429 



Devitrification (Ger. Entglasung) The process by which glass assumes a 

 lithoid character. According as the devitrification of the glassy base of 

 rocks takes place during, or subsequent to, their consolidation, it may be 

 termed original or secondary. Syn. Promorphisme (Levy). 



Diaelase See Joint. 



DialytiC A term applied by Nautnann to such rocks as are derived from the 

 chemical decomposition of pre-existing rocks as, for example, kaolin, 

 clay, &c. Syn. Cimmatic. 



Diehroism Se<? Pleochroism. 

 Dimorphism See Polymorphism. 



Dip (Fr. inclinaison ; Ger. Einfall) A term used to express the relation between 

 the plane of a bed or stratum and the plane of the horizon. It is 

 measured by the angle which the plane of the bed makes with the 

 plane of the horizon. 



Dislocationsmetamorphismus (Ger.) See Mechanical metamorphism. 



Disomatie A term applied by Seiffert and Sochting to a crystal and its 

 inclusions when each belongs to a different mineral species. See Mono- 

 somatic. 



Dome A term used in crystallography for forms which may be considered as 

 lateral prisms, i.e., they run parallel to a lateral instead of to the vertical 

 axis. Like the prisms, too, they may be treated as special cases of the 

 pyramids, namely, those in which the pyramidal faces cut one of the 

 lateral axes at an infinite distance. Corresponding to each class of 

 pyramid, in rhombic, monoclinic, and triclinic systems respectively, we 

 have consequently macro-, brachy-, orlho-, and c/wio-domes. See Pyramid. 



D6me (-PV.) The name given to certain hills of rounded form (Ger. Kuppe), 

 generally of volcanic origin, and owing their shape, when composed of 

 igneous material, to the viscid nature of the lava at the moment of eruption. 



Double-spheric Structure (Ger. Doppelt-spharische Structur) The name 

 given to a rock structure in which a number of small spherulites are 

 arranged in concentric layers around real or imaginary nuclei to form 

 spheroidal aggregates. 



Druse A term used in descriptive petrology for cavities in rocks which are 

 sprinkled over with crystals, or into which freely terminated crystals 

 project. 



Dyke (Ger. Gang) A term applied to intrusive rocks which occur in iv all-like 

 masses, cutting through the adjacent rocks. 



Dynamic Metamorphism See Mechanical Metamorphism. 



Effusive -A term lately used abroad for those rocks which have been poured 

 out at the surface, the word eruptive now being generally used for the 

 whole group of massive rocks. 



Einsprengling' (Ger.) A word used by German authors to designate the larger 

 mineral constituents disseminated in the groundmass of a rock. The only 

 English equivalent is " Porphyritic constituent." 



Elasticity (optic) Of crystals In uniaxial crystals the elasticity is different in 

 different directions, but is the same for all directions which make the same 

 angle with the optic axis. The greatest difference of elasticity exists between 

 the direction of the optic axis and a plane perpendicular to this. When 

 the optic axis coincides with the direction of greatest elasticity the crystal 

 is optic negative ; when of least, optic positive. In the former case the extra- 

 ordinary is less refracted than the ordinary ray (the crystal is repulsive 

 calcite) ; in the latter the extraordinary is more refracted than the 

 ordinary ray (the crystal is attractive quartz). In biaxial crystals, on the 



