31 



Perthite is an interluimnated aggregate of orthoelase or microcline and 

 albite or oligoclase. The laminae are parallel to the ortho-pinacoid of the 

 orthoclase, and are therefore seen as fine stripes on the two principal 

 cleavage planes. The microscopic equivalent of perthite, termed micro- 

 perthite by F. BECKE, (I) occurs in certain granulites and gneisses. 



" Centric " or pegmatitic structures analogous to those which arise in 

 consequence of intergrowths of quartz and felspar, have been observed by 

 BECKE to be produced by the intercrystallisatioii of garnet and felspar, 

 oinphacite and felspar and orthoclase and plagioclase, in augite-gneiss and 

 eklogite. These rocks are of course not known to be igneous, but as the 

 structures resemble those found in igneous rocks, it seems advisable to refer 

 to them in this connection. 



Another very interesting case of the definite intergrowth of two distinct 

 minerals is that furnished by the rhombic and monoclinic pyroxenes.' 2 ' 



The two minerals occur in lamellae, and are so arranged that the ortho- 

 pinacoid of the one lies parallel to the macropinacoid of the other. 



Magnetite and ilmenite may also be mentioned as occurring in definite 

 intergrowths. w 



Augite and hornblende furnish another illustration of the same principle. 

 The interpretation of the structure in this case is, however, complicated by 

 the fact that hornblende frequently arises, as a secondary product, by the 

 alteration of augite ; and when this is the case, it is, of course, incorrect to 

 describe the relation as due to the intergrowth of two minerals. 



Many other illustrations of the same kind might be mentioned, but the 

 general nature of the subject is sufficiently illustrated by those which have 

 been referred to. On reviewing the facts a conclusion which may prove to 

 be of considerable general interest naturally suggests itself. We know that 

 the most powerful object glasses do not enable us to approach the limits of 

 molecular structure and there is, therefore, 110 reason to believe that the 

 microscopic limit of visibility corresponds with anything definite in the nature 

 of crystalline growth. It follows then that definite associations of distinct 

 minerals, analogous to those which may be observed both macroscopically and 

 microscopically, may occur also on an ultra-microscopic scale. 



ZONAL STRUCTURE. A typical crystal is perfectly homogeneous. Each 

 portion of it possesses the same chemical and physical characters as every 

 other portion. In the crystalline constituents of igneous rocks there are, 

 however, important deviations from this rule, in consequence of isomorphous 

 replacements. 



(I) Die Gneissformation des niedero.tereicheschen Waldviertels. T.M.M., 1882, p. 199. 

 (2; 0. TRIPPKK. N.J., 1878, p. 673. 



E. COHEN, Sammlung von Mikrophographien. Plate LVIII.. fig. "2. 



TKALL. Microscopical and chemical characters of the Whin Sill. Q.J.G.S., vol. 

 XL., p. 050. 



(3) NEEF. Uber seltenere krystallinische Diluvium-Geschiebe der Marke. Z.D.G.G , 

 vol. XXXIV., p. 470. 



KiicH. Beitrag /ur Petrographie des westafrikanischen Schiefergebirges. T.M.M., 

 Vol. VI., 1885, p. 129. 



