439 



Metastasis A. word suggested by Professor Bonney (Proc. Geol. Soc., 1886, 

 p. 59) to denote "changes rather of a paramorphic character, such, for 

 example, as the crystallization of a limestone, the devitrification of a 

 glassy rock." 



Miarolitic A term applied by Eosenbusch (Die massigen Gesteine, p. 39) to 

 granitic rocks which are characterized by the presence of small irregular 

 cavities, into which the corners and edges of the crystallized mineral 

 components project. 



Micaceous Containing a notable proportion of mica. 



MiCPOClastic An epithet applied by Naumann to such clastic rocks as are 

 composed of small fragments. 



Mierodine Structure The mineral microcline shows in certain sections, under 

 the microscope, a cross-hatching (Crer. Gitterstruktur) which is remarkable 

 for its irregular and ill-defined appearance. It is generally referred to 

 lamellar twinning simultaneously on the albite and pericline types. It is 

 distinguished, however, from ordinary cross-twinning by the fact that the 

 lamellfB are spindle-shaped. 



Microfelsite A term first introduced by Zirkel, and since used in a slightly 

 varying sense by different authors. (See Rosenbusch, Die massigen 

 Gesteine, p. 370.) It is now generally defined as a microscopic substance, 

 forming the base of some porphyries, which is characterized by the 

 possession of a granular, scaly, and fibrous structure without the power of 

 exerting any definite action on polarized light. Rosenbusch states, how- 

 ever, that when aggregated in parallel or radial masses, microfelsite 

 shows weak double refraction. 



Mierofluxion-Strueture See Fluxion structure. 



Micro-granitic A term applied to a rock, or portion of a 1'ock, which is holo- 

 crystalline, and is composed of minute allotriomorphic granules. 



Microgranulitic (structure microgranulitique of Levy) See Granulitic. 



Microlite By most authors this term, first proposed by Vogelsang (Philos. d. 

 Geol., 1867, p. 139), is now generally restricted to those minute crystalline 

 bodies, mostly lath-shaped, but sometimes bifid or with ragged termina- 

 tions, which can be identified as belonging to some definite mineral 

 species. Rosenbusch, in his later editions, does not distinguish between 

 microlites and crystallites. See Fig. 5 of text. 



Micropegmatite As generally used, this term is applied to the microscopic 

 representative of graphic granite (peymatite, Haiiy) ; i.e., to an intimate 

 intergrowth of quartz and felspar in which each mineral exhibits, 

 independently of the other, uniform optic characters over considerable 

 areas. It is sometimes used to express a corresponding intergrowth of 

 any two minerals, e.y,, plagioclase and orthoclase, augite and nepheline. 



Microperthitic Microperthite is the name given by Becke (Tschermak's Min. 

 u. Pet. Mitth. 1882, IV., p. 197) to the so-called " streifige" orthoclase, the 

 striated structure of which was shown by microscopic examination to be 

 produced by the intimate intergrowth of a triclinic felspar (oligoclase or 

 albite) with the orthoclase substance, thin lamellas of the former being 

 intercalated along orthopinacoidal or prismatic planes of the latter, as in 

 the mineral perthite. 



Microplacite A term used by Schrauff for tabular microlites. 



Migration-Structur (Ger.}K name given by Giimbel to the pseudo-fluxion 

 structure shown by some diabase-tuffs (Schalsteine), in which crystalline 

 needles and granules are arranged in bands or zones around the larger 

 constituents. This structure is said by him to be caused by the alteration 

 of old and development of new material, which thus constitutes a 

 " migration " of mineral matter. 



