424 



Anticlinal A term applied to strata which dip away from a central axis. See 

 Synclinal. 



Aphanitic A texture in which the component grains are so minute as to 

 become visible only under the microscope. See Cryptomerous. 



Apophysis (Ger. Gangauslaufer) A vein or branch from the main mass (boss 

 or dyke) of an igneous rock. 



Aqueous Aqueous rocks are those which have been produced through the 

 agency of water, either by the formation of chemical precipitates, or by 

 the accumulation and deposition of rock-material suspended therein. 



This term embraces, or is synonymous with, the following : neptunic, 

 hydatogenic, sedimentary, derivative, regenerated, indigenous, catogenic, and 

 deuterogenic. 



Arenaceous Composed of sandy material. Syn. Psammitic. 



Argillaceous (Fr. argileuse ; Ger. thonig ; lehmig) A term applied to rocks 

 composed chiefly of clay. Syn. Pelitic or Pelolithic. 



Assise (Fr. assise, couches ; Ger. Schichten) The name given to two or more 

 beds or zones which contain a number of fossils in common, but separated 

 from the next following, lower or higher, beds, by a marked paleeontological 

 difference, e.g., the Micraster beds of the Cretaceous system. See Stratum. 



Asymmetric See Triclinic. 



Atmogenic A term applied to metamorphism produced by fumarole activity. 



Ausweichungselivage (Ger.) A term applied by Heim (Mechanismus der 

 Gebirgsbildung, Band II., p. 53) to Sorby's second type of cleavage 

 (Q.J.G.S., Vol. XXXVI., Proc., p. 72). The cleavage-planes, in this case, 

 are due to definite, inosculating planes of movement (shear-planes) 

 which separate the rock into flat lenticles, inside which comparatively 

 little deformation has taken place. The pre-existent planes of sedimenta- 

 tion or foliation are often recognizable in the lenticles. Ausn-eichungscli- 

 vage 'may be regarded as an advanced stage of minute puckering, some or 

 all of the surfaces of contrary flexure having become shear-planes. 



The expression " strain-slip-chavage " has been used in this country with 

 the same meaning. 



Authigenic A term applied by Kalkowsky (N.J., 1880, I., p. 4) to a mineral 

 constituent, to denote that it came into existence with, or after, the rock 

 containing it. In the one case it is primary ; in the other, secondary. See 

 Allogenic. 



Automorphic See Idiomorphic. 



Avanturine lustre Spangled lustre, like that of some oligoclase. 



Axes of depolarization (Fr. lignes d'extinction) Directions of extinction in 

 any mineral section between crossed nicols. The two directions may be 

 distinguished as the major and minor axes of depolarization. 



Axes Of Elasticity See Elasticity (optic) of crystals. 



Axiolite A name given by Zirkel (Mic. Petrog. of 40th Paral., p. 173) to the 

 elongated, sometimes branched, spherulitic bodies found in many rhyolites. 

 They possess a fibrous structure, the fibres being normal to the long axis. 



Axotomous Cleavable in one direction. 

 Bacillar (Ger. stangelig) Kod-shaped. 



Bank A word applied in stratigraphy to a bed which is thicker or more 

 coherent than those in which it is intercalated. 



