436 



Intratelluric A term applied by Eosenbusch (Physiog. der massig. Gest., p. 8) to 

 that period in the formation of a rock which immediately precedes eruption 

 (effusion) ; also to the minerals which separate out during that period. 



Intrusive (Ger. durchgreifende Lagerung) Applied to those igneous rocks 

 which have been injected or intruded into other rocks. In this meaning 

 it embraces perhaps all igneous rocks which have not flowed at the surface. 



Irruptive A term used by some authors in contradistinction to eruptive for 

 those intrusive rocks which do not reach the surface. 



Isomerous An epithet applied to the granular structure of a rock when the 

 grains are equal size. 



Isometric See Kegular. 



Isotropic Applied to substances which are not doubly refractive, and 

 consequently always remain dark under crossed nicols. 



Joint (Ger. Absonderung, Fuge, Zerkliiftung) The name given to the divisional 

 planes, other than those of bedding or schistosity, which separate rocks 

 into more or less regular blocks. Daubree (" Les eaux souterraines & 

 1'epoqae actuelle," Paris, 1887) uses the word lithoclase to embrace all 

 kinds of rock-fractures. These he further subdivides as follows 

 diadases (joints), paradases (faults), syndases (joints due to contraction), 

 leptodases (minute fissures). 



Kaolinized A term applied to altered felspar when the decomposition has 

 resulted in the formation of kaolin a hydrated silicate of alumina. 

 This mineral occurs, under the microscope, in cumulous aggregates of 

 minute scales and granules, producing turbidity, or even opacity in 

 the felspar. 



Kelyphit-StructUP (Ger.) A variety of centric structure (q.v.) in which crystals 

 of garnet are surrounded by a shell of radiating needles of augite or 

 hornblende. 



Knotted A term applied in petrology to certain nodular schists (Knoten- 

 schiefer) which contain little concretionary knots. These are sometimes 

 imperfectly developed (incipient) crystals of chiastolite, andalusite, &c., 

 sometimes pseudomorphs after cordierite or andalusite. 



Kramenzel-StFUCtur (Ger.) The name given to a variety of interlaced structure 

 (q.v.) developed in certain sedimentary rocks typically in those of the 

 Devonian age. In the meshes of a network of slaty material are enclosed 

 small flat nodular masses of limestone (Kramenzel-schiefer). 



Krithic Structure This term has been applied by Becke (T. M. M., II., p. 43) 

 to a structure characterizing certain mica schists in which grains of 

 orthoclase occur wrapped round by thin layers of quartz and mica. It is 

 probably a phenomenon of dynamic metamorphism. 



Laccolite A term proposed by G. E. Gilbert (Geology of the Henry Mts., 

 1880) for large masses of rock which, while molten, have risen from 

 below, and not being able to find their way to the surface, have spread 

 out laterally and elevated the superincumbent strata, so as to produce 

 bosses having the form of plano-convex lenses. 



Lacustrine A term applied to sedimentary rocks which have been deposited in 

 lakes. 



Lamella A term used in microscopic petrography for a thin plate. Hence 

 lamellar, lamellated, as, for example, the lamellar twinning in plagioclase 

 felspar. 



Lamina A thin leaf-like plate. Hence laminar, laminated, as, for example, the 

 lamination of shales. 



