Page:EB1911 - Volume 20.djvu/146

 Megalosaurus), pterosaurs (Rhamphocephalus), and in the Stonesfield slate the jaws of marsupial mammals (Amphitherium, Amphilestes, Phascolotherium) occur.

The building stones of the Great Oolite are mainly oolitic freestones, viz. the varieties of “Bath stone” quarried and mined in the neighbourhood of that city (Corsham Down, Monks Park, Coombe Down, Odd Down, Box Ground, &c.) and more shelly limestones like the Taynton and Milton stone. The Stonesfield slate has been largely worked near Woodstock in Oxfordshire and in Gloucestershire for roofing, &c. The “slates” are brown calcareous sandstone, grey and slightly oolitic calcareous sandstone, and blue and grey oolitic limestone. A curious modification of the Great Oolite—White Limestone division—is characterized by irregular ramifying tubular cavities, usually filled with ochreous material; this rock occurs in blocks and layers, and is used for rockeries under the name of “Dagham stone” from Dagham Down north of Cirencester. (See also .)

OOSTERZEE, JAN JACOB VAN (1817–1882), Dutch divine, was born at Rotterdam on the 1st of April 1817. After acting as pastor at Alkmaar and Rotterdam, in 1863 he was made professor of biblical and practical theology at the university of Utrecht. Oosterzee earned a reputation as a preacher, was editor of the ''Theolog. Jahrbücher'' from 1845, wrote a number of noteworthy books on religious history, and published poems in Dutch (1882). He died on the 29th of July 1882.

OOTACAMUND, or, a town of British India, headquarters of the Nilgiris district in Madras, approached by a rack railway from the Mettapolliem station on the Madras railway. Pop. (1901) 18,596. It is the principal sanatorium of southern India, and summer headquarters of the Madras government. It is placed on a plateau about 7230 ft. above the sea, with a fine artificial lake, and mountains rising above 8000 ft. The mean annual temperature is 58° F., with a minimum of 38° in January and a maximum of 76° in May; average annual rainfall, 49 in. The houses are scattered on the hillsides amid luxuriant gardens, and there are extensive carriage drives. In the neighbourhood are plantations of coffee, tea and cinchona. There are a brewery and two dairy farms. The Lawrence asylum for the children of European soldiers was founded in 1858, and there are also the Breeks memorial and Basel Mission high schools.

OOZE (O. Eng. wáse, cognate with an obsolete waise, mud; cf. O. Nor. veisa, muddy pool), the slime or mud at the bottom of a river, stream, especially of a tidal river or estuary, and so particularly used in deep-sea soundings of the deposit of fine calcareous mud, in which remains of foraminifera are largely present. The word “ooze” is also used as a technical term in tanning, of the liquor in a tan vat in which the hides are steeped, made of a solution of oak bark or other substances which yield tannin. This word is in origin different from “ooze” in its first sense. It appears in O. Eng. as wós, and meant the juice of plants, fruits, &c.

OPAH (Lampris luna), a pelagic fish, the affinities of which are still a puzzle to ichthyologists. The body is compressed and deep (more so than in the bream) and the scales are minute. A long dorsal fin, high and pointed anteriorly, runs along nearly the whole length of the back; the caudal is strong and deeply cleft. The ventral fin is also elongated, and all the fins are destitute of spines. The pelvic fins are abdominal in position, long and pointed in shape, and the pelvic bones are connected with the caracoids. These fins contain numerous (15-17) rays, a feature in which the fish differs from the Acanthopterygians.

In its gorgeous colours the opah surpasses even the dolphins, all the fins being of a bright scarlet. The sides are bluish green above, violet in the middle, red beneath, variegated with oval spots of brilliant silver. It is only occasionally found near the shore; its real home is the Atlantic, especially near Madeira and the Azores, but many captures are recorded from Great Britain, Ireland and Scandinavia; it strays as far north as Iceland and Newfoundland, and probably southwards to the latitudes of the coast of Guinea. It is rare in the Mediterranean. The name opah, which is now generally used, is derived from the statement of a native of the coast of Guinea who happened to be in England when the first specimen was exhibited (1750), and who thought he recognized in it a fish well known by that name in his native country. From its habit of coming to the surface in calm weather, showing its high dorsal fin above the water, it has also received the name of “sun-fish,” which it shares with Orthagoriscus and the basking shark. It grows to a length of 4 to 5 ft. and a weight exceeding 100 ℔, and is highly esteemed on account of the excellent flavour of its flesh.

OPAL, an amorphous or non-crystalline mineral consisting of hydrated silica, occasionally displaying a beautiful play of colour, whence its value as a gem-stone. It is named from Lat. opalus, Gr. , with which may be compared Sansk. upala, a precious stone. Opal commonly occurs in nodular or stalactitic masses, in the cavities of volcanic rocks, having been deposited in a gelatinous or colloidal condition. It is inferior to quartz in hardness (H. 5·5 to 6·5) and in density (S. G. 1·9 to 2·3), whilst it differs also by its solubility in caustic alkalis. The proportion of water in opal varies usually from 3 to 12%, and it is said that occasionally no water can be detected, the mineral having apparently suffered dehydration. Though normally isotropic, opal is frequently doubly refracting, the anomaly being due to tension set up during consolidation. The mineral when pure is transparent and colourless, as well seen in the variety which, from its vitreous appearance, was called by A. G. Werner hyalite (Gr. , glass), or popularly “Müller’s glass,” a name said to have been taken from its discoverer. This pellucid opaline silica occurs as an incrustation in small globules, and is by no means a common mineral, being chiefly found at certain localities in Bohemia, Mexico and Colorado, U.S.A. (Cripple Creek).

The beautiful variety known as “noble” or “precious opal” owes its value to the brilliant flashes of colour which it displays by reflected light. The colours are not due to the presence of any material pigment, but result from certain structural peculiarities in the stone, perhaps from microscopic fissures or pores or from delicate striae, but more probably from very thin lamellae of foreign matter, or of opaline silica, having a different index of refraction from that of the matrix. The origin of the colours in opal has been studied by Sir D. Brewster, Sir W. Crookes, Lord Rayleigh and H. Behrens. In the variety known to jewellers as “harlequin opal,” the rainbow-like tints are flashed forth from small angular surfaces, forming a kind of polychromatic mosaic, whilst in other varieties the colours are disposed in broad bands or irregular patches of comparatively large area. By moving the stone, a brilliant succession of fiery flashes may sometimes be obtained. The opal is usually cut with a convex surface, and, being a soft stone, should be protected from friction likely to produce abrasion; nor should it be exposed to sudden alternations of temperature. The loss of water, sometimes effected by heat, greatly impairs the colour, though moderate warmth may improve it. According to Pliny the opal ranked next in value to the emerald, and he relates that the rich Roman senator Nonius was exiled by Mark Antony for sake of his magnificent opal, as large as a hazel nut. The opal, on account of its unique characters, has been the subject of remarkable superstition, and even in modern times has often been regarded as an unlucky stone, but in recent years it has regained popular favour and is now when fine, among the most highly valued gem-stones.

Precious opal is a mineral of very limited distribution. Though ancient writers state that it was brought from India, and fine stones are still called in trade “Oriental opal,” its occurrence is not known in the East. The finest opals seem to have been always obtained from Hungary, where the mineral occurs, associated with much common opal, in nests in an altered andesitic rock. The fine opals occur only at the Dubnyik mine, near the village of Vörösvágás (Czerwenitza). The workings