446 



Reticulated Having the appearance of network ; traversed by two sets of 

 parallel lines. 



Rhombic The name given to one of the six systems of crystals. It is charac- 

 terized by three planes of symmetry. The forms of the rhombic system 

 are referred to three unequal and dissimilar axes, which stand at right 

 angles to one another. Any one of these may be taken as the vertical 

 axis. Syn. Orthorhombic, Prismatic, Anisometric, Trimetric, Ein-und- 

 einaxiges. 



Rhombic dodecahedron One of the forms belonging to the regular system of 

 crystals. It is contained by twelve equal and similar rhombs (the diagonals 

 of which bear to one another the ratio of 1 : i / 2). Each face cuts two axes 

 at unit distance and is parallel to the third. Six planes of symmetry of 

 the regular system coincide in direction with the rhombic dodecahedral 

 faces. 



Rhombohedral See Hexagonal. 



Rhombohedron The rhombohedra are the hemihedral forms derived from the 

 hexagonal protopyramids by the disappearance of alternate faces three 

 at the top, and three at the bottom. Two series of rhombohedra, differing 

 only in position, are thus obtained. One of these is known as the positive, 

 the other as the negative, series. 



Rock (Fr. roche ; Ger. Gestein, Felsart, Erdglied, Gebirgsart) An aggregate 

 of one or more mineral substances, possessing a more or less definite and 

 persistent geological character, which is maintained over some considerable 

 area. Ore and mineral veins, vein-stones, &c., are mineral aggregates ; 

 but these differ from rocks in not being characterized by invariability as 

 regards structure and substance. 



Saccharoid Applied to crystalline limestones and other rocks which have a 

 texture resembling loaf-sugar. 



Salband (Ger.} A thin crust at the margin of a dyke or vein. 



Scale (Fr. ecaille ; Ger. Blattchen, Schiippchen) A minute lamina. Mica, 

 chlorite, and kaolin are examples of minerals which occur in scales. 



Scalenohedron The scalenohedra are the hemihedral forms derived from the 

 dihexagonal pyramids by the disappearance of alternate pairs of faces. Two 

 series of scalenohedra are thus obtained, which differ only in position 

 from each other. They are known respectively as the positive and negative 

 series. 



Schiller (Ger.) The submetallic lustre given by the crystals of some minerals 

 (bronzite, hypersthene, &c.) when held in certain positions. 



Schillerization The name given by Prof. Judd (Q.J.G.S., XLL, 1885, p. 383) 

 to a change which the mineral constituents of deep-seated rocks undergo. 

 According to Judd this consists "in the development along certain planes 

 within the crystal of tabular, bacillar, or stellar enclosures, which give rise 

 to the peculiar phenomena expressed by the term schiller." 



Schiller-plane A plane in a crystal in which occur the enclosures giving rise 

 to the phenomenon of schiller. 



SchistOSity (Ger. schieferig) This term denotes the fissility of the crystalline 

 schists usually, if not always, the result of mechanical metamorphisni. 



N.B. It should be remembered that some authors use the term 

 schistose simply in the sense of fissile. Thus, according to them, slate 

 would be a schistose rock. 



Sehliere (Ger.) A word much used by German petrographists to denote the 

 streaks and bands produced by differential movements in rock-magmas. 



Scoriaceous Having the cindery appearance of scoriae. 



