450 



Translucent Applied to minerals which allow light to pass through them 

 without being perfectly transparent. 



Trappean The word trap, as used by the older writers, had no very precise 

 signification ; it appears to have been applied to all igneous rocks which 

 occurred in step-like masses. Geikie (Q.J.G.S., 1871, p. 280) proposes 

 to use it as a " convenient synonym for truly volcanic rocks which are 

 found in Paleozoic, Secondary, or Tertiary formations." 



TriaklSOCtahedron The triakisoctahedra are a series of forms in the regular 

 system contained by twenty-four isoceles triangles, the limits of which 

 are formed by the octahedron and the rhombic dodecahedron. Each face 

 cuts two axes at unit distance, the third at a distance equal to a 

 rational quantity m, greater than unity. Syn. Pyramidal octahedron. 



Trichite A name proposed by Zirkel (Mikros. Beschaf., 1873, p. 89), for hair- 

 like crystallites which occur often in curiously bent, curved, or twisted 

 forms in vitreous rocks. 



Trichroism See Pleochroism. 



Tridinic The name given to one of the six systems of crystals. There is no 

 plane of symmetry, and the forms of this system are referred to three 

 unequal and dissimilar axes, all cutting one another obliquely. Syn. Asym- 

 metric, Anorthic, Triclinohedral, Doubly Oblique, Ein-und-eingliedrig. 



Trimorphism See Polymorphism. 



Tritopyramid ; Tritoprism See Pyramid and Prism. 



Tufted (Ger. buschelformig) Applied to microlites and fibres occurring in bundles 

 or tufts. Syn. Fascicular. 



Twinned (Fr. he"mitrope, made; Ger. verzwillingt). A crystal is twinned when 

 two portions of the same individual or two different individuals are grown 

 together in such position that a revolution 180 is necessary to bring them 

 into complete parallelism. The axis of rotation is termed the twinning 

 axis ; and this is normal to the twinning plane, which must be a possible 

 face of the crystal. 



Care must be taken not to confound the twinning-plane with the face 

 of composition, the latter being a plane common to both individuals. 



Unconformity (Ger. discordante Auflagerung) A term used in stratigraphy 

 to express an interrupted relation between two sets of beds, the upper 

 series having a different strike and dip from that of the lower. Such incon- 

 formity may be brought about either by the earlier deposited strata being 

 upheaved and denuded or by their being denuded without upheaval, before 

 the later beds were laid down. 



Uniaxial See Optic Axis. 



Vacuole The name given to the minute amygdaloidal cavities, filled with 

 decomposition-products, found in variolite. 



Variole The name given to spheroidal bodies, varying in size from a pin's head 

 to a pea, which are found projecting from the weathered surface of vari- 

 olite (the altered marginal portion of diabase). They are very differently 

 constituted, consisting sometimes of radiating fibres of some actinolitic 

 mineral, sometimes of felspathic material, with or without pyroxenic or 

 hornblendic granules. 



VariolitlC A name given to a rock-structure which is characterized by the 

 presence of numerous spheroidal bodies as in variolite. See Variole. 



Vein (Fr. filon ; Ger. Gang, Trumm) A general term for any mass of mineral 

 matter filling up a crack or fissure. Geologists distinguish between 

 mineral veins, eruptive veins, and segregation veins. 



