438 



Macroclastic A word used by Naumann for such clastic rocks as are made up 



of large-sized fragments. 



Macrodiagonal The name sometimes given to the longer of the two lateral 

 axes in the rhombic and triclinic systems of crystals. 



Macropinaeoid One of the three chief crystallographic planes in the rhombic 

 and triclinic systems. It is parallel to the vertical axis and the longer 

 of the two lateral axes. 



Maeropyramid ; Maeroprism ; Macrodome See Pyramid, Prism, and Dome 

 respectively. 



Macroscopic A term used in contradistinction to microscopic, to imply that the 



character in question is visible to the naked eye. Syn. Megascopic. 



Maculated Spotted ; applied to rocks which have a spotted appearance. 



Magma (Fr. pate; Ger Teig) The molten mass or paste which by consolidation 

 gives rise to a rock. 



Margarita The name given by Vogelsang (Die Krystalliten, p. 19) to the 

 linear arrangement, like strings of beads, assumed by globulites ('/.*'.). 



MarmorOSiS (tier. Marmorisirung) The word proposed by Dr. A. Geikie 

 (Text-book of Geology, 1882, p. 577) to express the metamorphosis of 

 ordinary limestones into crystalline marble. 



Massive (Fr. massif; Ger. massig, richtungslos, ungeschichtet) Unstratified ; 

 occurring in homogeneous masses. By some authors this word is used 

 synonymously with eruptive. 



Matrix (Fr. moule) The groundmass of an igneous rock ; also the calcareous 

 or siliceous paste in which the pebbles of a conglomerate are embedded. 

 See Base and Mesostasis. 



Mechanical metamorphism The metamorphism produced in a rock by 

 mechanical processes earth movements, shearing, &c., involving always 

 a structural and generally a mineralogical change. 



Syns. Dynamic metamorphism, Pressure-metamorphism, Dislo- 

 cationsmetamorphismus (Ger.}. 



Megascopic See Macroscopic. 



Merohedrism A crystallographic term embracing both hcmihedritm and 

 tetartohcdrism (q.v.). 



Mesh Structure (Ger. Maschenstructur) The reticulated structure peculiar to 

 the serpentines derived chiefly from the alteration of olivine. 



Mesostasis (Ger. Zwischenklemmungsmasse) The interstitial matter (glassy, 

 microfelsitic, or cryptocrystalline) of hypocrystalline rocks. 



Metacrasis A word suggested by Prof. Bonney (Proc. Geol. Soc., 1886, p. 59) 

 to denote "changes like the conversion of a mud into a mass of quartz 

 with mica and other silicates." 



Metamorphism This word is usually restricted in geological literature to 

 changes which a rock undergoes in mineralogical or chemical composition 

 and internal structure through the operation of heat, heated water, or 

 vapour, and mechanical agencies. It is either local or regional. 



Metapepsis A term suggested by Kinahan (in his Geology of Ireland, 1878), 

 for metamorphism which "seems to be due to intensely heated water or 

 steam, which, as it were, stews the rock." 



Ketasomatosis The change of material, due to chemical agencies, undergone 

 by rocks subsequent to their formation. See Parainorphosis. 



