425 



Basal Plane In the tetragonal and hexagonal systems the plane which cuts 

 the vertical morphological axis at right angles, and runs parallel to 

 the lateral axes. In the rhombic, monoclinic, and triclinic systems 

 the term is applied to one of the three pinacoidal planes [see Pinacoid]. 

 In each case it is parallel to the two lateral axes macro- and brachy- 

 ortho- and c?mo-diagonal respectively. 



Base (Ger. Basis) A term introduced by Zirkel (Mikrosk. Beschaffenh., 1873, 

 p. 268) for the unindividualized ground-paste (Grundteig) of porphyritic 

 rocks, and which may be, according to the nature of the substance com- 

 posing it, either microfelsitic or glassy. It must not be confounded with 

 "groundmass " (q.v.), which is used solely in a macroscopic sense. 



Bausehanalyse (Fr. analyse en bloc) The German expression for the bulk 

 analysis of a rock. 



Bed (Fr. couche ; Ger. Lage) The English committee of the International 

 Commission for the Unification of Nomenclature suggested that the 

 word bed be defined as that portion of a sedimentary deposit which is limited 

 more or less distinctly by two successive divisional planes due t to the 

 mode of deposit. The word layer is applied to thin beds (Fr. lit), or 

 to those portions into which a bed subdivides parallel to the stratification. 



Belonite A term used by Zirkel for a needle-shaped crystallite. Also applied 

 by Allport to the hornblende microlites in the Arran pitchstone. 



Belonospherite A term proposed by Vogelsang (Die Krystalliten, 1875, p. 134) 

 for spheroidal aggregates of crystalline material, showing radial structure; 

 as, for instance, the concretionary bodies in the Napoleonite of Corsica, 

 which consist of hornblende and anorthite. Syn. Needle-spherulite. 



Biaxial See Optic axis. 



Bicuspid With two prongs ; forked. 



Bifurcated (Fr. bifurque ; Ger. gezweigt, gegarbelt) Terminally divided into 

 two prongs, as, for instance, some microlites. 



Bisectrices The directions bisecting the angles formed by the optic axes. The 

 two directions are distinguished as the acute and obtuse bisectrices ; also 

 called the first and second mean lines. 



Blade (Fr. lame ; Ger. Blatt) Used in petrography to designate a long narrow 

 plate or lamella. 



Bomb The name given to masses of lava thrown out from a volcanic vent. 

 These masses assume a spheroidal form in consequence of their having 

 been ejected in a semi-molten condition. 



BOSS (Fr. amas, typlion ; Ger. Stock) The name given to an irregularly shaped 

 mass of intrusive rock ; also sometimes to masses of stratified rock which 

 protrude through younger beds. 



Botryoidal In clusters, resembling bunches of grapes. 



Brachydiagonal The name sometimes given to the shorter of the two lateral 

 axes in the rhombic and triclinic systems of crystals. 



Brachypinacoid One of the three principal crystallographic planes in the 

 rhombic and triclinic systems. It runs parallel to the vertical axis and 

 the shorter of the two lateral axes. 



Brachy pyramid ; brachyprism ; brachydome See Pyramid, Prism, and 

 Dome respectively. 



Brecciated Applied to a rock composed of angular fragments, in contra- 

 distinction to conglomerate. 



