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LEFT ONYX 27 OPAL and 231 feet above the tide level of the St. Lawrence. Its depth is said to average 490 feet; but in some places it is upwards of 600 feet in depth, and it is navigable throughout its v/hole extent for vessels <Xf the largest size. The St. Lawrence (un- der the name of the Niagara river) enters it near its S. W. and leaves it at its N. E. extremity, where it is much encumbered with small islands. Lake Ontario has many good harbors; and as it never freezes, except at the sides, where the water is shallow, its naviga- tion is not interrupted like that of Lake Erie. It is, however, subject to violent storms and heavy swells. Toronto, Kingston, Newcastle, and Niagara are the principal towns on the British side; and Oswego, Genesee, and Sackett's Har- bor on the American bank. _ This lake re- ceives numerous rivers, including the Trent and Humber on its N., and the Black, Genesee, and Oswego from its S. shores. It communicates by the Genesee river and Oswego canal with the Erie canal, and, consequently, with the Hud- son river_ and New York City; the Niagara river and the Welland canal, at its S. W. extremity, unite it with Lake Erie, and the Rideau canal connects it with the Ottawa at Ottawa city. Nu- merous sailing vessels and steamers of large size navigate this lake, which is the center of an extensive commerce. ONYX, a semi-pellucid gem with vari- ously colored zones or veins. Any stone exhibiting layers of tv-o or more colors strongly contrasted is called an onyx, as banded jasper, chalcedony, etc., but more particularly the latter when it is marked ■with white and stratified with opaque and translucent lines. The ancients valued it very highly, and used it much tor cameos, many of the finest cameos in (existence being of onyx. ONYX MARBLE, a very beautiful translucent limestone of stalagmitic for- mation discovered by the French in the province of Oran, Algeria, and first brought into general notice at the London exhibition of 1862. It is used for the manufacture of mantel pieces, urns, and interior decorations, also for small orna- ments, such as ink-stands, paper-weights. Much Mexican onyx or Tecalli marble is used for these purposes. OOLITE, in petrology, a variety of limestone, composed of grains, like the roe of a fish, each of which has usually a small fragment of some organism or a grain of a mineral as a nucleus, around ■which concentric layers of calcareous matter have accumulated. In geology and palseortology, the term is not now generally petrological, but is chiefly chronological, being applied to a certain considerable portion of the Secondary period and to the strata then deposited. Some of the oolite limestones are excel- lent for building. OORI, LIMPOPO, or CROCODILE RIVER, a river of S. E. Africa, has its sources in the heart of the Transvaal, between Pretoria and Potchefstrom, de- scribes a huge curve to the N. and joins the Indian Ocean a little N. of Delagoa Bay. Its course exceeds 800 miles, and it has numerous tributaries, the most im- portant being the Olifant from the right. The Limpopo has been ascended 50 miles by steamboat; but its upper reaches are obstructed by rapids and falls. OOSTERHOUT. a city of Holland, in the province of North Brabant. It con- tains a town hall, a large Roman Cath- olic church, and a convent. It has manu- factories of beet sugar, pottery, shoes, and iron. It has considerable trade in agricultural products and linen. Pop. about 15,000. OOTACAMUND, or UTAKAMAND, the chief town in the Neilgherry Hills, the principal sanatorium of the Madras presidency, and the summer headquar- ters of the governor of Madras. It stands on a plateau, in an amphitheater surrounded by hills, 7,228 feet above the sea, 350 miles from Madras city, and 24 from the nearest railway station on the Madras line. There are a public library (1859), the Lawrence Asylum (1858) for the children of British soldiers, botanical gardens, the Breeks Memorial and Basel Mission buildings. Pop. 18,829. OOZE, in geology, a stratum consist- ing of minute calcareous and siliceous tests derived from various foraminifers, etc., the wreckage of land, with volcanic ash here and there, found on ocean beds. It was discovered in the Atlantic, where it exists between 5,000 and 15,000 feet in depth, whence it is often called Atlantic ooze; but it occurs also in the Pacific. The ooze is an appropriate habitation for sea lilies, sponges, etc. It is identical with the material of which chalk is com- posed, and its deposition has gone on from Cretaceous times till now. In tan- ning, a solution of tannin obtained by infusing or boiling oak bark, sumac, cate- chu, or other tannin-jrielding vegetable; the liquor of a tan-vat. OPAL, a precious stone of various colors, which comes under the class of pellucid gems. It consists of silica with about 10 per cent, of water, and is very brittle. It is characterized by its irides-