435 



ICOSitetrahedron The icositetrahedra are a series of forms contained by 

 twenty-four deltoids (a plane quadrilateral figure bounded by pairs of 

 adjacent equal straight lines), the limits of which are formed by the 

 octahedron and cube respectively. Each icositetrahedral face cuts one 

 axis at unit distance, the other two at a distance measured by a rational 

 quantity m. 



Idiochromatie A term applied, in contradistinction to allochromatic, to those 

 minerals which possess an essential, and therefore characteristic colour, 

 e.g., the metals and many of their salts. 



Idiomorphic A term introduced by Kosenbusch ("Die massigen Gesteme, 

 p. 11). The mineral constituents of rocks are said to be idiomorphic in so 

 far as they are bounded by faces peculiar to the species. The word is 

 synonymous with Bohrbach's " automorpkic " (T.M.M., VII., p. 18). 



Igneous A term employed in petrology to include rocks which have been formed 

 by the consolidation of molten material. The words pyroyenic, inyenltic, or 

 exotic are preferred by some authors. 



Imbricated (Fr. imbrique ; Get: dachziegelartig) Overlapping like the slates 

 of a roof. Tridymite occurs in rocks in imbricated plates. 



Inclusions This term is used in petrography, first with regard to minerals, 

 secondly with regard to rocks. (1) Most of the minerals composing a 

 rock contain inclusions (interpositions) either of gas, liquid, glass (fresh 

 or devitrified), and crystals or microlites of other minerals (See Figs. 8, 9, 

 10, and 11 of text). (2) As applied to rocks, the term refers to pieces of other 

 rocks which have been caught up by the molten magma and remain em- 

 bedded in the rock when it consolidates. They must be distinguished 

 from certain concretionary patches in granite and other rocks, which they 

 much resemble. 



Indigenous -S^ Aqueous. 



IngenitiC See Igneous. 



In situ (Ger. anstehend) In place. 



Interbedded Bedded between ; (1) as applied to igneous rocks, the word is 

 synonymous with contemporaneous, and implies that the rock must have 

 flowed at the surface ; (2) as applied to sedimentary rocks, it merely 

 expresses an alternation of beds. 



Intercept The distance from the origin (or point of intersection of the axes) at 

 which a face cuts a crystallographic axis. 



Interference-figure (Ger. Axenbild) The figure which is obtained when a 

 section of a doubly refractive crystal is examined in convergent polarized 

 light. 



Interlaced Structure (Fr. structure entrelacee ; Ger. durchflochtene Structur) 

 A structure occurring in schistose rocks, in which lamellated or granu- 

 lated mineral-layers meet and intersect so as to produce a kind of network, 

 in the meshes of which lie the lenticular portions (eyes) of the rock. 



Intersertalstructur (Ger.) The name given by Eosenbusch (Mikros. 

 Physiog. der massig. Gest., p. 504) to a rock-structure found especially in 

 a certain class of porphyrites (Tholeiites). It is characterised by the 

 presence of an interstitial substance (mesostasis) which is hypocrystalline, 

 but not rich in micro-felsitic or glassy base. This substance occurs in 

 irregular masses wedged in between the lath-shaped felspathic constituent. 



