440 



Mikrolithenfilz (Ger.) A. felted aggregate of microlites or slender needle-like 

 crystals, such as is formed by the felspars of the groundmass of many 

 rocks notably the andesites. 



Mimetic (G-er. mimetisch) Tschermak proposed to call those crystals 

 mimetic which possess externally a high degree of symmetry, but are 

 built up by polysynthetic twinning of crystals having a low grade of 

 symmetry. Thus chabasite is termed a mimetic rhombohedral crystal. 



Moire" The name given to the wavy appearance, comparable to that of watered 

 silk, presented by thin plates of mica under crossed nicols. 



Monoclinic The name given to one of the six systems of crystals. It is 

 characterized by one plane of symmetry. Its forms are referred to 

 three unequal and dissimilar axes ; of these one is at right angles to 

 the other two, which cut one another obliquely. 



Syn. Clinobasic, Clinorhombic, Monoclinohedral, Monosymmetric, 

 Oblique, Zwei-und-eingliedrig. 



Monogenous A word applied by some authors to clastic rocks the fragments 

 of which are derived solely from one rock. See Polygenous. 



MonosomatiC A term applied to crystals and their inclusions when both 

 belong to the same mineral species. See Disomatic. 



Mortelstructur (Ger.) A term proposed by Tornebohm (Geol. Foren. 

 Forhandl, 1881, V., p. 233), for a structure especially exemplified in certain 

 gneisses. This structure is characterized by grains of felspar and quartz 

 being cemented by a fine mortar of the same material. 



MylonitlC Prof. Lapworth (Brit. Ass. Keport, Aberdeen, 1885, p. 1026), has 

 suggested that finely laminated schists, which have been produced by a 

 crushing, dragging, and grinding (in short, dynamic metamorphism) of 

 the original rocks, should be termed mylonites (from mylon, the Greek for a 

 mill). Such rocks are "composed of shattered fragments of the original 

 crystals set in a polarizing cement of the secondary quartz " (and 

 presumably felspar). The lamination is due to the presence of thin 

 ribbon-like and tissue-like bands and fibres of mica and similar minerals. 

 See Cataclastic structure. 



Neck The volcanic material which has accumulated around and in the pipes 

 or funnels of ancient volcanic vents. 



Needle (Fr. aiguille ; Ger. Nadelchen) A term used in petrography to denote 

 the acicular prismatic forms in which many minerals, notably mesotype, 

 occur in rocks. 



Negative (optic) See Elasticity of crystals. 



Neovoleanic Applied by Kosenbusch (Physiogr. der Massig. Gest., 1886, p. 6) 

 to volcanic rocks of post-cretaceous age. See Palasovolcanic. 



NeptuniC See Aqueous. 



Netted Structure (Ger. gestrickte-Structur) In serpentines (antigorite-serpen- 

 tines) which have been formed by the alteration of augite, the chemical 

 change has proceeded along cleavage-cracks which cross one another at an 

 angle of about 90", thus producing a plexus of bladed forms which some- 

 times bears a resemblance to network. It is also known in Germany as 

 " Balken-struktur." See Mesh and Lattice structure. 



Nodule (Ger. Steinwulste, Schlangenstein) Any irregular aggregation of rock- 

 material, with or without a nucleus, which may be organic or inorganic. 

 Flint and ironstone are substances which frequently form nodular 

 concretions. 



