GLOSSARY 



OF 



TERMS USED IN DESCRIBING ROCKS. 



BY 



FREDERICK H. HATCH, PH. D., F.G.S. 



Accessory A term applied to those mineral constituents whose presence or 

 absence does not affect the character of a rock. Used in contradistinction 

 to essential. 



Acicular In slender needle-like crystals, as mesotype. 



Ad&Ogene (F>'-) A word used by Haiiy for finely granular. It is synonymous 

 with aphanitic and cryptomerous. 



JEolian A term applied to accumulations formed by the wind. 



Aggregate polarization The dappled appearance produced by a mineral 

 aggregate under crossed nicols, due to the different orientation of the 

 various particles. 



Allochromatic A term applied, in contradistinction to idiochromatic, to those 

 minerals which owe their colour to the presence of a chemically dissolved, 

 or mechanically included, pigment. Amethyst, fluor-spar, and rose-quartz 

 are examples. 



Allogenic ( Allothigenic) Applied to such minerals as are of more ancient origin 

 than the rocks which they compose ; as, for instance, the quartz-grains in 

 a sandstone. 



Allomorphism See Paramorphism. 



Allotriomorphic A term applied by Rosenbusch (Mikroskop. Physiog. der 

 massigen Gest., 1886, p. 11) in contradistinction to idiom orphic, to those 

 mineral components of a rock which owe their form not to their own 

 molecular arrangement, but to some other cause, i.e., to such constituents 

 as are not bounded by crystalline faces. 



It is synonymous with xenomorphic (Rohrbach, T. M. M., VII., p. 18). 



Amorphous Structureless ; non-crystalline. 



A body is amorphous when it possesses the same elasticity in every 

 direction. 



Amygdaloidal (Ger. mandelformig) A term applied to eruptive rocks containing 

 almond-shaped vesicular cavities (amygdules) which have become filled 

 with infiltration-products (zeolites, calcite, quartz, chlorite, delessite, &c.). 



Anisomerous A name applied to the granular structure of a rock when the 

 grains are of different sizes. 



Anisotropic Doubly refractive. 



AnOgenic A term applied in geology, in contradistinction to catof/enic, to rocks 

 whose material has come up from below ; it is thus synonymous with 

 eruptive. 



