Rocks

“ Lofty mountains, lifted up in rounded forms of granite, gneiss and other igneous rocks, massive and grand, like mighty boulders welded together, with monster trees in the valley below, and tall and straight ones high above wherever a ledge or a fissure affords their hardy roots chance to take hold, flank the road for the first ninety miles. ”

“ Thou hast set a bound that they may not pass over; that they turn not again to cover the earth.—It is demonstrable that, were all the rocks which compose our mountain ranges and dry land to be dissolved and carried into the sea, the waters of the globe are sufficient again to cover and conceal from view their vast and multiform materials, and to replace them in those depths whence they originally arose. ”

“ PETROLOGY, the science of rocks (Gr. πέτρος) , the branch of geology which is concerned with the investigation of the composition, structure and history of the rock masses which make up the accessible portions of the earth's crust. Rocks have been defined as “aggregates of minerals.” They are the units with which the geologist deals in investigating the structure of a district. Some varieties cover enormous areas and are among the commonest and most familiar objects of nature. Granite, sandstone, clay, limestone, slate often form whole provinces and build up lofty mountains. ”

“ For the present we shall pass over these more humble whisperers of things curious and strange, and turn to those massive fragments of rocks which, far removed from their original site, are now scattered either singly or in groups over a large portion of the earth’s surface, resting sometimes on the slopes of hills composed of materials totally different from their own, seen sometimes on the sand and gravel of extensive plains, and distant from the mountains of which they were once a part, sometimes from one to three hundred miles: they are Boulders. ”

“ The subsequent changes which the earth's crust has undergone—aqueous, volcanic, and organic—the working up of the conglomerates and sandstones, the depositing of the deep-sea beds, the overflowing of the traps and lavas, the storing away of the carboniferous treasures, are all the story of every hand-book of geology, and pertain no more to one theory than another of the origin of the rocks. ”

“ Upheaved and contorted, rifted and fissured, pierced by dykes of molten matter or worn away over vast areas by aqueous action, the older rocks appeared to bear witness to a state of things far different from that exhibited by the peaceful epoch on which the lot of man has fallen. ”

“ No one told me that the rocks were positively a part of the bill of fare, but I pointed to ten miles of rocks enclosed by a wall and asked a man why they fenced in the rocks, and he said it was a pasture, and he ought to know, as he is a native and to the manner born. ”

“ Our earth, we find, by the records preserved in the foundation-stones of her mountains, has existed through countless ages, and through them all exhibited the same active energies that prevail at the present moment. By precisely similar influences to those now in operation, have rocks been formed, which, under like agencies, have been covered with vegetation, and sported over by, to us, strange varieties of animal life. Every plant that has grown upon the earliest rocks which presented their faces to the life-giving sun, has had its influence on the subsequent changes of our planet. ”

“ 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. ”

“ The old rock stands for the mountains of the Old World, like the Welsh mountains, or the Mendip Hills. The slates stand for the new rocks, which have been piled up against these, one over the other. But, you see, each slate is lower than the one before it, and slopes more; till the last slate which you are putting on is the lowest of all, though it overlies all. ”

“ We must reckon the roots of plants and trees among the agents which break rocks into pieces. The tiny rootlet in its search for food and moisture inserts itself into some minute rift, and as it grows slowly wedges the rock apart. ”

“ These rocks may have been molten in the depths beneath; but no innate powers of nature raised them unto mountains, and separated the hard from the soft, lifting the heavier substance into the highest places, and scooping out the hollows for the lighter. ”

“ The igneous rocks are divided into two general classes,—the volcanic or eruptive rocks, which have been outpoured in open air or on the floor of the sea, and the intrusive rocks, which have been intruded within the rocks of the crust and have solidified below the surface. ”

“ In the splitting of a flagstone, the long-buried skeleton of the fish came to light.
Under our feet the earth lies in layers. Under the soil lie loose beds of clay and sand and gravel, and under these loose kinds of earth are close-packed clays, sandstones, limestones, shales, often strangely [Pg 8] tilted away from the horizontal line, but variously fitted, one layer to another. Under these rocks lie the foundations of the earth—the fire-formed rocks, like granite. The depth of this original rock is unknown. It is the substance out of which the earth is made, we think. ”

“ Next are taken up the rocks as they are formed, either sedimentary or igneous, and so on through the whole rock-world, going step by step in a most natural way from the simplest to the most complex, from the elements to their compounds. ”

“ From the foregoing sketch of the processes of rock-making it may easily be inferred that the varieties of rocks may be almost unlimited, and that they may be defined, named, and classified on many different bases ”

“ 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. ”

“ Among the bare hill-tops, or on the face of cliffs exposed to the weather, the strongest and hardest rocks are every winter split and broken, just as if quarrymen had been at work at them. In the summer the rain-water gets into the little cracks and rifts in the stone and lodges there. Then the winter comes with its cold and freezes the water. And the water bursts the rocks asunder just as it bursts our waterpipes. ”

“ Rocks in themselves having insufficient drawing power, as the dramatic critics say, a maze has been added, together with swings, a seesaw, arbours, a croquet lawn, and all the proper adjuncts of a natural phenomenon. The effect is to make the rocks appear more unreal than any rocks ever seen upon the stage. ”

“ The geologist has travelled over great part of the earth’s surface, has examined the nature of the rocks, which everywhere repose beneath the soil, and has found them to be very unlike in character, in composition, and in hardness—in different countries and districts. ”

“ Thus we learn that when one agency fails, Nature makes use of another to take up the important work of rock-building. How the other rocks which we mentioned in our list were formed,—such as granite, basalt, and the metamorphic rocks,—we must explain in a future chapter dealing with volcanoes and their work. ”

“ As all the stratified rocks have been formed by the wearing down of the original upper slopes and summits, we cannot expect to be able to recognize the original skeleton form of our water-washed globe. ”

“ Mountain ranges observe a singular uniformity of direction, and the cleavage planes of rock are evidently determined by some all-pervading power. Mineral bodies are not distributed in all rocks indiscriminately. ”

“ In various parts of the country there are, amongst the granitic masses, rocks which have fallen across each other, leaving small openings, or there are holes, low and narrow, extending under a pile of rocks. ”

“ Yet, with all their differences, the shapes rocks take—sandstone compared with granite, for example—are so characteristic that one soon learns to tell a long way off what kind of rock a distant landscape is made of. ”

“ PRIMARY ROCKS.
Although the earth has not been actually penetrated to a greater depth than three thousand feet, the nature of its substance can, in many instances, be inferred for the depth of many miles by other means of observation. We see a mountain composed of a particular substance, with strata, or beds of other rock, lying against its sloped sides; we, of course, infer that the substance of the mountain dips away under the strata which we see lying against it. ”

“ Threading the slender passages between the perpendicular rocks, or creeping-in close under their overhanging landward sides, where no ray of the sun has ever penetrated, we hang on by the points and groins, and eagerly peer below. ”

“ 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. ”

“ The woods, the vegetable mould, and the soil beneath, protect the rocks they cover from the direct action of heat and cold, and from the expansion and contraction which accompany them. Most rocks, while covered with earth, contain a considerable quantity of water. ”

“ The Stratified Rocks, of which a very large part of the continents is made, appear to have been built up [pg 13] slowly, layer upon layer, out of the gravel, sand, and mud, washed away from the land and dropped on the shore of the ocean. ”

“ And gradually we perceive that it was not Nature's hand which made this rocky scaffolding so high. Those monochromatic rocks, piled one atop the other, forming a wall all round, and seeming to prolong the mountain range, are the work of human hands. ”

“ 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 ”

“ To obtain a general idea of the nature and method of our science before beginning its study in detail, we may visit some valley, such as that illustrated in the frontispiece, on whose sides are rocky ledges. Here the rocks lie in horizontal layers. Although only their edges are exposed, we may infer that these layers run into the upland on either side and underlie the entire district; they are part of the foundation of solid rock which everywhere is found beneath the loose materials of the surface. ”

“ Led by this observation, the geologist, after comparing the rocks of different countries with one another, compared next the soils of various districts with the rocks on which they immediately rest. The [Pg 81] general result of this comparison has been, that in almost every country the soils have as close a resemblance to the rocks beneath them—as the loose earth derived from the crumbling of a rock before our eyes, bears to the rock of which it lately formed a part. ”

“ The rocks hitherto spoken of contain none of those petrified remains of vegetables and animals which abound so much in subsequently formed rocks, and tell so wondrous a tale of the past history of our globe. ”

“ When beds of sand, clay, and marl, containing shells and vegetable matter, are found arranged in a similar manner in the interior of the earth, we ascribe to them a similar origin; and the more we examine their characters in minute detail, the more exact do we find the resemblance. Thus, for example, at various heights and depths in the earth, and often far from seas, lakes, and rivers, we meet with layers of rounded pebbles composed of flint, limestone, granite, or other rocks, resembling the shingles of a sea-beach or the gravel in a torrent’s bed. ”

“ Mountains internally are principally composed of stone and rocks in parallel beds: between the horizontal beds small strata of a softer matter than stone is found, and the perpendicular clefts are filled with sand, crystals, minerals, metals, &c. these last matters are of a more modern formation than the horizontal beds in which we find sea-shells. ”

“ On a great plain, devoid of hills or rivers, composed of different rocks, and subjected to the agents of{102} disintegration, we can conceive that over each kind of rock a soil would be formed corresponding closely to the materials of which that rock is composed. ”

“ I hope to show in the sequel that the rocks or minerals which lie beneath the thin coating of earth and vegetation have always exerted an immense though often unsuspected influence upon the history of man. ”

“ The same substances which we see around us—granite, porphyry, and basalts, which are called igneous or Vulcanian rocks, as contrasted with the Neptunean rocks, such as gypsum or lime, clay and sandstone, the agglomeration of which is attributed to water. The science which deals with these subjects is called geology, a study with which, some day, you will be delighted. ”

“ These causes very generally impede the view of the natural rock. Wherever it can be seen it appears to possess the disposition which is I believe invariably characteristic of this class of rocks, forming large irregular beds, much rifted and fissured, and never continuous in inclination and extent for any considerable space. ”

“ Beneath this coverlet of soil, extending as far as man has penetrated into the earth, is rock. Excepting in regions overflowed by lava poured out from beneath, or along the backbones of continents where the surface rocks have been upheaved into folds and carried away by denudation, the rocks immediately beneath the soil, to a thickness often of thousands of feet, are in the form of layers, or sheets, arranged one above another. ”