesia, commonly associated with gypsuln, rock-salt, and other results of the evaporation of saturated saline waters. It is dull granular to ﬁnely crystalline in texture, sometimes full of cavities lined with crystals of dolomite, sometimes aggregated into botryoidal, mannnillated, and other concrctionary forms. Dolomite also occurs as the result of a chemical transforma- tion of ordinary limcsto11c, carbonate of magnesia replacing carbon- ate of lime. This process, known as (loIonn't2':at1'o-n, was largely insisted on by Von Buch and has been discussed by llisehof. The metamorphic variety of the rock is generally quite crystalline, resembling saecharoid marble in texture, and sometimes even in colour, though yellowish tints are apt to predominate. As a proof of the dolomitization or conversion of limestone into dolomite the. fact may be noticed that fossil shells, and other organisms Ccflisistillg originally of calcite, have been altered into dolomite. On a small scale a similar change may be observed in a limestone where it is traversed by some igneous dyke. Even along the vertical joints of limestone where no igneous matter has penetrated, and where percolating water l1as probably been the only agent of change, the limestonc is changed for some distance on either side into a dull yellow dolomite, locally termed “ dunstone.” Gypsum occurs as a rock in the form of beds and conerctions as well as in strings and veins. It is associated with red strata, often with dolomite, rock-salt, and anhydrite. Rock-salt, massive chloride of sodium, occurs in beds some- times 60 to 90 feet in thickness, rudely crystalline, usually stained red from an admixture of red sediment, like that of the red clays and sandstones among which the salt-beds occur. Ironstone.—Besides the iron ores met with in veins associated with other accompaniments of metallifcrous lodes, there are many which were doubtless formed as chemical precipitates on the ﬂoors of lakes and other sheets of water. Some of these deposits (either peroxide or carbonate of the protoxide of iron) arise from pre- cipitation in water or on moist ground where organic matter, espe- cially of vegetable origin, has decomposed. The hard crust of hydrous peroxide of iron which forms under wet or bogg ' soil (moor- band pan, bog-iron-orc) is an example of suel1 a deposit now in course of formation. Vhcre the peroxide has been reduced and become carbonate, it occurs in beds or nodules usually mixed with a variable proportion of clay (clay ironstone), aml sometimes with a good deal of carbonaceous matter from associated vegetation (black- bancl iroazsto-nc). Clay iron-ore is one of the most valuable ores of the metal, and occurs largely in beds and nodules in the Carboni- ferous system, as well as in parts of the Jurassic series i11 Britain. In some of the oldest geological formations extensive beds occur of haematite and magnetite. Scrpcntinc.—Tl1is mineral occurs massive, forming large bands of rock. In some places it may have been an original deposit from oceanic water, comparable to the glauconite found ﬁlling the chambers of Fora’/m'mfe7-a, and occurring extensively both in old geological formations and on the present ocean-floor. The serpen- tines associated with the gncisses and other crystalline rocks have had this origin assigned to them by Sterry Hunt. There can be no doubt, however, that many, probably most, scrpentincs are the results of the alteration of pre-existing rocks. Tscliermak pointed out that much serpentine has been roduccd by the hydration and alteration of olivine, and this view has been conﬁrmed by illustra- tions from all parts of the world. In many scrpentincs the forms of the original crystals of olivine may still be detected. Hence the difficulty in understanding l1ow there could be intrusive masses of scrpcntinc—a hydrated magnesian silicate-——is now removed, for we see that the original olivine-rocks may have been intruded as. molten masses which would preserve their external characters as eruptive rocks though undergoing an internal conversion into serpentine. With many Palaeozoic limestones, and more particularly with the crystalline beds which occur among the schistose ro«-ks, serpentine is frequently associated. Some of this may represent. the result of an alteration of dolomite, though, as above stated. it may with more probability be connected with some original oceanic deposit of a magnesian silicate. 2. 0O7)l[70lt7l(l Il0cl's. Nearly the whole of the rocks in this division consist of two or more minerals. A few examples occur, however, where, at least in some parts of their mass, the rocks are- formed of only one mineral. Strictly speaking, these excep- tions should be placed among the simple rocks. But they are so closely linked with the compound masses that to separate them would do much more violence to geological continuity than any harm likely to arise from the present