John Smith Flett

“ Although frequently insufficient in themselves to determine the true nature of a rock, they usually serve for a preliminary classification and often give all the information which is really needed. With a small bottle of acid to test for carbonate of lime, a knife to ascertain the hardness of rocks and minerals, and a pocket lens to magnify their structure, the field geologist is rarely at a loss to What, group a rock belongs. The fine grained species are often indeterminable in this Way, and the minute mineral components of all rocks can usually be ascertained only by microscopic examination. ”

“ QUARTZITE, in petrology, a sandstone which by the deposit of crystalline quartz between its grains has been compacted into a solid quartz rock. As distinguished from sandstones, quartzites are free from pores and have a smooth fracture, since when struck with the hammer they break through the sand grains, while in sandstones the fracture passes through the cementing material and the rounded faces of the grains are exposed, giving the broken surface a rough or granular appearance. The conversion of sandstone into quartzite is sometimes the work of percolating water under ordinary conditions. ”

“ In the history of rock masses changes of many kinds are inevitable. Loose sands, clays and heaps of shells are gradually converted into sandstones, shales and limestones by the action of percolating water and the pressure of over-lying accumulations. All rocks exposed at the earth’s surface or traversed by waters circulating through the earth’s crust, undergo changes in their component minerals due to weathering and the chemical action of the atmosphere and of rain. ”

“ Kernels, lumps or phacoids of massive rock are often found embedded in schists, and it is clear upon inspection that the phacoids represent the original state of the rock, while the schist is the effect of metamorphism. At other times a rock mass, such as an intrusive sill, is schistose at its edges and surrounded by schistose sediments, while near its centre it is almost entirely massive. The hard igneous rock has proved more rigid than the soft and plastic sediments; in folding, the latter have yielded to the stresses, and internal movement has produced foliation. ”

“ Few obsidians are entirely vitreous; usually they have small crystals of felspar, quartz, biotite or iron oxides, and when these are numerous the rock is called a porphyritic obsidian (or hyalo-liparite) . These crystals have, as a rule, very good crystalline form, but the quartz and felspar are often filled with enclosures of glass.
All obsidians have a low specific gravity (about 2.4) both because they are acid rocks and because they are non-crystalline. Their lustre is vitreous except when they contain many minute crystals; they are then velvety or even resinous in appearance. ”

“ The chemical change is often complete, as when a limestone is replaced by chert or otherwise silicified, but it is probably more usually incomplete, part of the substance of the original rock having been retained though possibly in new mineral combinations. When a limestone is replaced by ironstone (e.g. carbonate of iron or siderite) part at least of the carbonic acid may be that of the limestone. A dolomite, formed from a limestone, contains more than one-half of its weight of carbonate of lime presumably derived from the limestone itself ”

“ Only rarely, however, does metamorphism produce much difference in the chemical composition of the rocks affected. Sandstones become quartzite’s and quartz schists, limestones are converted into marbles, granite passes into gneiss, and so on, without their bulk composition being greatly modified. From all that we know it seems established that however great the heat and pressure to which metamorphic rocks have been exposed they have very rarely been melted or reduced to the liquid state. ”

“ But in the volcanoes of the Sandwich Islands the craters are filled at times with liquid basalt which suddenly escapes, without the appearance of any lava at the surface. The molten rock, in such a case, must have found a passage underground, following some bedding plane or fissure, and giving rise to a dike or sill among the older lavas or in the sedimentary rocks beneath. Many of the great sills, however, may have been connected with no actual volcanoes, and may represent great supplies of igneous magma which rose from beneath but never actually reached the earth's surface. ”

“ When solidification is nearly complete the gases can no longer be retained in the rock and make their escape through fissures towards the surface. They are powerful agents in attacking the minerals of the rocks which they traverse, and instances of their operation are found in the kaolinization of granites, tourmalinization and formation of greisen, deposit of quartz veins, stanniferous and auriferous veins, apatite veins, and the group of changes known as propylitization. ”

“ In the Cretaceous rocks of the south of England soft argillaceous limestones of marine origin, which may be described as marls, occur on several horizons. At its base the white chalk is often mixed with clay, and the “chalk marl” is a rock of this kind; it is known in Cambridgeshire, at Folkestone, in the Isle of Wight, &c. The chloritic marl, which underlies the chalk and is well developed in the Isle of Wight, is a greenish argillaceous limestone, the colour being due to the presence of glauconite, not of chlorite; it is often very fossiliferous. ”

“ The less pure rocks have often eroded or pitted surfaces, showing bands or patches rendered more resistant to the action of the weather by the presence of insoluble materials such as sand, clay or chert. These surfaces are often known from the crust of hydrous oxides of iron produced by the action of the atmosphere on any ferriferous ingredients of the rock; they are sometimes black when the limestone is carbonaceous; a thin layer of gritty sand grains may be left on the surface of limestones which are slightly arenaceous. ”

“ In the weathering of granite the quartz suffers little change; the felspar passes into dull cloudy, soft aggregates of kaolin, muscovite and secondary quartz, while chlorite, quartz and calcite replace the biotite, hornblende and augite. The rock often assumes a rusty brown colour from the liberation of the oxides of iron, and the decomposed mass is friable and can easily be dug with a spade; where the granite has been cut by joint planes not too close together weathering proceeds from their surfaces and large rounded blocks may be left embedded in rotted materials. ”

“ It seems clear that another factor must be taken into account, and in all probability that factor is the action of water in rocks at a comparatively high temperature. All rock masses contain interstitial water, and many also consist of minerals in some of which water exists in combination. Hence all metamorphism must be regarded as taking place in presence of water. It is almost equally certain that metamorphism must be accompanied by a rise of temperature in nearly every case—in fact it is difficult to imagine such a process going on without considerable heat. ”

“ Sands may be uniform when they have been sorted out by some agency such as a gentle current of water or the wind blowing steadily across smooth arid lands, but usually they vary much both in the coarseness of their grains and in their mineral composition. The great source of natural sands is the action of the atmosphere, frost, rain, plants and other agencies in breaking up the surfaces of rocks and reducing them to the condition of fine powder ”

“ A granite arises by the consolidation of a molten magma (a fused rock mass; Gr. μάγμα, from μασσειν, to knead) at high temperatures and great pressures and its component minerals are such as are formed in such circumstances. Exposed to moisture, carbonic acid and other sub aerial agents at the ordinary temperatures of the earth's surface, some of these original minerals, such as quartz and white mica are permanent and remain unaffected, others “weather” or decay and are replaced by new combinations. ”

“ The active gases were evidently in solution in the magma as it rose to the surface. Some geologists believe it is of subterranean origin like the lava itself, and is an essential or original component of the magma. They point to the existence of gases in considerable quantity in meteorites, and, comparing the earth to a great aerolite, insist that it should contain gases in solution like the smaller masses of the same kind. Others hold it more probable that the water has percolated in from the surface, or seeing that many volcanoes stand near the sea. ”

“ The distinction between mineral veins and other veins is to a large extent artificial. With improvement of methods of mining and extraction deposits formerly unprofitable become payable, and in all cases veins vary considerably in the amount of ore they carry. The rich parts are sometimes called shcots or bonanzas, while the barren portions are often left standing in the mine. Near the ground surface the vein stones become oxidized and the metallic minerals are represented by oxides, carbonates, hydrates, or in the case of gold and silver veins they may be rich in the metals themselves. ”

“ Like all metamorphic rocks, mica-schists are principally found in Archean areas; the great majority of them are of pre-Cambrian age. There are, however, in the Alps, Himalayas, &c., many rocks of this sort which are believed to be secondary or even tertiary; the evidence for this is not in all cases satisfactory, as of course the fossils, which if preserved would be sufficient to prove it, are nearly always destroyed by the metamorphism. Mica-schists are rarely of economic value, being too fissile for building-stones and too brittle for roofing-slates. ”

“ In some rocks the whole ground mass consists of such spherulitic growths of fibrous micropegmatite (see Quartz-Porphyry) ; in their centres there is often a quartz or felspar crystal; the outer boundaries of the spherulites are not usually circular but irregular owing to the interlocking of adjacent spherulites at their margins (“granophyric structure”) . Micrographic structures may occur in other minerals, e.g. quartz and garnet, cordierite, epidote or hornblende, augite and felspar, but are less common, and the name micropegmatite is usually reserved for aggregates of quartz and felspar. ”

“ In the Carboniferous rocks the Whin Sill lies almost like an interstratified bed, following the same horizon for many miles and hardly varying more in thickness than the sedimentary bands which accompany it. ”

“ The ores occur not only in veins and shoots, but also in great masses replacing the limestone, and the geologists who have examined these mining districts are nearly unanimous as to the metasomatic nature of a large part of these deposits. Other rocks such as tuff, volcanic breccia, shale, porphyry and granite may also be impregnated with metalliferous ores, but the largest ore bodies are found in the limestones. Secondary enrichment has often taken place on a considerable scale. ”

“ Sections perpendicular to an optic axis of a biaxial mineral under the same conditions show a dark bar which on rotation becomes curved to a hyperbolic shape. If the section is perpendicular to a “bisectrix” (see Crystallography) a black cross is seen which on rotation opens out to form two hyperbolas, the apices of which are turned towards one another. The optic axes emerge at the apices of the hyperbolas and may be surrounded by coloured rings, though owing to the thinness of minerals in rock sections these are only seen when the double refraction of the mineral is strong. ”

“ Something depends also on the hardness of the minerals resent in the sands, yet even the diamonds and other gems found in sand deposits have often their corners worn and smoothed. Minerals with very perfect cleavage, such as mica, split up into thin plates under the shock of impact with adjacent grains, and are never rounded like quartz or tourmaline. In deserts the transport of the sands is effected by the wind, and owing to the low viscosity of air even the smallest grains are not held in suspension but are rolled along the ground ”

“ Some minerals decompose readily and become turbid and semi-transparent (e.g. felspar) ; others remain always perfectly fresh and clear (e.g. quartz) , others yield characteristic secondary products (such as green chlorite after biotite) . The inclusions in the crystals are of great interest; one mineral may enclose another, or may contain spaces occupied by glass, by fluids or by gases.
Lastly the structure of the rock, that is to say, the relation of its components to one another, is usually clearly indicated, whether it be fragmental or massive ”

“ Other rock species are of rare occurrence and may be known in only one or two localities in distant parts of the earth’s surface. Nearly all rocks consist of minerals, whether in a crystalline or non-crystalline state, but the insoluble and imperishable parts of the skeletons of animals and plants may constitute a considerable portion of rocks, as for example, coral limestone, lignite beds and chalk. ”

“ Great masses of the strata which the granite has invaded are often floated off, and are found lying in the heart of the granite much altered by the heat to which they have been exposed, and traversed by the igneous rock in ramifying threads. Such granite intrusions are generally known as bosses from their rounded surfaces, and the frequency with which they form flattish dome-shaped hills, rising above the older rocks surrounding them. ”

“ Artificial glass which has not the right composition, or is retained for too long a time in a furnace, sometimes crystallizes, and contains spherulites which may be as large as a marble. As the glass has little similarity in chemical composition to volcanic obsidians these spherulites when analysed throw little light on the mineral nature of spherulites in rocks. They show, however that in viscous semi-solid glasses near their fusion point crystallization tends to originate at certain centres and to spread outwards, producing spherulitic structures. ”

“ In thermal or contact alteration the rocks are baked, indurated, and often in large measure recrystallized. In regional metamorphism recrystallization also goes on, but the final products are usually schists and gneisses. It is as a rule not difficult to distinguish the two classes of metamorphic rocks at a glance, and they may conveniently be considered separately.
When a rock is contact altered by an igneous intrusion it very frequently becomes harder, more crystalline and more lustrous, owing to the development of many small crystals in its mass. ”