Rock (geology)

“ The scenery too is here of the most magnificent description; rocks overhang rocks in ruinous grandeur, and appear so fearfully equipoised, that, although secure in their immensity, they create in the mind the most awful apprehension of their instability, whilst the mighty roar of the ocean beneath, unites in effect with the scenery above.—All around is sublime.——But the Geology, enough of the picturesque.
The body of the rock is composed of Slate and Granite ”

“ The student can hardly learn much about the mineralogical composition of rocks, without at the same time acquiring some knowledge of the manner of their occurrence in nature. We have already briefly described certain sedimentary rocks, such as conglomerate, sandstone, and shale, and have in some measure touched upon their structure as rock-masses. These rocks, as we have seen, are arranged in more or less thick layers or beds, which are piled one on the top of the other. Rocks which are so arranged are said to be stratified, and are termed strata. ”

“ The beginner, however, must constantly bear in mind that the term rock by no means implies that a mineral mass is in an indurated or stony condition.
The most natural and convenient mode of classifying the various rocks which compose the earth’s crust, is to refer, in the first place, to their origin, and in the second to their relative age. ”

“ It was formerly held that coarsely crystalline rocks such as granite are the oldest portions of the crust, but modern geology has conclusively proved that many of the so-called fundamental masses of rock are merely piles of ancient sediments which have been subjected to the repeated operation of powerful physical and chemical forces, and have undergone a complete rearrangement of their substance. ”

“ They cannot be derived from the underlying rock by weathering, but have been brought from elsewhere. Thus where a region is underlain by sedimentary rocks, as is the drift-covered area from the Hudson River to the Missouri, the drift contains not only fragments of limestone, sandstone, and shale of local derivation, but also pebbles of many igneous and metamorphic rocks, such as granites, gneisses, schists, dike rocks, quartzites, and the quartz of mineral veins, whose nearest source is the Archean area of Canada and the states of our northern border. ”

“ When large erratic blocks are found in the immediate neighborhood of the formation from which they have been derived, then it is easy to explain their origin and to trace their course. But it often happens that the nearest rock of the same mineral composition, and therefore, the nearest rock from which they can possibly have been derived, is separated from the site which they now occupy by a lofty chain of mountains. ”

“ In a previous chapter we have compared the layers of the undermost and harder rocks of the earth to the lines on a page of this book as they would appear if the pages were crumpled up into a ball. Sometimes the beds of solid rock have been so distorted that they look like waves of the sea; sometimes they have been completely overturned; hardly ever have they been suffered to lie down flat. ”

“ The layered appearance of sedimentary rocks is one of their most characteristic features and these rocks are said to be bedded or composed of many individual beds of sedimentary rock. Sandstone, composed of sand size particles; shale, originally mud; and limestone, once lime-rich mud, are examples of sedimentary rocks.
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Metamorphic rocks result when igneous or sedimentary rocks are subjected to abnormal heat and pressure. Folding or faulting [2] of rocks within the earth’s crust or deep burial beneath overlying rocks or sediments commonly produce metamorphism. ”

“ The older strata,—the strata which have been most deeply buried,—and especially those which have suffered most from folding and from fracture, show the largest amount of igneous intrusions. The molten rock which has been driven from the earth's interior to within the crust or to the surface during geologic time must be reckoned in millions of cubic miles. ”

“ The ideas here expressed will be much better appreciated by the reader after following through the salient points in the history of North America as set forth in the succeeding pages.
Again, the reader may ask, by what line of reasoning do we conclude that these stratified rocks are so exceedingly ancient? All rocks of Archeozoic Age, including strata as well as certain younger igneous rocks (see below) , invariably occupy a basal position in relation to all other rock systems. ”

“ STRUCTURES DEVELOPED IN COMPRESSED ROCKS Deformed rocks show the effects of the stresses to which they have yielded, not only in the immense folds into which they have been thrown but in their smallest parts as well. A hand specimen of slate, or even a particle under the microscope, may show plications similar in form and origin to the foldings which have produced ranges of mountains. A tiny flake of mica in the rocks of the Alps may be puckered by the same resistless forces which have folded miles of solid rock to form that lofty range. ”

“ Rocks are not permanent in their condition, but at practically all times and places are undergoing some kind of metamorphism which tends to adapt them to their environment. The conception of [Pg 28] rocks as representing phases or stages in a progressive series of changes called the metamorphic cycle aids greatly in correlating and holding in mind many details of rock nature and origin, and brings into some sort of perspective the conditions which have produced rocks. ”

“ Thus mountain chains and mountain systems were created. Often their summits, worn down by frosts and rains, disclose the core of rock which, ages before, then hot and fluid, had underlain the crust and bent it upward into mountain form. Now, cold and hard, these masses are disclosed as the granite of to-day's landscape, or as other igneous rocks of earth's interior which now cover broad surface areas, mingled with the stratified or water-made rocks which the surface only produces. ”

“ Rocks, we have to remember, are in themselves considerable masses, and have relations with others far away. The coarseness of a sandstone at one point, or even over square miles of country, implies the deposition of finer material somewhere else. The lava-flow implies the existence of mysterious cauldrons in the crust. It is, however, fortunate that the primary classification of rocks was promulgated without regard for theories of rock-origins. The work was done by men who were masters and pioneers in mineralogy. ”

“ We have seen how many of the rocks are made in the sea,—the sandstones and the clays,—but there are two other kinds of rocks, about which we must say a little. The first are the Igneous rocks, which means rocks made by fire. These rocks have solidified, most frequently in crystalline forms, from a molten mass. Lava, which flows hot and fluid, from a volcano, and cooling becomes a sheet of solid rock, is an igneous rock. Some igneous rocks solidify under ground under great pressure, and become crystalline rocks such as granite. ”

“ Above high-water mark and thus away from the dash of the waves, we can often trace the progress of decay among such sedimentary rocks as sandstones and conglomerates. Worn into holes and scars, projecting cusps and tapering pinnacles, or eaten away into the rude semblance of a human form, headless perchance, or into the shape of a huge table poised on a narrow pedestal, the rock affords an endless variety of aspects and a continual source of pleasure. ”

“ Minerals do not lose their identities when they make up a rock. Instead, they are merely associated together in varying proportions. Some rocks, as we will find later, instead of being composed of the minerals themselves, are made up of fragments of earlier-formed rocks.
Ordinarily, we think of rocks as hard and solid substances, such as limestone and granite, but some geologists consider loose and uncemented materials, such as sand, gravel, or volcanic ash, to be rocks also. The words sediments or deposits are often used to describe this uncemented or loose material. ”

“ I also had a clear view of the coast mountains for a hundred miles or thereabouts, all of which are tellingly glaciated in harmony with the above generalization. Most of the rock is granite with cleavage planes that cause it to weather rapidly into flat blocks. One conical black hill, fifteen hundred feet high, is volcanic rock, close-grained and dense like some kinds of iron ore. ”

“ And hence if the geologist would form an accurate conception of the origin and structure of many of the stratified rocks, he must study them not in hand specimens neatly trimmed and arranged along the shelves and drawers of a cabinet, nor even in the ponderous blocks daily exhumed by the quarryman, but along some surf-beaten cliff-line or down some precipitous ravine where the rock for centuries has been exposed to the wear and tear of the elements. ”

“ Classification of rocks according to the scale of the phenomena of structure and metamorphosis, which are still passing before our eyes. Rocks of eruption, sedimentary rocks, changed (metamorphosed) rocks, conglomerates — compound rocks are definite associations of cryctognostically simple fossils. There are four phases in the formative condition; rocks of eruption, p 20 endogenous (granite, sienite, porphyry, greenstone, hyperathene, rock, euphotide, melaphyre, basalt, and phonolithe) ; sedimentary rocks (silurian schist, coal measures, limestone, travertino, infusorial deposit) ”

“ All igneous and sedimentary rocks, and earlier-formed metamorphic rocks too, can be changed, without being moved to some other place, 13 into new and different rocks. As they are changed, they may become harder, new minerals may form, and they may look entirely different. For example, granite, an igneous rock, can be changed into the metamorphic rock known as gneiss; limestone, a sedimentary rock, can be changed into marble; shale, a sedimentary rock, can be changed into slate. These changes occur because the earth is a big and complex chemical system. ”

“ Under the drift lie layers of stratified rock, and [Pg 56] under these are the granites and other fire-formed rocks, the beginning of those rock masses which form the solid bulk of the globe. We know little about the core of the earth, but the granites that are exposed in mountain ridges are found to have a great capacity for absorbing water, so it is not unlikely that much surface water soaks into the rock foundations and is never drained away into the sea. ”

“ The nature of the rock, whether sandstone, shale, limestone, or greenstone, must be set down at the same place, and, to save room, a system of marks for the different rocks may be conveniently used. When a sufficient area of ground has been thus traversed, the student may find, say a row of arrows on his map all pointing due west, and indicating a set of quarries about a quarter of a mile distant from one another, the rock in each of them dipping to the west. ”

“ In order to make the age of the North American rough-stone implements clear, we must study the geological evidence.
Clay and gravel are made, we know, from the primitive rocks. The atmosphere and the rain loosen large pieces of rock from the mountains, and they are broken in the beds of streams by the action of the water. The fine particles are produced by the rubbing of the stones together; the water grinds the broken rocks down smooth, and makes pebbles of them. Gravel, sand, and clay, produced in this manner, become sorted out by the action of the water, or stratified. ”

“ Yet, to try out the theory, geologists in the universities make layers of different material, corresponding to the strata of different kinds of stone, and then subject this composition to pressure at both ends, as the earth crust is supposed to be pressed in the crumpling process. The result is that these artificial strata take similar forms to those we see in mountain rock. And that's the answer!
Notice the similarity of the rock wrinkles in the baby mountain in Virginia and these imitation mountains of the laboratory.
WHY MOUNTAINS RUN NORTH AND SOUTH
Look at your relief map once more. ”

“ We see in one place the primary rock, with its veins of granite and quartz,—its dizzy precipices of gneiss, and its huge masses of hornblende; we find the secondary rock in another, with its bed of sandstone and shale,—its spars, its clays, and its nodular limestones. ”

“ Scientists tell us that these vast mountains of primitive rock are composed of granite, gneiss and syenite, with veins of trap. But regarding the formation of these towering masses of rock both geologists and laymen have only speculated and guessed for more than a century, as their descendants and successors will continue to do for ages to come, and leave the maze of mystery as dark and deep as ever. We can only wonder and admire, while scientists wrestle with the mighty problem of creation here presented. ”

“ Rocks.—Every one knows that the crust of the earth is composed of very various substances, some of which are hard and crystalline in texture, like granite; others less indurated and non-crystalline, such as sandstone, chalk, shale, &c.; while yet others are more or less soft and incoherent masses, as gravel, sand, clay, peat, &c. ”

“ Metamorphic rocks are usually associated with the scenery of mountain, moor, and forest. The highly altered siliceous masses furnish but indifferent soils. The connexion between metamorphic rocks and earth-crumpling, and their frequent penetration by granite, lead to the production of rugged ridges and high moorlands, among which denudation has - 161 - cut romantic glens. The schists weather out on the valley-walls along their foliation-surfaces, and scarps arise like those of stratified rocks. ”