431 



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

 which have been formed by deposition around a nucleus of any kind. It 

 is used in contradistinction to entoolitic. 



Exotic See Igneous. 



Extinction (Ger. Ausloschung) A mineral section becomes dark (extinguishes) 

 under crossed nicols when the axes of the ellipse in which it cuts the 

 ellipsoid of elasticity (axes of depolarization) coincide with the chief planes 

 of the nicols. 



Extinction-angle (Ausloschungsschiefe) The angle between a position of 

 extinction (axis of depolarization) and the trace of some definite crystallo- 

 graphic plane. 



Eye-Structure (tier. Angenstrnoturj In this structure, which is characteristic 

 of the metamorphic rocks, more particularly of the coarsely crystalline 

 ones, the foliated and secondary minerals are arranged in layers round 

 the larger original constituents, producing lenticular forms which often 

 bear a striking resemblance to eyes. 



Face The bounding plane surfaces of crystals are termed faces. 



FaSClCUlar (Ger. buschelformig) Aggregated in tufts or bundles of needles. 

 Syn. Tufted. 



Fault (Fr. faille ; Ger. Verwerfung, Wechsel, Sprung, Verschiebung) A dis- 

 placement of rocks along planes usually inclined at a high angle (the 

 hade) to the horizontal surface. In a normal fault the downward displace- 

 ment is in the direction of the hade. Faults in which this is not the case 

 are termed reversed faults. Syn, Throw, Heave, Slip, Trouble, Dyke, 

 Paraclase. 



Felsitic (Fr. petrosiliceuse) The name applied to the macroscopically homo- 

 geneous groundmass of many porphyries, and by most English petro- 

 graphers to the rocks themselves. Under crossed nicols, felsitic (not 

 microfelsitic} matter gives a micro- or crypto-crystalline reaction. 



Felsophyric A term applied to certain quartz-porphyries of which the ground- 

 mass consists mainly of microfelsite, but may also contain crypto- 

 crystalline aggregates. 



FelSOSpherite (Fr. spherolites petrosiliceux a croix noire) See Spherulite. 



Felspar-mosaic The name given to felspar when occurring in aggregates of 

 clear, variously orientated granules, packed together like the pieces com- 

 posing a Eoman mosaic. It is especially characteristic of rocks which 

 have undergone mechanical rnetamorphism. 



Felspathic Containing a large proportion of felspar ; generally applied to the 

 groundmass of a rock. 



Felted (Ger. filzig ; filzartig) Applied to any aggregate of microlites (mikroli- 

 thenjilz] forming a compact mass like the hairs in felt. 



Fibrous Composed of fibres or threads, as in asbestos, amianthus. 

 Fissile Admitting of being split into laminae. 

 Fluid-Cavity See Inclusions. 



Fluxion- or fluidal Structure (Fr. texture fluidale ; texture d'ecoulernent) A 

 structure characteristic of eruptive rocks, and especially of the more vitreous 

 types. It assumes one or more of the following forms : (1) a parallelism 

 of differently coloured glass bands (Schlieren) ; (2) an arrangement of 

 crystallites and microlites with their long axes parallel ; (3) an arrange- 

 ment of different crystallites in parallel layers ; (4) the smaller and later 

 constituents sweeping round the larger and first-formed crystals. All 



