Dyke (geology)

“ On each side of the dyke, betwixt it and the regular strata, there is a thin layer of clay, or, as it is called, a 'gut' or 'core,' about 6 inches thick, which turns the water from the rise to the dip side of the dyke, and forces it to the surface in several springs, in the direction of the dyke, where it crosses the country. ”

“ Why, for example, should it almost never occur that the substance of porphyritic and basaltic dykes, whether they pass through slate, coal, sandstone, or limestone, is extended from the main body into the numerous small cracks and fissures which margin the dyke; while, on the other hand, there are few situations where granite comes in contact with gneiss, clay slate, limestone, mica slate, or hornblende slate, without throwing off many branches into those rocks? ”

“ Galena has been found in a dyke in Willington colliery, and a small string of the same ore has been observed in the main dyke at Whitley. A salt spring issues from a slip in Birtley colliery.
The dykes are an endless source of difficulty and expense to the coal owner, throwing the seams out of their levels, and filling the mines with water and fire damp. At the same time they are not without their use; when veins are filled, as is often the case, with stiff clay, numerous springs are damned up and brought to the surface ”

“ Where the dyke reaches the surface a quarry of the basalt was formerly worked, which has lately been cleared. A small seam of coal meets the basalt at no great depth from the quarry-head, but the place of contact is at present inaccessible. In the neighbouring colliery both portions of the vein hold their course through the seam there worked, and the coal is charred by their influence. ”

“ The Thistle pit dyke which is a downcast of eight fathoms to the south, and traverses the Coal-field from west to east, appears to have been as well known to the miners who lived nearly a century since, as to those of the present day. ”

“ How plants grow—and how animals feed—why some thrive and others are stunted—are questions as yet imperfectly answered: but they are the subject-matter of the science of Physiology; a science, the comprehension of which implies an accurate knowledge of Chemistry and of the general principles of Natural Philosophy. The constitution of the soil, the varieties of strata, are taught by Mineralogy and Geology. Draining, in its present advanced state, depends on the laws of Hydrostatics. The economy of farm-labour requires the skilful application of the laws of Mechanics. ”

“ The Plas Newydd dyke crosses the Menai. The cliff which bounds the dyke at Plas Newydd is composed of clay shale, and argillaceous limestone. The lowest portion (thin calcareous shaly bed) , on approaching the dyke, undergoes various changes. At 15 feet from the contact it forms a compact bluish grey mass, with spots of a fainter colour. In contact it is bluish green, very compact and hard. ”

“ It is noticed, as a circumstance of common occurrence, that mineral veins are no otherwise different from faults than by reason of the fissures which these have opened in the rocks being filled by sparry and metallic matters. This filling of a fissure constitutes a mineral vein; a similar fissure filled by basaltic or other rocks would be called a rock dyke; if occupied by clay and soft materials, a clay dyke. The point of importance in each of these cases is the mechanical formation of a fissure of the rocks along the plane of the fault ”

“ Independent of this circumstance, we know, first, that the forms of plants, crinoidea, and shells, do remain among limestones rendered completely saccharine by heat (Teesdale) ; among shales indurated to a great degree (Coley Hill Dyke) ; among coarse and fine slaty rocks (Snowdon, Coniston,) which have undergone metamorphosis; and, secondly, as we ascend in the series of strata, organic remains gradually appear (at first very few) , and become continually more and more numerous, as the circumstances of the land and sea more approximated to the present. ”

“ Intermediate between the extruded volcanic rocks, and the highly crystalline varieties of the internal reservoirs of molten material, are the dyke or hypabyssal rocks, which as narrow vertical or nearly vertical intrusions pass in all probability from the deep-seated source towards the surface of the earth, when they reach it giving rise to volcanic effects. ”

“ The crystals are much cracked. Ophitic structures are not conspicuous. Brown biotite is very sparingly present in small flakes. Magnetite is liberally scattered in small grains in the augite of the ground mass. Apatite occurs in minute prisms included in the felspars.
Fig. 39.—Diabase, from a dyke in Wadi Kreiga [12,110] , × 17. pl, plagioclase felspar; a, augite; h, hornblende; l, limonite strings.
The diabase dykes which penetrate the granite in Wadi Kreiga [12,110] differ from the rock last described in their freedom from olivine and in showing marked ophitic structure. ”

“ A geologist might naturally expect that such abundant intrusions of igneous rock as those of the dykes should be accompanied with plentiful proofs of contact-metamorphism. But in actual fact, evidence of any serious amount of alteration is singularly scarce. A slight induration of the rocks on either side of a dyke is generally all the change that can be detected.
Fig. 255.—Section of coal rendered columnar by intrusive basalt, shore, Saltcoats, Ayrshire. ”

“ Where two dykes cross each other, it is sometimes not difficult to decide upon their relative antiquity. In intrusive rocks, the finest-grained parts are those which lie nearest the outer margin, where the molten material was rapidly chilled by coming in contact with cool surfaces of rock. Such "chilled margins" of closer grain are common characteristics of dykes. Wherever a dyke carries its chilled margin across another dyke, it must be the younger of the two, and wherever such a margin is interrupted by another dyke, it must belong to the older. ”

“ Temperature at the well 8° above that of the adjoining springs.
↑ Plate 4, fig. 1. ↑ The exportation of lime from Sunderland is chiefly to Scotland, and amounts to from forty-two to forty-five thousand chaldrons of 36 bushels each, annually. ↑ See Plate 2. ↑ The Ninety-fathom Dyke described hereafter. ↑ See a paper by Dr. Lucas Hodgson, on the Salt sublimed, in the Phil. ”

“ Survey, vol. ii. pt. i. pp. 73—75. ↑ British Palæozoic Rocks, 1851. ↑ The reader will remark that the terms "clay slate," and "grauwacke," are used to designate rocks, not formations. ↑ Sedgwick, in Geol. Proceedings. ↑ Address to Geol. Soc. 1849. ↑ Along the side of the Craven fault in Giggleswick Scar (Settle) , the mountain limestone is crossed by many divisional planes, which cause it to split parallel to the fault, with a kind of rude cleavage. By the side of the Coleyhill dyke, heat has produced a similar effect on shale ”

“ But the breadth of igneous rock is not perceptibly greater in the valleys than on the ridges. The depth of boulder clay and other superficial deposits on the valley bottoms, however, too frequently conceals the dykes at their lowest levels. Perhaps the best sections in the country for the study of this interesting part of dyke-structure are to be found among the higher hills of the Inner Hebrides, such as the quartzites of Jura and the granophyres and gabbros of Skye. ”

“ A central light-coloured band, about eight to ten feet broad, consists of a spherulitic granophyre in which the spherulites are crowded together and project from the weathered surface like peas, though they do not here show the curious rod-like aggregation so marked in some other dykes. On either side of this acid centre a narrow basalt dyke intervenes as a wall next to the Torridon sandstone which here forms the country-rock. Such compound dykes have sometimes a total width of 100 feet or more. ”

“ Some marked examples of this alteration of intrusive igneous material are to be observed among the basalt dykes which cut the Lower Lias Shales of Skye. These shales, where black and carbonaceous, as in the island of Pabba, have exercised an unmistakable influence on the abundant dykes which intersect them. The chilled selvage of each dyke has assumed the dull earthy pale-grey or yellowish aspect, which extends for a few inches from the wall into the interior, where it rapidly passes into the ordinary black crystalline basalt. ”

“ They are likewise often markedly acid in composition, including such rocks as granophyre, felsite and pitchstone.
[163] In the Blath Bheinn group of gabbro-hills, however, it is the youngest dykes which have been found by Mr. Harker to possess the lowest hade.
(1) External Characters.—As regards the grain of the rock, every gradation may be found, from a coarsely crystalline mass, in which the component minerals are distinctly traceable with the naked eye, to a black lustrous basalt-glass. Each dyke generally preserves the same character throughout its extent. ”

“ Many dykes exist regarding which there is no evidence to connect them with any actual volcanic rocks. They have been injected into fissures, but whether this took place during volcanic paroxysms, or owing to some subterranean movements which never culminated in any eruption, cannot be decided.
The question of the age of dykes, like that of intrusive masses of all kinds, is often difficult or impossible to decide. A dyke must of course be younger than the rocks which it traverses, and a limit to its antiquity is thus easily fixed. ”

“ The dykes are about two metres wide, of a white colour, and resemble limestone in appearance; being soft, they have weathered more rapidly than the granite around them, leaving vertical-sided chasms. But for the manner of its occurrence, the rock would have been easily mistaken in the field for a sedimentary one. ”

“ But dykes, when seen in perfection, as in the Island of Arran, the coal-field of Durham and Newcastle, the limestone of Teesdale, the lias near Stokesley, the silurian rocks of Shropshire, or the slates of Snowdonia, present characters of greater symmetry, and claim a somewhat different origin. The fissures which inclose these trap dykes present often no trace of violent movement of the strata, which, on the contrary, sometimes appear level and undisturbed on both sides ”

“ In these central bands the cavities are largest and depart farthest from the regular spherical form, so that for short spaces they may equal in bulk the mass of enclosing rock. In some rare instances, a whole dyke is composed of cellular basalt, like one of the lava-sheets in the plateaux, as may be seen on the north flank of Beinn Suardal, Skye. Mr. Harker has observed that an amygdaloidal structure is more common among the earlier than among the later dykes of that district. ”

“ In those districts where the small and crowded dykes of the gregarious type are developed, one cannot usually trace them for more than a short distance. The longest examples known to me are those which have been mapped with much patience and skill by Mr. Clough in Eastern Argyleshire. Some of them he has been able to track over hill and valley for four or five miles, though the great majority are much shorter. In Arran and in the Inner Hebrides, it is seldom possible to follow what we can be sure is the same dyke for more than a few hundred yards. ”

“ But when we pass to the great solitary dykes, that run so far and so continuously across the country, we encounter much thicker masses of igneous rock. Most of the measurements of these dykes have been made at the surface, and the variations noted in their breadth occur along their horizontal extension. The Cleveland dyke, which is the longest in Britain, varies from 15 feet to more than 100 feet, with perhaps an average width of between 70 and 90 feet. [182] Some of the great dykes that cross Scotland are of larger dimensions. ”

“ Among the rocks which emerge from under the Carboniferous volcanic plateaux, dykes are sometimes to be observed in considerable numbers. They may be compared to the far more extensive series connected - 407 - with the Tertiary basalt-plateaux, like which they may have had a close relation to the actual building up of the successive sheets of andesite, trachyte and basalt that were erupted at the surface. They are particularly well developed in the Clyde plateau, where by extensive denudation they have been admirably exposed. ”

“ At least, we know, that many dykes do not reach the present surface, and that those which do have shared in the enormous denudation of the surrounding country. That even in the same dyke the lava rose hundreds of feet higher at one place than at another is abundantly proved. When, however, we consider the vast number of dykes that now come to the light of day, and reflect that the visible portions of some of them differ more than 3000 feet from each other in altitude, we can hardly escape the conviction that it would be incredible that nowhere should the lava have flowed out at the surface. ”

“ As in the modern eruptions of Iceland, new fissures are successively opened through the older lava-sheets, so in the Tertiary volcanic areas, renewed ruptures of the earth's crust allowed later dykes to be formed. The basalt-plateaux are traversed by such dykes, even up to their highest sheets. It is impossible to say how often the process of dyke-making may have been repeated. ”