Foliation (geology)

“ On the other hand, where vegetation is scant or absent, transportation and corrasion are favored, while weathering is retarded. There is no accumulation of disintegrated material. The rate of erosion is limited by the rate of weathering, and that varies with the diversity of rock texture. The soft are eaten away faster than the hard; and the structure is embodied in the topographic forms.
Thus a moist climate by stimulating vegetation produces a sculpture independent of diversities of rock texture, and a dry climate by repressing vegetation produces a sculpture dependent on those diversities. ”

“ Such rounded cores, known as bowlders of weathering, are often left to strew the surface.
Differential weathering. This term covers all cases in which a rock mass weathers differently in different portions. Any weaker spots or layers are etched out on the surface, leaving the more resistant in relief. Thus massive limestones become pitted where the weather drills out the weaker portions. In these pits, when once they are formed, moisture gathers, a little soil collects, vegetation takes root, and thus they are further enlarged until the limestone may be deeply honeycombed. ”

“ In many cases the new structure is parallel with the old; but in others, as in the "strain-slip" cleavage of a phyllite, the newer can be seen distinctly cutting across the older mineral banding. To put it briefly, I assert, as the result of examining numbers of specimens, that, though in certain cases the new structure is dominant, a practiced eye seldom fails to detect traces of the older foliation, while in a large number of instances it is still as definite as the stripe in a slate. We have got, then, thus far—that pressure acting on rocks previously crystallized can produce a foliation ”

“ As the roots penetrate into the clay, they more or less open up a way into it for the rains. Now the rains in nearly all lands, when they have a passage downwards, have a tendency to carry down the clay along with them. They do so, it has been observed, on sandy and peaty soils, and more quickly when these soils are laid down to grass. Hence the mechanical action of the rains,—slowly in many localities, yet surely,—has a tendency to lighten the soil, by removing a portion of its clay. ”

“ Sandstones by weathering yield only a sandy soil. Limestones lose their calcium carbonate by solution, leaving only clay with fragments of quartz or chert as impurities. A foot of soil may represent the weathering of a hundred feet of limestone. Shales may weather into products more nearly like those of the weathering of igneous rocks. Silicates in the shales are broken down to form clay, which is mixed with the iron oxide and quartz. ”

“ Possibly this may explain the mineral banding of much of our older granitoid rock, already mentioned, and the coincidence of foliation with what appears to be stratification in the later Archæan schists, as well as the certainly common coincidence of micro-foliation with bedding in the oldest indubitable sediments. Pressure, no doubt, has always been a most important factor in the metamorphism of rocks ”

“ The proportion of peculiar species, many of them of tertiary aspect, is confessedly large; and great aqueous erosion suffered by the white chalk, before the pisolitic limestone was formed, affords an additional indication of the two deposits being widely separated in time. ”

“ In a word, it would look exactly as vast tracts of the English, Scotch, and Irish lowlands must have looked before returning vegetation coated their dreary sands and clays with a layer of brown vegetable soil.
Thus, and I believe thus only, can we explain the facts connected with these boulder pebbles. No agent known on earth can have stuck them in the clay, save ice, which is known to do so still elsewhere. ”

“ The Pinney Hollow formation, which is an estimated 2,300 or more feet thick, is primarily composed of pale-green schist [5] with abundant white quartz layers and lenses sandwiched between the dominantly greenish colored rock. Originally these metamorphic rocks were sand, mud and sandy mud deposited on a relatively shallow sea floor. After burial these sediments hardened into thin layers and lenses of sandstone, shale, and sandy shale. ”

“ Its aggregate thickness amounts sometimes to 700 or 800 feet. It usually occurs in regular horizontal beds, and is occasionally intersected by deep valleys, such as those of Sortino and Pentalica, in which are numerous caverns. The fossils are in every stage of preservation, from shells retaining portions of their animal matter and colour to others which are mere casts. The limestone passes downward into a sandstone and conglomerate, below which is clay and blue marl, from which perfect shells and corals may be disengaged. The clay sometimes alternates with yellow sand. ”

“ The upper sand forms a very productive arable soil, and the clay impervious, wet [Pg 94] and cold lands chiefly in pasture. The lower sand is generally unproductive.
It is an important agricultural remark, that where the clay (plastic clay) comes in contact with the top of the chalk, an improved soil is produced, and that where the chalk and the green sand mix, extremely fertile patches of country present themselves. (See pages 86 and 87.)
4. The Wealden formation, nearly 1000 feet thick, consists of 400 feet of sand, covered by 300 of clay, and resting upon 250 of marls and limestones. ”

“ In proportion as vegetation is profuse the solvent power of percolating water is increased, and on the other hand the ground is sheltered from the mechanical action of rains and rills. The removal of disintegrated rock is greatly impeded by the conservative power of roots 105and fallen leaves, and a soil is thus preserved. ”

“ In the upper coal measures of Lancashire, and in the shales of the peculiar oolitic strata of Yorkshire, we have found thin leaves yet retaining their elasticity, and changed to a brown translucent pellicle, in which the impressions of the superficial respiratory pores might be clearly seen. In other cases the nervures and seed vessels of fern leaves are perfectly retained in shale, fine sandstone, and ironstone.
The distribution of fossil plants in the earth is remarkable on many accounts. ”

“ In some localities the soil may accumulate to a considerable depth, allowing the processes of weathering to go to an extreme; in others the processes may be interrupted by erosion, which sweeps off the weathered products at intermediate stages of decomposition and may leave a very thin and little decomposed soil. ”

“ Sand does not at all diminish in bulk in drying, but peat shrinks in one-fifth, and agricultural clay nearly as much. The roots are thus compressed, and air is excluded, especially from the hardened clays, and thus the plant is placed in a condition unfavourable to its growth. ”

“ Where deposited under water, soil materials may be brought above the water by physiographic changes, and exposed at the surface in condition for immediate use. Or, they may become buried by other sediments and not be exposed again until after they have been pretty well hardened and cemented,—in which case they must again undergo the softening processes of weathering before they become available for use. Where soils become buried under other rocks and become hardened, they are classed as sedimentary rocks and form a part of the geologic record. ”

“ In many of these orthogneisses the foliation is primitive, being an original character of the rock which was produced either by fluxion movements in a highly viscous, semi-solid mass injected at great pressure into the surrounding strata, or by folding stresses acting immediately after consolidation. ”

“ The peaty soil on which they rest contains numerous flattened stems of reeds and other plants, and is penetrated by roots, probably of some aquatic or marshy plants which spread over the site of the forest as it became gradually submerged. A lower forest-bed rests directly on a foundation of boulder clay. ”

“ The increased thickness of the limestones in those regions, as in the Alps and Jura, where the clays are comparatively thin, arises from the calcareous matter having been derived from species of corals and other organic beings which live in clear water, far from land, to the growth of which the influx of mud would be unfavourable. ”

“ Thus a group of shales followed by a fossiliferous limestone would almost always mark the lapse of a much longer period than an equal depth of sandy strata. A thick mass of limestone, made up of organic remains which lived and died upon the spot, and whose remains are crowded together generation above generation, must have demanded many years or centuries for its formation. ”

“ They urge that the platy minerals, mica and chlorite, are produced during the alteration of the rock, and can spread with ease in directions perpendicular to that of compression; they thus give rise to slaty cleavage at a late stage in the deformation of the rock. These authors, it will be seen, have developed one of Darwin's principal propositions, as to the close connexion between rock-cleavage and foliation, and, in opposition to Sorby, consider the platiness of the original constituents to be of less importance. ”

“ Soc. Trans.”) and Mr. Darwin (“Geological Observations in South America”) on these obscure subjects. I may add that wherever I met with the gneiss, mica, schists, and slates, in Sikkim, very near one another, I invariably found that their cleavage and foliation were conformable. This, for example, may be seen in the bed of the great Rungeet, below Dorjiling, where the slates overlie mica schists, and where the latter contain beds of conglomerate. In these volumes I have often used the more familiar term of stratification, for foliation. ”

“ The crystalline arrangement of the lime in the organic structures enables them to resist disintegration better than the general mechanically aggregated matrix of the rock. An experienced fossil collector will always search well such weathered surfaces, for he often finds there, delicately picked out by the weather, minute and frail fossils which are wholly invisible on a freshly broken surface of the stone. Many rocks weather with a thick crust, or even decay inwards for many feet or yards. ”

“ In soft clays concretions may, as they grow, press the clay aside. In many other rocks concretions are made by the process of REPLACEMENT. Molecule by molecule the rock is removed and the mineral of the concretion substituted in its place. The concretion may in this way preserve intact the lamination lines or other structures of the rock. Clays and shales often contain concretions of lime carbonate, of iron carbonate, or of iron sulphide. Some fossil, such as a leaf or shell, frequently forms the nucleus around which the concretion grows. ”

“ Not only does the natural coating of vegetation, such as our forests impose upon the country, protect the soil from washing away, but the roots of the larger plants are continually at work in various ways to increase the fertility and depth of the stratum. In the form of slender fibrils these underground branches enter the joints and bed planes of the rock, and there growing they disrupt the materials, giving them a larger surface on which decay may operate. ”

“ They consist of blue clays with some subordinate layers of lignite, and exhibit a richer flora than the overlying Newer Pliocene beds, and one receding farther from the existing vegetation of Europe. They also comprise more species common to the antecedent Miocene period. Among the genera of flowering plants, M. Gaudin enumerates pine, oak, evergreen oak, plum, plane, alder, elm, fig, laurel, maple, walnut, birch, buckthorn, hickory, sumach, sarsaparilla, sassafras, cinnamon, Glyptostrobus, Taxodium, Sequoia, Persea, Oreodaphne (Fig. 134) , Cassia, and Psoralea, and some others. ”

“ Should permeable soil lie between the top stratum and a bed of clay, water will accumulate upon the clay, make it slimy and cause a flow upon the bed; for example, when a thin bed of vegetable earth rests upon gravel, sand, or peat, and that upon clay, water will percolate, and perhaps air, through the top soils, and may cause them to slip upon the clay-bed. Also should a layer of clay overlie permeable strata, as clay upon sand, or clay upon gravel, unless it is sufficiently thick and solid to prevent infiltration, it may slide upon the permeable soil as its lower surface becomes wet. ”

“ The feeding tips of roots secrete acids that eat away lime and other substances that occur in rocks. Dead leaves and other discarded portions of the trees rot about their roots, and form soil of increasing depth. The largest trees grow on the rocky soil deposited at the base of the slope. The tree's roots prevent the river from carrying it off.
When granite crumbles, its different mineral elements are separated. Clear, glassy particles of quartz we call sand. Dark particles of feldspar become clay, and may harden into slate. Sand may become sandstone. ”

“ But the Big Trees are only found in certain favoured spots—sheltered places watered by snow-fed streams, at an average of from 5,000 to 7,000 feet above the sea. Eight distinct groves have been discovered, all growing in rich, deep, vegetable mould, on a foundation of powdered granite. Broad gaps lie between the principal groves, and it is observed that these invariably lie in the track of the great ice-rivers, where the accumulation of powdered rock and gravel formed the earliest commencement of the soil, which by slow degrees became rich, and deep, and fertile. ”

“ Surface-erosion cuts through the finer superficial layer, into the pebble-layer beneath, leaving, however, portions of the superficial layer as mounds. The size of the mounds depends on the thickness of the superficial layer; their shape depends much on the slope of the surface. The process once started, small shrubs and weeds take possession of the mounds, as the better soil, and hold them by their roots, and thus increase their size, by preventing or retarding erosion. ”