432 



these forms of fluxion-structure owe their origin to differential movements 

 in the molten magma. 



Foliaceous A lamellar structure in which the lamina are thin and easily 

 separable. 



Foliated Composed of thin layers or folia. Applied to the crystalline schists, 

 in which the minerals are arranged in parallel, sometimes wavy, layers. 

 See Schistosity. 



Formation This word has been applied to a group of rocks, less than a system, 

 having in common certain petrological or paleontological characters, but 

 not necessarily separated by a break from the next following series above 

 or below it. In this sense it is synonymous with the word proposed for 

 this statigraphical subdivision at the Bologna meeting of the International 

 Geological Congress, viz., series, or section (Fr. series ; Ger. Abtheilung). 

 The word formation has also been used to express the idea of origin, and 

 not of time ; thus, it would be correct to speak of an eruptive formation, 

 a sedimentary formation, marine, lacustrine, or detrital formations, but 

 not of a Cretaceous, Jurassic, or Carboniferous formation. 



Fracture (Fr. cassure ; Ger. Bruch) The freshly broken surface of a mineral 

 or rock. It may be smooth, rough, splintery, hackly, conchoidal, &c. 



Friable Easily crumbled ; reducible to powder. 



Fusiform Spindle-shaped, as the banding seen in microcline under crossed 

 nicols. 



Gang 1 The German term for a dyke, vein, or lode of a ruuk or mineral. 

 Gas-cavity See Inclusion. 



Geode Originally applied to nodules of indurated clay or ironstone. Now used 

 to denote any hollow nodular body, whether empty or lined with 

 crystals. 



Glass-Cavity See Inclusion. 



Glassy Base (Fr. pate vitreuse ; Ger. Glasbasis) This term is employed to 

 denote the residual uncrystallized, and therefore structureless and 

 isotropic, portions of a rock. 



Gliding-plane (Ger. Gleitflache) A definite direction in a crystal in which the 

 molecules glide over one another when it is subjected to a suitably 

 directed pressure. 



Globospherite A term used by Vogelsang (Die Krystalliten, p. 184) to designate 

 those spherulites which consist of globulites in radial arrangement. 



Glotmlite The name given by Vogelsang (Die Krystalliten, p. 134) to minute 

 crystallites having a spherical drop-like form. See Crystallite. 



Glomero-porphyritic The name given by Judd (Q.J.G.S., 1886, p. 71) to a 

 porphyritic rock- structure in which the porphyritic constituents are granu- 

 lar aggregates. 



Granitic (Fr. structure granitique, Levy) A term applied p to holocrystalline 

 rocks having the allotriomorphic-granular structure of the granites. 



Granitoid Resembling granite in structure. 

 Granito-trachytic See Ophitic. 



Granophyric A structure characteristic of certain quartz-porphyries (grano- 

 phyres) and of some granitic rocks, in which the quartz and felspar of the 

 ground-mass, having consolidated simultaneously, occur in micropegmatitic 

 intergrowth, often forming radially fibrous spheroidal bodies (pseudo- 

 spherulites). 



