437 



Lanceolate Lance-shaped ; tapering gradually. 



Lapilli A general term applied to the irregular small fragments of eruptive 

 rocks (and, by some authors, also to the broken crystals) which occur in 

 volcanic ashes. 



Lattice Structure (Ger. Gitter-, or Fenster-structur) IK serpentines, derived 

 from hornblende-bearing rocks, the alteration has proceeded along cleavage- 

 cracks which intersect one another at an angle of about 124, thus pro- 

 ducing a lattice-work with lozenge-shaped interstices. This structure 

 was first described by Weigand (T.M.M., 1875, p. 183). See Mesh and 

 Netted structure. 



Lava Lava, in its general sense, denotes all rocks which have flowed at the 

 surface while in the molten state. 



Lenticular (Ger. linsenformig) Shaped like a lens. 



Leptomorph (Ger.} A word used by Giimbelto designate a body which, though 

 crystallized, is not bounded by crystallographic faces. It was first applied 

 by him to the so-called " nepheline-glass " of some of the nepheline 

 basalts. See Allotriomorphic. 



Libella (Fr. bulle ; Ger. Blaschen) The bubble of air or gas contained in liquid 

 and glass inclusions, and which may be vibratile or fixed. 



Linear-parallel StrUCtur (Ger.) That structure in rocks in which the mineral 

 components are elongated in one direction (Ger. Streckung). This may 

 be due (1) to differential movement in the magma while still molten 

 (fluxion) ; (2) to mechanical deformation of the consolidated rock. 



Lithoclase See Joint. 



Lithogeny That department of petrology which treats of the formation of 

 rocks. 



Lithoid Stony, a term often used in the description of devitrified glass. 



Lithology The study of rocks ; used by some authors to designate more 

 especially the study of rocks when confined to the laboratory. See 

 Petrography. 



Lithophysae (Ger. gekammerte Spiiroliten) The name given by Kichthofen 

 (J.G.B., 1860, p. 180) to a concentrically chambered variety of hollow 

 spherulites. Iddings has lately proposed that the term should be 

 extended to all forms of hollow spherulites. 



Littoral A term applied to sedimentary rocks, which are formed on the 

 sea-shore. 



Longulite A term proposed by Vogelsang (Die Krystalliten, p. 21) for 

 crystallites of an elongated cylindrical, elliptical, or conical shape. 



Lustre (Ger. Glanz) A term used in describing the character of the reflections 

 obtained from the fractured surfaces of minerals and rocks. There are 

 several kinds of lustre : metallic, adamantine, yreasy, vitreous, resinous, 

 pearly, silky, &c. 



Lustre-mottling A term proposed by Pumpelly (Proc. Amer. Acad., Vol. 

 XIII., p. 260) for the intermittent lustre obtained from the large cleavage- 

 faces of a mineral containing numerous inclusions. The ophitic masses 

 of augite in some dolerites present this phenomenon. Synonymous 

 with G-. E. William's " poicilitic " 



Made Twin (q.v.). 



