PETRIFACTION 



PETROGRAPHY 



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privately. His earliest explorations lx>re fruit, in his 

 Stonehenge ( 1880), anil lie next turned liis attention 

 to the pyramids anil temples of (Jhizeh ( l>ook, 1887 ), 

 subsequently, with the aid of the Egypt Exploration 

 Fund, to the immnds of Said the Scripture Zoar, 

 the forgotten city of Naukratis, Am, and Defenneli. 

 His Memoirs on Tanis ( 1885-89), on Naukratis 

 ( 1886), on Tel el Hesty, the site of Lachisli ( 1891 ), on 

 Mi'ijiini ( 189-2), and on 7V )V,/V Itir/r/ingx ( 1892), 

 are all important. 



PftrifiK'tloili a name given to organic remains 

 found in the strata of the earth, lieeause they are 

 generally more or less mineralised or made into 

 -I i me. The word has fallen very much into disuse, 

 having given place to the terms Fossil (q.v.) and 

 < trganic Remains. The name petrifaction is also 

 loosely used of an ohject which, from being exposed 

 to the action of limy or other water, becomes 

 covered with .. crystalline deposit. See (under 

 Fossil) FOSSIL FORKSTS. 



Petro-Alcxandrovsk, the seat of adminis- 

 tration of the Amu-Daria district, is little more 

 than a fiirt, and is situated on the Lower Amu- 

 Daria or Oxus (q.v.), 30 miles E. of Khiva. 



Petrography i- that branch of geological 

 science which deals with rocks viewed as aggregates 

 f mineral matter. It is a study, therefore, which 

 i.- rarrii-d on chiefly indoors, its object being to 

 ascertain the mineralogical composition, the texture, 

 and other physical characters of rocks, for which 

 various appliances and apparatus are required. 

 Although |>etrograpliy is pro|>erly only a description 

 <if rocks, it is hardly possible to ilescrilie rocks 

 without reference Ui their geological relations and 

 mode of origin. Hence by many geologists the 

 term Petrology is preferred as a name for this 

 tinindi of their science, while others use Lithology 

 in a imilar sense. For a general account of rocks 

 from the (mint of view of their origin, reference may 

 lie made to the section Petrology under (iKoLOQY. 



Petrographers are hardly yet agreed on any 

 particular classification of rocks, not certainly 

 from the want of materials, for a very large 

 ii i nnlier of so-called rock-species have l>eendescril)ed. 

 lint in the case of the crystalline igneous rocks 

 * many gradations exist l>etween one kind and 

 another that the definition of rock-species is often 

 very difficult. As careful descriptions multiply 

 ano comparisons arc made it is probable that many 

 of the igneous rocks which flourish at present as 

 species will come to lie included as mere varieties 

 Ota few well-marked tvpcw. 



In examining a rock the petrographer notes first 

 those characters which can lie seen by the naked 

 ye i iiini-i'i .11 n/iif rlinrnrtm), such as its structure 

 (whether cri/nliil/iin-, c/7, <,*, rnm/Mirf. or clastic, 

 \'c. i ; its state of aggregation or relative hardness ; 

 its colour; its composition ; and specific gravity 

 which may vary from O'tJ among the liydro-car)>n 

 compounds to .'t'l among the basalts, the average 

 pacific gravity of rock* in general l>eing about 2'5 

 or a little more. In the case of many coarse- 

 grained rocks, especially those belonging to the 

 derivative division, it is hardly requisite to go 

 licyond a macroscopic examination. But when the 

 rock appears to lie homogeneous it is necessary to 

 subject it to closer scrutiny. Thin slices are there- 

 fore prepared for microscopic study, after which it 

 i- frequently found that the apparently smooth 

 compact mass turns out to lie composed wholly 

 or largely either of crystalline or of fragmental 

 material*. Kven in the case of rocks which are 

 manifestly crystalline, and the mineral ingredients 

 of which can IK- determined macroscopically, it is 

 necewmry that a microscopic examination should 

 ' made. When this is done the rock will some- 

 times be shown to contain minute crystalline 

 371 



granules and crystals, or small quantities of non- 

 ditierentiated matter and glass which quite escape 

 the unassisted eye. The minute structure of the 

 various rock-forming minerals is likewise investi- 

 gated by means of the microscope, and the 

 chemical changes which they may have undergone 

 since the time of their formation are carefully 

 studied. In this way much light has been thrown 

 on the genesis of rocks and the changes which these 

 have suleequently experienced. 

 Of the minerals known to science comparatively 

 few are rock-formers ; the larger number of these 

 are practically confined to the igneous and schis- 

 tose rocks, very few entering into the forma- 

 tion of the derivative class. The mineral con- 

 stituents of the igneous rocks are grouped as 

 essential, accessor;/ or adventitious, and secondary. 

 The essential minerals are the most important, as 

 it is upon their presence that the various species of 

 rocks depend. Accessory minerals are accidental 

 ingredients, the presence or al>sence of which does 

 not affect the specific character of a rock ; if 

 sufficiently prominent or remarkable they merely 

 give rise to varieties. Secondary minerals are the 

 products of chemical changes subsequent to the 

 formation of the rock in which they occur the 

 essential and accessory myierals Iieing primary or 

 original constituents. Among the more important 

 essential minerals of iffiifoiix rocks are quartz, 

 felspar, nepheline, lencite, pyroxene, hornblende, 

 mica, and olivine. All these also occur as acces- 

 sory ingredients, and there are very many other 

 adventitious minerals, but only'the following need 

 l>e named magnetite, ilmenite (see IRON), apatite, 

 schorl, tit unite (sphene), haiiyne (nosean), zircon, 

 &c. Amongst the secondary minerals and decom- 

 position products in igneous rocks are quartz, 

 chalcedony, calcite, oxides of iron (see IRON), 

 zeolites, epidote, chlorite, serpentine, green-earth, 

 &c. The chief mineral constituents of the Schis- 

 tose rocks are the following : Quartz, mica, felspar, 

 talc, chlorite, hornblende, actiiiolite (see AMPHI- 

 BOLE), omphacite, smaragdite.' Less prominent in- 

 gredients are andalnsite and chiastolite, staurolite, 

 ottrelite, kyanite, magnetite, schorl, sphene, epi- 

 dote, pyrite, specular iron, &c. The <lrrii-nth-e 

 rocks having been formed out of the debris of pre- 

 existing rock-masses, whether igneous, schistose, or 

 sedimentary, it is obvious that they may contain 

 many of the minerals already mentioned. Thus, 

 there are some sandstones composed of quartz, 

 felspar, and mica the debris of granite or 

 gneiss. But most of the minerals which aqueous 

 rocks have derived from crystalline igneous and 

 schistose rocks are more or less altered the 

 felspars are kaolinised, the micas are reduced to 

 non-elastic scales or folia of a dull gray colour 

 and much diminished lustre, the pyroxenes, amphi- 

 boles, oliviue, &c. are either unrecognisable or 

 represented by decomposition products. Quartz, 

 as might have been anticipated, owing to its resist- 

 ance to the chemical action of water and its superior 

 hardness, is the most common mineral constituent 

 of derivative rocks. The clay-rocks consist in large 

 measure of the insoluble residue of the various 

 silicates of alumina and the alkalies and alkaline 

 earths, of which igneous and schistose rocks are so 

 largely composed. The readily soluble and readily 

 precipitated minerals calcite, dolomite, rock-salt, 

 and gypsum are also important rock-formers in 

 certain groups of derivative rocks. As binding 

 materials (i.e. the mineral cements which hold the 

 grains of many sedimentary rocks together) we 

 have quartz, chalcedony, opal, &c. , calcite, haema- 

 tite, and Union ite (see IRON), dolomite, siderite, 

 &c. The rocks which are mainly composed of 

 organic debris necessarily consist chiefly of cal- 

 careous and carbonaceous materials. 



