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O P H 



escarpment from the Humber to the Avon , 

 may be regarded as the wasting effects of 

 water on the subjacent red marls and lias 

 clays. Bakewell. De La Beche. LyelL 

 Cleaveland. MantelL Phillips. 



OOLI'TIC. Composed of oolite ; resembling 

 oolite. The name of a large group of 

 strata commencing with the Portland beds 

 above, and terminating in the inferior 

 oolite below. 



OOLITI'FEROUS. Producing oolite, or roe- 

 stone. 



OPA'CITY. (opacitas, darkness, Lat. opa- 

 cite, Fr. opacita, It.) Opaqueness ; 

 darkness. The quality of opacity is not 

 a contrary or antagonist quality to that of 

 transparency, but only its extreme lowest 

 degree. 



OPAL. The quartz resinite of Werner ; 

 untheilbarer quartz of Mohs. A sub- 

 species of indivisible quartz. Of this 

 there are many varieties, the principal of 

 which are, 1. The precious opal, a milk- 

 white variety, with a beautiful play of 

 various rich colours. 2. Fire-opal ; a 

 transparent variety, brought from Mexico, 

 with a carmine-red and apple-green iri- 

 descence of great beauty. 3. Common 

 opal ; a variety differing but little from 

 the precious opal in many of its charac- 

 ters, but not presenting that effulgence, 

 or play of colours, by which the precious 

 opal is distinguished. Its colour is 

 white, shaded with grey, green, or yellow, 

 sometimes milk-white. When viewed 

 by transmitted light, the milk-white and 

 greenish varieties often change their 

 colours. 4. Semi-opal ; a feebly trans- 

 lucent variety, having a conchoidal frac- 

 ture ; colours white, grey, and brown. 

 Prof. Ehrenberg states that nodules of 

 semi-opal, which occur in the Poliers- 

 chiefer, are composed of silex derived 

 from infusorial remains that have been 

 dissolved and formed into siliceous con- 

 cretions, having dispersed through them 

 numbers of infusorial shields, partially 

 dissolved, together with others that are 

 unaltered. Ehrenberg also thinks that 

 he has found indications of microscopic 

 organic bodies of a spherical form in 

 semi-opal from Champigny, and also 

 in semi- opal from the dolerite of Stein - 

 heim, and in precious opal from the 

 porphyry of Kaschan. 5. Menilite ; a 

 variety occurring in small, irregular, 

 roundish masses, often tuberose, or 

 marked with little edges on the surface. 

 The exterior is often bluish or striped, 

 but the interior has a brown or dark 



ry appearance. Fracture conchoidal. 

 is translucent. These varieties con- 

 sist of silex in various proportion, from 

 86 to 95 per cent., combined with 

 oxide of iron and water. The semi- 



opal contains about 3 per cent, of 

 alumina. 



OPALE'SCENT. Resembling opal ; exhibit- 

 ing a play of various colours ; displaying 

 iridescence. 



O'PALIZED. Converted into a substance 

 resembling opal. 



O'PALIZED WOOD. This has the form and 

 texture of wood ; the vegetable matter 

 having gradually given place to a siliceous 

 deposite possessing the characters of 

 semi-opal. Its texture is fibrous ; frac- 

 ture conchoidal, with a moderate lustre. 

 It does not strike fire with steel. Spe- 

 cific gravity between 2'0 and 2'6. Co- 

 lours white and grey, often shaded with 

 yellow or red, and passing into yellow or 

 brown. Translucent at the edges. It 

 has been found in Hungary, near Schem- 

 nitz . Cleaveland. 



OPE'RCULAR. Having a lid, or cover, or 

 operculum. 



OPE'RCULUM. (operculum, Lat. from 

 operio, to close or shmt.) 



1. A lid, by means of which many of the 

 molluscous animals close the aperture of 

 their shells. It is in some animals tes- 

 taceous; in others, homy or cartilaginous. 

 It is affixed to the animal. The oper- 

 culum of multivalves is composed of two 

 or four pieces. The operculum is cal- 

 culated for the protection of the animal 

 when it retires within its dwelling, of 

 which it may be termed the door ; it is 

 adapted to the shape of the aperture, 

 which it closes completely. The carti- 

 laginous operculum of the common peri- 

 winkle is a familiar example. 



2. The flap which covers the gill, or organ 

 of respiration, in fishes. " A large flap, 

 termed the operculum," says Dr. Roget, 

 " extends over the whole gill, defending it 

 from injury, and leaving below a wide 

 fissure for the escape of the water, which 

 has performed its office in respiration." 



OPHI'DIA. (from o^if, a serpent, Gr.) 

 The third order in the class Reptilia, in 

 Cuvier's arrangement, comprising three 

 families, Anguina, Serpentia. and Nuda. 

 In the structure of the skeleton of the 

 serpents, the first of the true reptiles, we 

 may observe a beautiful illustration of 

 the simple means employed in organic 

 structures to accomplish the most numer- 

 ous and diversified ends, and of the re- 

 sources of nature in adapting the forms of 

 bones, in all their essential and common 

 parts, to the various uses the animal is to 

 make of them in the living state. We 

 have here animals destitute of anterior 

 and posterior extremities, destitute of 

 arms and legs, of hands and feet, yet ca- 

 pable of a great variety of those active 



, movements which we see in animals the 

 most gifted with those parts. We see 



