A few hornblendeschists are metamorphosed gabbros; others have developed from dikes or sills of lamprophyre.
Neuwerk, containing some marshland protected by dikes, has two lighthouses and a lifeboat station.
The most fertile districts lie on the banks of the Elbe and near the North Sea, where, as in Holland, rich meadows are preserved from encroachment of the sea by broad dikes and deep ditches, kept in repair at great expense.
When it is remembered that the woodwork is infested by the pile worm (Teredo navalis), the ravages of which were discovered in 1731, the labour and expense incurred in the construction and maintenance of the sea dikes now existing may be imagined.
Granite is the most widely spread of the crystalline rocks; but dikes of various kinds occur, and gneiss, schist and marble are also met with.
Practically all the remaining area in these islands is occupied by metamorphic schists and gneisses which occur in great variety and with which are associated numerous dikes and masses of intrusive igneous rock.
These beds, as well as the Cretaceous series, from which they are as yet only imperfectly distinguished, are associated with sheets of basalt, which penetrate them in great dikes, and in some places, owing to the wearing away of the softer sedimentary rocks, stand out in long walls running across the beds.
Many vitreous rocks show alteration of this type in certain parts where either the glass has been of unstable nature or where agencies of change such as percolating water have had easiest access (as along joints, perlitic cracks and the margins of dikes and sills).
In Arran there are pitchstone dikes, some of which are very completely vitreous, while others are changed to spherulitic felsites more or less silicified.
Most of the island is occupied by the band of the old rocks, which include mica, glaucophane and sericite-schists and slates; there are small intrusions of granite, and numerous dikes and masses of basic eruptive rocks.
The main mineral deposits are the nickel ores, occurring as veins of garnierite, associated with peridotite dikes, in the ancient rocks of the eastern slope of the island.
Lying for the most part below sea-level, the islands are protected by a continuous line of artificial dikes, which hide them from view on the seaward side, whence only an occasional church steeple is seen.
The Cordillera Negra in this region is in many places cut by numerous dikes of diorite, and it is near these dikes that silver ores are chiefly 1 See L.
The gain would be the addition to the kingdom of a new and fertile province of the area of North Brabant, a saving of expenses on dikes, diminution of inundations, improvement of communication between the south and the north of the kingdom, protection of isles of the sea, &c. The costs were calculated as follows: (I) enclosing dike, sluices, and regulation of Zwolsche Diep, £1,760,000; (2) reclamation of four polders, £5,200,000; (3) defensive works, £400,000; (4) indemnity to fishermen, £180,000; total, £7,540,000.
Kerguelen Island is of undoubted volcanic origin, the prevailing rock being basaltic lavas, intersected occasionally by dikes, and an active volcano and hot springs are said to exist in the south-west of the island.
Igneous intrusions consist only of unimportant dikes of trap. The most striking and uniformly characteristic geologic feature of the mountains is their internal structure, consisting of innumerable parallel, long and narrow folds, always closely appressed in the eastern part of any crosssection (Piedmont Plateau to Great Valley), less so along a central zone (Great Valley and Valley Ridges), and increasingly open on the west (Allegheny and Cumberland Plateaus).
Besieged from May until October, it was at length relieved by the cutting of the dikes, thus enabling ships to carry provisions to the inhabitants of the flooded town.
It had been protected under the native kings by a system of dikes, which were added to under the earlier viceroys, but serious inundations in 1553 and 1580 flooded the city, and the latter suggested the relief of the highest lake, that of Zumpango, by a tunnel carrying its chief affluent into a tributary of the Panuco, and so to the Atlantic. This, however, was not then undertaken, and when mooted again in 1603 was opposed as certain to involve a heavy sacrifice of Indian life.
But it did nothing for the southern lakes, so that a further system of dikes was recommended in preference, in 1614, by the Dutch engineer Adrian Boot; it was inadequate for its work and, not being lined with masonry, it was liable to be choked by falls.
The sedimentary rocks are affected by many dikes and sheets of igneous rock, some of the latter being extrusive and some intrusive.
The principal sea-inlets in the north are the Texel Gat or Marsdiep and the Vlie, which lead past the chain of the Frisian Islands into the large inland sea or gulf called the Zuider Zee, and the Wadden or " shallows," which extend along the shores of Friesland and Groningen as far as the Dollart and the mouth of the Ems. The inland sea-board thus formed consists of low coasts of sea-clay protected by dikes, and of some high diluvial strata which rise far enough above the level of the sea to make dikes unnecessary, as in the case of the Gooi hills between Naarden and the Eem, the Veluwe hills between Nykerk and Elburg, and the steep cliffs of the Gaasterland between Oude Mirdum and Stavoren.
These are represented by the famous sea dikes called the Westkapelle dike and the Hondsbossche Zeewering, or sea-defence, which were begun respectively in the first and second halves of the 15th century.
These two sea dikes were reconstructed by the state at great expense between the year 1860 and 1884, having consisted before that time of little more than a protected sand dike.
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.
The firm and regular dunes which now run from Petten to Kallantsoog (formerly an island), and thence northwards to Huisduinen, were thus formed about the Zyper (1617) and Koegras (1610) dikes respectively.
In more than one quarter the dikes have been repeatedly extended so as to enclose land conquered from the sea, the work of reclamation being aided by a natural process.
Layer upon layer of clay is deposited by the sea in front of the dikes, until new fringe has been added to the coast-line on which sea grasses begin to grow.
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.
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.
A section of artificially drained land), by surrounding it with dikes or quays for the twofold purpose of protecting it from all further inundation from outside and of controlling the amount of water inside.
As the system of impoldering extended, the small sluggish rivers were gradually cut off by dikes from the marshy lands through which they flowed, and by sluices from the waters with which they communicated.
Sea-aster flourishes in the Wadden of Friesland and Groningen, the Dollart and the Zeeland estuaries, giving place nearer the shore to sandspurry (Spergularia), or sea-poa or floating meadow grass (Glyceria maritima), which grows up to the dikes, and affords pasture for cattle and sheep. Along the coast of Overysel and in the Biesbosch lake club-rush, or scirpus, is planted in considerable quantities for the hat-making industry, and common sea-wrack (Zostera marina) is found in large patches in the northern half of the Zuider Zee, where it is gathered for trade purposes during the months of June, July and August.
The mineral resources of Holland give no encouragement to industrial activity, with the exception of the coal-mining in Limburg, the smelting of iron ore in a few furnaces in Overysel and Gelderland, the use of stone and gravel in the making of dikes and roads, and of clay in brickworks and potteries, the quarrying of stone at St Pietersberg, &c. Nevertheless the industry of the country has developed in a remarkable manner since the separation from Belgium.
The remarkable stability of the mountain appears to be due to the innumerable dikes which penetrate the lava flows and tuff beds in all directions and thus bind the whole mass together.
Yet large as the terms were, the emperor would probably have been well advised to grant them; but Honorius was one of those timid and feeble folk who are equally unable to make war or peace, and refused to look beyond the question of his own personal safety, guaranteed as it was by the dikes and marshes of Ravenna.
Canals and dikes have been constructed to control and distribute the much-needed water, and the officials are housed in new buildings of substantial appearance.
A great difference, however, is to be remarked between the coasts of the North Sea and those of the Baltic. On the former, where the sea has broken up the ranges of dunes formed in bygone times, and divided them into separate islands, the mainland has to be protected by massive dikes, while the Frisian Islands are being gradually washed away by the waters.
In places where perennial irrigation is impossible, the land is divided by rectangular dikes into basins.
The oldest rocks consist of gneiss and schist, penetrated by dikes and bosses of granite, syenite, porphyry and other intrusive rocks.
Through these, again, pierce other granites in dikes or lava flows, and overlying the whole are limestones of Cretaceous and Tertiary age, themselves cut through by later volcanic eruptions.
In 1667 was compelled to beat an ignominious retreat through its defenders opening the dikes and flooding the country.
As Antiochia, was now refortified with dikes by Spasines, and christened Spasinu Charax (the wall of Spasines), or simply Charax (Plin.
Intrusive dikes - locally known as ironstone - by preventing erosion are often the cause of the flat-topped hills which are a common feature of the landscape.
The harbour, which is important as a harbour of refuge, is protected on the east by land, and the Federal government has strengthened this protection by dikes and groins and other sand-catching devices; it has five lighthouses.
The researches of Helbig (Die Italiker in der Po-Ebene, Leipzig, 1879) show that the lower valley of the Po was at an early period occupied by people of the Palaeolithic and Neolithic stages of civilization, who built houses on piles along the swampy borders of the streams. It is possible that even they may have begun by crude dikes the great system by which the waters are now controlled; at least it is certain that these works date their origin from pre-Roman antiquity.
In the Rajmahal Hills basaltic lava flows are interbedded with the Gondwana deposits, and in the Karharbari coalfield the Gondwana beds are traversed by dikes of mica-peridotite and basalt, which are supposed to be of the same age as the Rajmahal lavas.
The granite is of the same formation or closely related to that of Madagascar and throughout the islands is closely uniform in its composition, but exhibits dikes of finer grain.
All over Iceland, in both the basalt and breccia formations, there occur small intrusive beds and dikes of liparite, and as this rock is of a lighter colour than the basalt, it is visible from a distance.
The numerous " felstone " dikes, often lamprophyric, occurring in the north and west of Ireland, are probably also of Devonian age.
And S.E., broke through the region now occupied by the British Isles, and basalt was pressed up along these cracks, forming thousands of dikes, from the coast of Down to the Dalradian ridges of Donegal.
The basalt again broke out, through dikes that cut even the Mourne granite, and some of the best-known columnar masses of lava overlie the red deposits of iron-ore and mark this second basaltic epoch.
The Devonian period, as in Victoria, was marked by a series of granitic intrusions, which altered the older beds on the contact, while the quartz-porphyry dikes, which are intrusive in the Silurian rocks at the Mount Bischoff tin mine, doubtless belong to this period.
The purpose was to decrease the floods which sometimes threaten to entirely submerge the city.
They are often intersected by dikes of chalcedony, formerly mistaken for lava.