Page:The American Cyclopædia (1879) Volume VI.djvu/119

Rh island in the South sea, where they had been left 13 years before. One of these men had learned that two vessels had been lost on one of the Vanikoro islands, and had discovered some of the articles found on the wrecks. Dillon recognized these as having belonged to the expedition of La Pérouse, and returning to Pondicherry was put in command of a vessel, which in July, 1827, reached Whannon, an island of the Vanikoro group, where were found additional traces of the expedition of La Pérouse. Dillon reached France on his return in February, 1829, and received from Charles X. a gift of 10,000 francs and a pension of 4,000. He wrote a narrative of his expedition, under the title of Voyage aux îles de la mer du Sud, 1827 et 1828, et relation de la découverte du sort de La Pérouse.  DILMAN, a town of Persia, in the province of Azerbijan, 75 m. W. by N. of Tabriz, on a stream flowing E. into the N. end of Lake Urumiah, 10 m. distant; pop. about 15,000. It is a modern town, situated in an extensive and fertile valley, and surrounded by gardens, with more cleanly streets than those of older Persian towns. About 4 m. W. is a decayed ancient town of the same name.  DILUVIUM, or Drift, the superficial deposits of clay, sand, gravel, and bowlders which in both hemispheres are spread more or less uniformly over the land of the polar regions and the adjacent portions of the temperate zones. Geologically this deposit is very recent, and is found overlying strata of later tertiary or pliocene age. Inasmuch as great portions of the material of which it is composed seem to have been transported or at least accumulated in their present position by some violent action, the name of diluvium was given to it by the earlier geologists. In the northern hemisphere the drift is found alike in Europe, Asia, and America, extending from the polar regions toward the equator, and disappearing on the continent of North America about lat. 38°; while in Europe all traces of it are said to be lost in the countries bordering on the Mediterranean. In South America it is recognized from Cape Horn northward into southern Chili, and according to some observers much further. The assertion that great deposits of such material occur even in tropical Brazil is denied by the more recent observers. This drifted or diluvial material is divided into diluvium proper, or unstratified drift, and stratified or modified drift, which is the result of a rearrangement of the former by water. Unstratified drift is met with at considerable elevations over the present sea level—3,000 ft. above the Baltic, and at a height of 4,000 ft. in the Grampians of Scotland. It is everywhere characterized by loose masses of rock, more or less rounded, which in many cases have evidently been transported for considerable distances from their parent beds. As already described in the article , they are often of great dimensions, and increase in size as the deposit is traced

toward its source to the northward. In Russia they have thus been identified with ledges more than 800 m. distant toward the north. Bowlders of the same kind of granite, easily recognized, traced from Moscow to St. Petersburg, vary from two or three feet in diameter at the former to as many yards at the latter point. Instances of these phenomena are everywhere to be seen in the northern United States. In southern Wisconsin pieces of native copper were often found in the superficial deposits long before the mines of this metal were discovered on the S. shore of Lake Superior, 300 m. to the north. The N. shores of Long Island are strewn with bowlders of red sandstone, and of granite and other primary rocks, arranged in groups which correspond with the position of the ledges of the same rocks in Connecticut. So on the European continent, the stratified rocks of which the whole region on the S. side of the gulf of Finland is composed are covered with granitic bowlders from the primary region of Scandinavia on the other side of the gulf. These bowlders, or erratic blocks, are in some places the only evidences of diluvial action, and are found resting directly upon the solid rocks; but in such cases they have been left in this position by the subsequent washing away of the finer portion of the original diluvium or unstratified drift, in which they were included. This may be described as a heterogeneous mass of clay with sand and gravel in varying proportions, enclosing the transported fragments of rock, of all dimensions, partially rounded or worn into wedge-shaped forms, and generally with surfaces furrowed or scratched, the whole material looking as if it had been scraped together. Such is the unstratified diluvium, or bowlder clay, as it is sometimes called; while in allusion to its supposed accumulation by the agency of ice, it is often designated glacial drift. The rocks beneath this deposit are worn smooth or polished, and more or less deeply grooved or striated in a manner which shows unmistakably that the drift has been made to move over the surface with great force, grinding, planing, and scoring the rocks beneath. Resting on this deposit are generally found accumulations of stratified clays and sands, evidently arranged by deposition from water, which are known as stratified drift, or modified drift.—The characters, relations, and distribution of these various products of diluvial action in North America have been studied with great care by Newberry and by Dawson. The latter has recently published a valuable summary under the title of “The Post-pliocene Geology of Canada.” Nowhere are the phenomena of this geological period better seen than in the valley of the St. Lawrence and its tributaries. Dawson divides the deposits of this drift formation into three parts: the first, or lowest, the so-called bowlder clay; the second, or leda clay, a fine deposit in deep waters; and the third, or saxicava sand, an accumulation of shallow-water sand