441 



Ocellar-struetur (G-er.) See Centric Structure. 



Octahedron The simplest form in the regular system of crystals. Its eight 

 faces are all equilateral triangles, each cutting the three axes at unit 

 distance. The angle between any two adjacent faces measured across 

 an edge is 109 28'. 



Oolitic (G-er. Koggensteinstructur) A term applied to rocks composed of small 

 spheroidal concretions which show, in most cases, both a radial and a 

 concentric structure. 



Opacite A term employed by some authors to designate the opaque substances, 

 occurring in many rocks, which do not admit of microscopic determination. 



Opalescent Exhibiting a play of colours similar to that of some varieties of opal. 



Ophitic (G-er. diabasisch-kornig, divergent-strahlig-kornig) The name given to a 

 structure characteristic of many dolerites (diabases of the German) in 

 which the augite appears under the microscope in plates extending over 

 large areas, but penetrated and cut up into small portions by lath-shaped 

 crystals and needles of felspar. The separated portions of the augites 

 show, however, that they are in crystalline continuity by the possession 

 of the same optic orientation. Other minerals have been observed to be 

 associated in a similar manner. Syn. Granito-trachytic (v. Lasaulx). 



Optic axis The direction in anisotropic crystals in which a ray of light under- 

 goes no double refraction. Crystals are termed uniaxial when they have 

 one, biaxial when they have two such axes. Crystals belonging to the 

 Tetragonal and Hexagonal systems are uniaxial, those belonging to the 

 Ehombic, Monoclinic, and Triclinic, biaxial. In the former the optic axis 

 coincides with the principal crystallographic axis. In the latter the 

 position of the axes varies with the mineralogical composition of the 

 crystal, sometimes even with the chemical composition of the mineral (as, 

 for instance, in felspar) ; it is also influenced by the temperature (as in 

 selenite). 



Optic normal The direction at right angles to the optic axial plane. It is 

 coincident with the mean axis of elasticity. Syn. Third mean line. 



Orbicular A term applied to rocks (granites, gabbros, and diorites) which 

 contain spheroidal concretions (e.g., the Napoleonite of Corsica). 



Organogenic A term applied to rocks which are derived from organic sub- 

 stances. See Phytogenic and Zoogenic. 



Orientation This term has reference in microscopic petrography to the relative 

 direction of crystal-axes, or of axes of elasticity, in two or more crystals ; 

 similarly oriented crystal sections extinguish simultaneously under 

 crossed nicols. 



Orogenetic That which relates to the formation of mountains. 



Orthodiagonal The name sometimes given to the axis in the monoclinic 

 system of crystals that is at right angles to the remaining two axes. 



Orthopinacoid One of the three principal crystallographic planes in the 

 monoclinic system. It is parallel to the vertical and to the horizontal 

 or ortho-axis. Sec Pinacoid. 



Orthopyramid ; Orthoprism ; Orthodome See Pyramid, Prism, and Dome 

 respectively. 



Orthorhombie See Ehombic. 



Oscillatory When the faces of two different forms of the same order are 

 alternately developed in a crystal, the result being a fine parallel striation, 

 the combination is said to be oscillatory. 



