Dike (Dutch, dijk, from the root of dig). I. Primarily a ditch, but now more commonly a wall or embankment intended to restrain the flow of water. Such earthworks were in former times a common means of defence, and were built around castles and fortresses. In Holland are the most remarkable dikes in the world, constructed to prevent the overflow of the lands reclaimed from the sea. Their importance may be appreciated from the fact that a single inundation from the sea in the year 1277 caused the destruction of 44 villages; and in 1287 80,000 persons were destroyed by another, and its present extent and shape were given to the Zuyder Zee. In the 15th century about 100,000 persons were destroyed through the imperfection of the dikes, when their construction was undertaken in the most thorough manner, and a law was enacted enforcing their being kept in order. At present this work is conducted on a systematic plan and at great cost. Embankments are made toward the sea with heavy timbers filled in with stone, and the surface is covered with bundles of flags and reeds fastened down by stakes. Piles also are driven into the sand, and protected by planking as well as by earth, turf, and stones.

These artificial dikes are often 40 ft. above ordinary high water, and wide enough at top for a common roadway. Frequently the slopes are covered with wickerwork made of willow twigs, and the willow tree is extensively cultivated to furnish supplies of these, which require frequent renewal, as also to bind together by its roots the loose sands. Walls of masonry are built in some of the most exposed situations, and rows of piles outside protect the dikes from the action of the waves. The expenditure in Holland for maintaining dikes and regulating the water levels is annually from $2,000,000 to $2,500,000. Engineers are constantly employed, and every provision is made of materials that may be required for immediate repairs. During the winter months watchmen patrol the dikes by day and night, and give alarm whenever the tide threatens to overflow. The people then hasten to the point, and with mats of straw and rushes and large sheets of sail cloth buried in the sand they raise a temporary bulwark, to be more securely built before the approach of the next tide. - Dikes constructed as barriers for reservoirs are built on several well established plans. The loose materials excavated for the channel or basin are piled up in a firm bank and consolidated by rolling with heavy rollers.

Sometimes they are rendered more secure by building within them along their central line a puddle bank of selected clayey earth, mixed with sufficient sand to give it tenacity, so as not to crack in drying. This should be carried down to a solid foundation, and may be advantageously bedded upon a layer of concrete. It is built up a little later than the bank on each side of it, and both are rolled on the addition of every layer of six inches with a heavily ribbed roller of cast iron. The use of any material of the nature of quicksand is to be carefully avoided in any part of the embankment. Next the water it is well to face the work with a layer of broken stone that will pass through a two-inch ring, and over this should be laid a sloping wall of flat stone at an inclination of 1 base to 1 vertical, or from that to one of 3 base to 1 vertical. The broken stone within is a guard against the embankment being penetrated by any small water animals. The dike around the great reservoir of 106 acres in the Central park, New York, is made on the plan given above, which is approved by the engineers of France and England. It is 16 ft. 8 in. wide at top, with an inner and outer slope of 1 1/2 base to 1 vertical.

The puddle bank of clay in the middle, which reaches to within a few feet of the top, is 16 ft. thick. The depth of water around the margin is 34 ft. At the surface of the water the thickness of the embankment is 24 ft. 9 in., and at 30 ft. below it is 114 ft. 9 in. The French engineers give the preference to this mode of construction over that of a wall of masonry alone or of an embankment within a wall. Stonework by settling is liable to injury that can be repaired only at great cost, especially if the structure be concealed within an embankment. Where room is an object, as in the streets of a city, the outer sides of the dike are conveniently held up by steep walls of stone, which add neither to the strength nor to the impermeability of the work. II. In geology, a wall of trap or other igneous rock, which traverses other rocks, and appears to have been produced by the flowing of melted matter into a deep rent or fissure. Dikes are distinguished from veins by the greater uniformity of their contents, by the parallelism of their sides, by their not ramifying into smaller veins, and by their usually larger dimensions.

The name was given them from their frequently projecting above the surface like a wall, owing to the degradation of the softer rock around them, dike being in the north of England and in Scotland a provincial name for wall. They are from a few inches to more than a mile in thickness. In volcanic eruptions they are seen in process of formation, as deep rents open and are filled with liquid lava. In the English coal mines trap dikes are occasionally met with, forming walls across the line of the coal beds, cutting them off, and causing them at times to be thrown out of place. In the United States they occur likewise in the gold mines of North Carolina. In the Connecticut valley, in fissures of sandstone, as well as in New Jersey" the trap dike contains copper ore, indicating that the copper veins in these rocks have a common origin with the dikes, and also with the barytes which forms the gangue or matrix of the vein. Prof. Dana remarks that the trias-sic formations along the Atlantic appear to be a repetition of the processes which occurred in the Huronian and Potsdam periods in the Lake Superior region.

The trap rocks of Lake Superior are often remarkable for the grandeur of their basaltic walls and columns.