433 



Granospherite A term applied by Vogelsang (Die Krystalliten, p. 135) to 

 spherulites which consist of an irregular aggregate of crystalline grains, 

 showing aggregate polarization under crossed nicols. 



Granular (Fr. grenu ; Ger. kornig) Composed of rounded grains or irregular 

 crystalline particles, as sandstone, marble, and granite. 



Granulation As used in petrography a term signifying a breaking-up into 

 granules of the mineral constituents (usually the quartz or felspar) of a 

 rock, a phenomenon often accompanying dynamic metamorphism. See 

 Cataclastic Structure. 



Granulitic Michel-Levy (B.S.G., 3 e serie, II., p. 177; III., p. 204) applies this 

 term to holocrystalline rocks composed of juxtaposed grains, all approxi- 

 mately of the same size and independently orientated. Under crossed nicols 

 such a rock has the appearance of a brilliantly coloured mosaic. When the 

 grains are of microscopic dimensions the structure is said to be micro- 

 granulitic. 



Groundmass The groundmass of a rock is the matrix in which the porphyritic 

 crystals are embedded. It may be holocrystalline, hypocrystalline, or 

 consist entirely of a glassy or microfelsitic base. 



Hackly Rough ; having fine, short and sharp points on the surface ; as a 

 hackly fracture, e.<j., the fracture of the native metals. 



Hardness (Fr. durete ; Ger. Harte) By hardness is understood the resistance 

 offered by a body to the separation of its particles. This resistance 

 is termed coherence. The hardness of a mineral is measured by the 

 force required to scratch it with a steel point or other sharp-pointed 

 fragment of some harder mineral. The results obtained from one and 

 the same mineral are found to vary slightly with the crystalline face experi- 

 mented on, and even with the direction in one and the same face. 

 Relative hardness may be expressed by reference to the following scale 

 (Mohs) : 



1. Tale. 6. Orthoclase. 



2. Selenite. 7. Quartz. 



3. Calcite. 8. Topaz. 



4. Fluor-spar. 9. Sapphire. 



5. Apatite. 10. Diamond. 



Hemiclastic (Ger. halbclastisch) A term applied to a rock composed of detrital 

 constituents united by a secondary cement ; as, for instance, many 

 quartzites. 



Hemicrystalline A term applied by Prof. Bonney to rocks possessing a struc- 

 ture intermediate between holocrystalline and hyaline. This intermediate 

 division contains " the rocks often called cryptocrystalline with some of 

 the microcrystaliine." The material of such rocks " appears to have lost 

 the usual property of a colloid, to have acquired, around innumerable 

 centres, polarities in one or more directions, without its being possible to 

 distinguish with precision what are the minerals into which it has 

 segregated." (Bonney, Address to the Geol. Soc., 1885, p. 38.) 



Hemihedral The term applied to crystals which present only half the full 

 number of faces, the others having disappeared in accordance with some 

 law of symmetry. 



Hemimorphism The property possessed by some crystals (not belonging to 

 the regular system) of presenting the faces of totally different forms at 

 the opposite ends of an axis of symmetry. Such crystals show electrical 

 polarity. 



Hemitrope The French expression for twinned (q.v.}. The etymology of the 

 word has reference to the revolution of 180, round an axis normal to the 

 twinning-plane, which the one portion of a crystal is assumed to make in. 

 order to bring it into the twinning position, 



