430 



other hand, there exists no direction around which the elasticity varies 

 similarly in all directions. There are, however, three directions, standing 

 at right angles to one another, which correspond to the greatest, least, and 

 mean elasticity. These are termed axes of elasticity. In Germany they 

 are distinguished by the German (or Old English) letters : a = axis of 

 greatest elasticity ; fc = axis of mean elasticity ; c = axis of least elasticity. 

 In France and England the Greek letters , ft, 7 are used in the same 

 order ; whereas in Germany these are used to express the indices of 

 refraction, being the least, ft the mean, and 7 the greatest index of 

 refraction. A biaxial crystal is negative when the axis of greatest elas- 

 ticity coincides with the acute bisectrix of the angle between the optic 

 axes ; it is positive when the axis of least elasticity occupies that position. 



EnantiomOPphic The name given to such hemihedral and tetartohedral 

 crystals as have no plane of symmetry. Of these there are always two, 

 a right and a left-handed, which have the same relation to one another 

 as an object to its image or a right glove to its fellow. 



Endogenous See Endomorphic. 



EndomorphiC Applied, first by Fournet, to contact-metamorphism when pro- 

 duced in the erupted rock. It is used in contradistinction to exomorphic. 

 Syn. Endogenous. 



EntOOlitic A term applied by Giimbel (N. J., 1873, p. 303) to oolitic grains 

 which have grown from outwards, and are therefore often hollow in the 

 interior. Oolitic grains of this kind are produced in the hot springs of 

 Carlsbad. Their growth is often started by carbonate of lime being 

 deposited round floating bubbles. The thin film thus formed, increasing 

 in thickness as the dancing bubble moves rapidly about, ultimately becomes 

 sufficiently heavy to carry the spherule to the bottom. 



EpigenetiC (Fr. epigenise) An expression applied to the formation of secondary 

 minerals by alteration processes. 



Erratic The name given to a block of any rock which has been detached from 

 the parent mass and transported to its present site by any agency, but 

 more especially by ice. 



Eruptive A term now used generally to designate all igneous rocks, whether 

 interstratified or not. By some writers, however, it is restricted to rocks 

 poured out at the surface. See Effusive. 



Essential The essential constituents of a rock are those which determine its 

 character, and which must therefore be present in its typical development. 



Estuarine A term applied to rocks deposited in the mouth of a river. 



Euritic A term used by some petrographers in the sense of cryjrtogranitic, 

 i.e., a granitic structure in which the individual grains are of microscopic 

 dimensions. N.B. The eurites are microgranites. 



Eutaxitic A name applied to lavas in which the texture changes rapidly in a 

 direction normal to the direction of flow. Eutaxitic lavas are thus often 

 composed of layers, each of which has its characteristic structure. The 

 term is sometimes also extended to banded igneous rocks which have 

 not flowed at the surface. 



Exfoliation A separation into laminae or scales, an effect often produced by 

 weathering on rocks. 



Exogenous See Exomorphic. 



Exomorphic Applied by Fournet to contact-metamorphism when produced in the 

 rock through which the molten mass is erupted. Used in contradistinction 

 to enJomori>hic. Syn. Exogenous. 



