Dike (geology)

“ Strata of shale and argillaceous limestone, through which it cuts perpendicularly, are altered to a distance of 30, or even, in some places, to 35 feet from the edge of the dike. The shale, as it approaches the trap, becomes gradually more compact, and is most indurated where nearest the junction. Here it loses part of its schistose structure, but the separation into parallel layers is still discernible. In several places the shale is converted into hard porcellanous jasper. In the most hardened part of the mass the fossil shells, principally Producti, are nearly obliterated ”

“ In the most hardened part of the mass the fossil shells, principally Producti, are nearly obliterated; yet even here their impressions may frequently be traced. The argillaceous limestone undergoes analogous mutations, losing its earthy texture as it approaches the dike, and becoming granular and crystalline. But the most extraordinary phenomenon is the appearance in the shale of numerous crystals of analcime and garnet, which are distinctly confined to those portions of the rock affected by the dike. ”

“ Clinging to the dikes are bodies of altered sandstone, but the great sandstone masses of the summit were unaltered and from them have been excavated the gorges. Along the dike-filled fissures there has been some faulting, but there is no reason to believe that the displacement is great in amount. Toward the flanks of the mountain there are a few sheets, the outermost of which is far within the line of maximum flexure. Their escarpments instead of facing upward like the revetting sheets of Mount Ellsworth, face downward; their buried and unknown edges are the edges toward the mountain. ”

“ They generally slope up stream pretty sharply. The tide comes in, loaded right up with fine mud, flows over and into and around the long lines of warping dike, then stops and begins to unload. Now, you see, when there are no warping dikes, the current has nothing to delay it, so it soon gets going on the ebb so fast that it washes away pretty near all it has deposited. But these warping dikes bring in a new state of affairs. They so hinder the ebb that there is more silt deposited, and at the same time there is less current on the flats to carry the mud away. ”

“ The dikes traverse the different stratified formations like gigantic walls, which are often encountered by the coal-miners, and on the surface are frequently left in relief by the erosion of the softer enclosing rock, so that in the west of Scotland, especially, they are actually made use of for enclosures. In other cases the dike has decayed faster than the enclosing rock, and its position is marked by a ditch-like depression. ”

“ Nearly all rocks are traversed by planes of division or cracks called joints, which usually run in several directions, dividing the rock into blocks. And it is probable that dike-fissures are most commonly produced, not by breaking the rocks anew, but by widening or opening the pre-existing joint-cracks. Hence the straight and regular jointing of slate, limestone and most sedimentary rocks is accompanied by wall-like dikes—the typical dikes (Fig. 10) ; while the more irregular jointing of granite and other massive rocks gives rise to sinuous, branching, variable dikes. ”

“ Hence, when we see a coarse-grained dike outcropping on the surface, we are justified in regarding it as very old, for all the fine-grained upper part has been gradually worn away by the action of the rain, frost, etc. Other things being equal, coarse-grained must be older than fine-grained dikes; and the texture of a dike is at once a measure of its age and of the amount of erosion which the region has suffered since it was formed. ”

“ In order to deflect the current and cause it to deepen the channel at the mouth of the river the United States built submerged dikes and sills, composed of willow mattresses weighted down with stone; also several spurs. Spur dike No. 5, located near the 'Head of the Passes,' consists of a cribwork of round piles, hewn walings, and sawn cross-braces, all securely bolted together, with a curtain of round piles bolted against the upstream face. ”

“ River dikes are as necessary as sea dikes, elevated banks being found only in a few places, as on the Lower Rhine. Owing to the unsuitability of the foundations, Dutch dikes are usually marked by a great width, which at the crown varies between 13 and 26 ft. The height of the dike ranges to 40 in. above high water-level. Between the dikes and the stream lie “forelands” (interwaarden) , which are usually submerged in winter, and frequently lie 1 or 2 yds, higher than the country within the dikes. These forelands also offer in course of time an opportunity for endiking and reclamation. ”

“ Along Grand Lake the rock is a compact amphibolite [3] with a strike S. 78 degrees E. cut by numerous pegmatite dikes, having a strike N. 30 degrees W. and a dip 79 degrees W.. These dikes vary in width from three to twenty feet. Half way to the head of the lake is a dike [1] having a total width of eight feet, consisting of a central band of segregated quartz, six feet wide, cut by numerous thin sheets of biotite, which probably mark the planes of shearing. The quartz is bordered on either side by a band of orthoclase,’ one foot in width. ”

“ I have heard so much about the dikes of Holland, that I expected to see a big thing when I came across one of them," added Paul.
"They are a big thing; but really there is very little to see."
"But what is a dike, sir?" asked Paul, curiously. "I never supposed it was anything more than a mud wall."
"It is nothing more than that, only it is on a very large scale, and it must be constructed with the nicest care; for the lives and property of the people depend upon its security. When they are going to build a dike, the first consideration, as in putting up a heavy building, is the foundation. ”

“ The sides of dikes are often as parallel and straight of those of built walls, the resemblance to human workmanship being heightened by the numerous joints which, intersecting each other along the face of a dike, remind us of well-fitted masonry. ”

“ But, since they are formed wholly below the surface, and usually at great depths in the earth, it is evident that they can appear on the surface only as the result of enormous erosion; and erosion is a slow process, demanding, in these cases, many thousands or millions of years. Therefore, the more ancient dike rocks alone are within our reach; those of recent formation being still deeply buried in the earth’s crust. ”

“ Hauke spread the plan of the new dike out on the table. "A few minutes ago," he said, "one of you asked where we should get all the necessary earth. You see here that as far as the foreland extends out into the shallows there is a strip of land left free outside the line of the dike; we can take the earth from there and from the foreland that runs along the dike, north and south from the new koog. If we only have a good thick layer of clay on the water side, we can fill in, on the inside or in the middle, with sand. ”

“ It slopes gently for miles in an easterly direction and consists largely of loose sand.
Between Mountain Springs and the summit is another illustration of the fact that lamination in a crystalline rock is no proof of its sedimentary origin. A small dike less than 2 inches thick cuts across a coarse biotite gneiss at an angle of 30°. It is separated from the gneiss by a thin layer of quartz and feldspar. It is made up of the same constituents as the gneiss, arranged so as to show a well-pronounced gneissoid structure. This is very similar to the large dikes seen in the cañon. ”

“ But the money spent on dikes is the salvation of Holland. The entire country would be washed away in a few years, if they were suffered to decay."
"I see there are trees growing on the shore, farther up the river," added Paul.
"Those trees are willows; and wherever it is possible for them to thrive, they encourage their growth for two reasons: first, because the roots of the trees strengthen the dike; and, secondly, because the willow twigs are wanted in repairing and securing the embankment. The foundations of sea-dikes vary from a hundred and twenty to one hundred and fifty feet in width. ”

“ The earthen dikes are protected by stone-slopes and by piles, and at the more dangerous points also by zinkstukken (sinking pieces) , artificial structures of brushwood laden with stones, and measuring some 400 yds. in circuit, by means of which the current is to some extent turned aside. The Westkapelle dike, 12,468 ft. long, has a seaward slope of 300 ft., and is protected by rows of piles and basalt blocks. On its ridge, 39 ft. broad, there is not only a roadway but a service railway. The cost of its upkeep is more than £6000 a year, and of the Hondsbossche Zeewering £2000 a year. ”

“ As a matter of fact, they are dredges, with living-quarters for the crews. Across the channel behind them, shutting them off from the sea, is a solid-looking clay dike.
“Colonel,” we asked, “how did you put those boats into that mud-puddle,—fly them or jump them over?”
“The dipper-dredge cut the way in, and the suction-dredge closed it behind them. That dike keeps them from being disturbed by the rise and fall of the tide. A great part of the canal between here and Limon Bay was excavated in the dry by steam-shovels working behind dikes forty feet below sea-level ”

“ If the edge of the ice meets an obstacle, it is apt to break at a foot or more from the shore, and the pieces, still carrying their load of gravel, are shoved up the bank to its top, until, as years roll by, the dike is made too high to receive further additions. ”

“ The lake was lower by a foot than I had ever before seen it in the autumn. In August it had washed the bushes on its dikes; now a yard or more of sand tempted a stroller to follow its fair rim past wood and meadow. Along my shore of the lake the natural dike is in places fully seven feet high. It has been made during the centuries by the “thrust” of the ice which results from the expansion of the ice-field by day following its contraction by night. On a sandy shore the expanding ice pushes up a little ridge of silt, and works it higher and higher as the ice mass rises during the winter. ”

“ Such effects may often be traced in dikes, which are only the cracks in rocks filled by materials poured into them at some period of eruption when the melted masses within the earth were thrown out and flowed like water into any inequality or depression of the surface around. The walls that inclose such a dike are often found to be completely altered by contact with its burning contents, and to have assumed a character quite different from the rocks of which they make a part ”

“ The columns lie horizontally across the dikes and are 12 to 15 inches in diameter. The cavities are wholly or in part filled with calcite. Metamorphism of the adjoining shales is apparent for 2 feet away, but the sea water has so decomposed the shale that it is not so strongly marked as it would otherwise be. The dike projects above the water in places for a distance of 1,000 feet, making its total exposed length about 1,800 feet. In the mesa southeast of Rose Cañon, and along the San Diego River, and back of San Diego, the formation belongs almost wholly to the late Tertiary. ”

“ Eight feet the dike rose above the level of the ground; only to the west towards the water where the sluice was to be laid a gap [Pg 303] had been left; also above, in front of the old dike, the water-course was still untouched. Thus, as for the last thirty years, the tide could flow into the koog without doing much damage there or to the new dike. And so the work of men's hands was consigned to the great God above, and placed under his protection until the spring sun should make its completion possible. ”

“ The Beadnell Dike is well exposed on the coast, rising like a Cyclopean wall through sandstone, limestone, shale, and coal beds, whose relative position is but slightly altered, but whose structural characters are greatly changed: coal for some distance from it is valueless, limestone near it will not burn into lime, and shale and sandstone are indurated. The mutual transference of qualities, between the basalt and the stratified rocks penetrated, is well illustrated in this dike. Its width is generally 25 feet; the direction is south 85° west ”

“ Whence, then, has the greenstone and basalt, forming these dikes, come? Are we to suppose, like some of the elder geologists, that a zone of trap is uniformly spread out beneath the granitic series, which composes, as far as we know, the foundations of the earth’s crust? Is it not more probable, that these dikes have been formed by fissures penetrating into partially cooled rocks of the granitic and metamorphic series, and by their more fluid parts, consisting chiefly of hornblende, oozing out, and being sucked into such fissures? ”

“ All that keeps the tide of the North Sea from flooding the country with from ten to a hundred feet of water every day are the dikes which man has built. Behind these huge embankments lies a country as flat as the flattest prairie in Kansas. A few sandhills and an occasional little rise of ground might stick out of the water if the dikes broke, but I doubt it. This “made” land has been fertilized and built up by the silt of the rivers, added to by the labor and science of man, until it is a vast market garden. The water of the rivers is diverted in every direction into canals. ”