Crystalline rock

“ It may be stated, as a general principle, that the greater the pressure under which a rock solidifies and the slower and more gradual the cooling the more perfect will be found the crystalline structure. Hence, it follows that the older and more deep-seated rocks which are forced up in the form of dikes, bosses, or intrusive sheets, into the overlying masses, and which have become exposed only through erosion and removal of the overlying rocks, are the more highly crystalline. ”

“ While the igneous rocks are classified according to their composition, the rate at which they cooled has much to do with their texture, and certain names apply to the texture. For instance when the molten lava cools very rapidly, there is no time for the formation of crystals, and the resulting rock is glassy or non-crystalline. If the cooling is slow as in large bodies, crystals have time to form and grow to considerable size as in granites. ”

“ 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. ”

“ In many cases the ground-mass itself becomes minutely crystalline and the porphyritic aspect is due simply to large distinct crystals set in a mass of minute obscure ones. The rock is then really holocrystalline, but the term porphyry is applied to it. In other rocks the crystals of the ground-mass become more and more distinct, the porphyritic aspect gradually disappears, and there is a graduation into the next class. ”

“ The matrix is very coarsely crystalline, and the included organic remains are chiefly stems of Crinoids. (Original.) annihilated, and the rock becomes completely crystalline throughout. This, for example, is the case with the ordinary white "statuary marble," slices of which exhibit under the microscope nothing but an aggregate of beautifully transparent crystals of carbonate of lime, without the smallest traces of fossils. ”

“ The rock itself has no crystalline form, and any structures that simulate such forms will be found on measurement to have none of the regularity that characterises genuine crystals. A rock, moreover, formed of several mineral species in association will by no means possess a constant chemical composition, and the variations from point to point form a feature of especial interest in the study of igneous masses, of - 8 - sediments deposited on a shore, or of alluvium in a valley stretching far between the hills. ”

“ The minutest crystals may show traces of crystalline outlines; undoubtedly they are of new formation and have originated in situ. This leads us to believe that the whole rock has been recrystallized at a high temperature and in the solid state, so that there was little freedom for the mineral molecules to build up well-individualized crystals. The regeneration of the rock has been sufficient to efface most of the original structures and to replace the former minerals more or less completely by new ones. ”

“ The crystals were all found in disintegrated crystalline rocks, consisting principally of coarse feldspathic granite, which have all decomposed to a greater depth than their position. Most of them are obtained by digging where one crystal has been found, or by driving a plow till some hard object is struck. Several dozen have been found weighing from twenty to thirty pounds each. ”

“ A series of specimens may be collected showing unaltered or at least quite recognizable sedimentry rocks at the one end, and thoroughly crystalline igneous rocks at the other. Thus the remarkable fact is brought home to the mind that ordinary sandstones, shales, and other sedimentary materials may in the course of ages be converted by underground changes into crystalline granite. The framework of the land, besides being knit together by masses of igneous rock intruded from below, has been strengthened by the welding and crystallization of its lowest rocks. ”

“ Although there still remain some difficulties to be explained, there is little doubt that the fluid condition in which all unstratified crystalline rocks originally existed, was owing to the solvent power of heat; a power whose effect in melting the most solid materials of the earth we witness in the fusion of the hardest metals, and of the flinty materials of glass. [4]
Beneath the whole series of stratified rocks that appear on the surface of the globe (see section Pl. 1) , there probably exists a foundation of unstratified rocks ”

“ But this law of crystallization gives to snow and ice their mechanical properties, and is connected with a multitude of adjustments of water in the solid state to its place in nature. The same law, varied in a vast number of ways in every distinct substance, builds up crystals of all kinds and crystalline rocks, and is connected with countless adaptations of different kinds of matter to mechanical and chemical uses in the arts. ”

“ Great absorption48 and slow conduction favor disruption. A given range of temperature is unequally effective on rocks of different mineral composition. In general crystalline rocks (igneous and metamorphic) are more subject to disruption by this means than sedimentary rocks, partly because they are more compact, but especially because they are made up of aggregates of crystals of different minerals which, under changes of temperature, expand and contract at different rates, while the common sedimentary rocks are made up largely of numerous particles of one mineral. ”

“ Deeply buried in the earth under newer sediments, they have been baked, until sandstones, gravels, and clays came out bright and crystalline, as gneiss, mica-schist, hornblende-schist, and quartzite—all hard crystalline rocks showing at first sight no resemblance to their original material, except in the regularly stratified or bedded arrangement which serves to distinguish them from igneous or volcanic rocks. ”

“ The force of crystallisation is a subject worthy of much consideration. If we examine our slate rocks, through which little veins filled with quartz crystals are spread, we shall see that the mechanical force exerted during the production of these crystals has been capable of rending those rocks in every direction. Those fissures formed by the first system of crystalline veins, in order of time, are filled in by another set of crystalline bodies, which equally exert their mechanical power, and thus produce those curious intersections and dislocations which were long a puzzle to the geologist. ”

“ Under the influence of percolating, heated waters, new minerals are formed in sedimentary or igneous rocks, and rocks once metamorphosed may undergo additional changes. Mineral matter previously widely disseminated through rocks is, under the action of percolating, heated water, brought together and the regeneration and crystallization of a large variety of ores and minerals result. The birthplace of a large variety of ores and minerals is in the zone of metamorphism. It is in metamorphic rocks that the geologist looks for gems, the precious metals, crystalline marble, magnetic iron, etc. ”

“ This eutectic proportion, when attained by two mineral substances in a magma, brings about a complete interlocking of their crystals, as is seen in the quartz and alkali-felspar of the rock known as "graphic granite." The order of crystallisation of minerals from an ordinary non-eutectic magma is profoundly affected by the proportions in which their constituents are present in the mass. ”

“ Being protruded from beneath in a melted state they are found injected through the superincumbent rocks in shafts or veins of various sizes, from several inches to several feet. Sometimes being unable to rend the solid rocks above they are injected between their strata. They are generally somewhat crystalline in structure, because deposited on the same principles as granite, when undisturbed. From their position, superficial extent, and quantity, we infer they are the products of all ages, and of immense igneous action, seated at an unknown distance beneath the surface of the earth. ”

“ I pointed out, I remember, that these delicate forms are, like the frost patterns on a window-pane, really expressions of the crystalline shape and symmetry of the mineral; each mineral consists of crystals, and therefore has its own peculiar crystalline shape which is one of its inherent properties; moreover, this persists unchanging through the ages, and under all conditions, and is in no way dependent upon the environment in which the mineral is situated. ”

“ When a mineral occurs as a well-formed individual crystal, it has a definite, precise shape. The kind of crystal shape it has depends on its own type of crystalline internal structure. A well-formed crystal has smooth, flat, outer surfaces called crystal faces, which are arranged together to form prisms, cubes, pyramids, and many other geometric shapes. For example, quartz, a common Texas mineral, is commonly found as a six-sided, prism-shaped crystal that is topped by pyramid-like forms. Pyrite, another common mineral, occurs as cube-shaped crystals. ”

“ If the quarry be of granite, you find the rocks to be an agglomeration of crystals, of quartz, mica, and felspar. If the rocks be sedimentary, you find them, for the most part, composed of crystalline particles derived from older rocks. If the quarry be marble, you find the fracture of the rocks to be what is called crystalline fracture. These crystals are, in fact, everywhere. If you break a sugar-loaf, you find the surface of fracture to be composed of small, shining, crystalline surfaces. In the fracture of cast-iron you notice the same thing ”

“ The deeper down in the earth the rocks are the more apt they are to be crystallized, because the rocks piled above them help to hold in the heat, just as thick blankets keep you warmest on a cold winter night.
KINDS OF "METAMORPHIC" ROCK
Rock of any kind may be changed to crystallized rock. Where the conditions are not favorable for crystallization the rock is made more solid, and material soaked out of the rocks above filters down into it. The lower layers of sandstone may become almost as solid as glass, and are then called "quartzite." Clay rocks are hardened into slate. ”

“ These rocks, when in their characteristic or normal state, are wholly devoid of organic remains, and contain no distinct fragments of other rocks, whether rounded or angular. They sometimes break out in the central parts of mountain chains, but in other cases extend over areas of vast dimensions, occupying, for example, nearly the whole of Norway and Sweden, where, as in Brazil, they appear alike in the lower and higher grounds. However crystalline these rocks may become in certain regions, they never, like granite or trap, send veins into contiguous formations. ”

“ Now the alterations above described as superinduced in rocks by volcanic dikes and granite veins prove incontestably that powers exist in nature capable of transforming fossiliferous into crystalline strata, a very few simple elements constituting the component materials common to both classes of rocks. These elements, which are enumerated in the table at p. 499, may be made to form new combinations by what has been termed Plutonic action, or those chemical changes which are no doubt connected with the passage of heat, unusually heated steam and waters, through the strata. ”

“ So that we have at our command a source of depression on a grand scale, at every period when granitic rocks have originated in the interior of the earth's crust. All mineralogists are agreed that the passage of voluminous masses, from a liquid or pasty to a solid and crystalline state, must be an extremely slow process. It may often happen that, in the same series of superimposed rocks, some are expanding while still solid or while partially melting, while others are at the same time crystallising and contracting ”

“ This centre must once have been a mass of melted matter far down in the earth, and as you trace the dykes back deeper and deeper into it, the rocks grow more and more stony, till at last they are composed entirely of [Pg 112] large crystals closely crowded together without any glassy matter between them. You know this crystalline structure well, for we have plenty of blocks of granite scattered about on Dartmoor, showing that at some time long ago molten matter must have been at work in the depths under Devonshire. ”

“ In a thick viscid liquid, this systematic arrangement of molecules into definite crystal forms takes place slowly, for the crystalline force in the silicates is far less energetic than that in water, which crystallizes into ice with much rapidity and with great force. Because of this slowness, the solidification of the lava may catch the process of crystallization at any stage. If the lava is cooled quickly, the result is a glass; if less quickly, part glass and part crystals; if slowly enough, all becomes crystalline. In general the slower the growth the larger the crystals. ”

“ In shape, crystallisation, fracture and hardness, it is the same, and often contains, in addition to the chromium, the further addition of traces of carbonate of lime, magnesia, and occasionally faint traces of hornblende and mica, which evidently result from its intimate association with the granite rock and gneiss, amongst which it is mostly found, the latter rocks being of a slaty nature, in layers or plates, and, like granite, containing mica, pyrites, felspar, quartz, etc. ”