THE AEOH^AX FORMATION OF THE ABUKUMA PLATEAU. 247 



but l)oth modificntious ni'e re.-illy one und tlie .sinie rock, and .should 

 not be interpreted ;is alternnrions of two. 



The sLibotance of the orthochxxe i.s for tlie greater part k-aolinized, 

 and lias a dirty as|>e'T ; there is therefore no difficulty in discriiniijat- 

 ing it from the quartz grains, wliidi ase otiiervvise not veiy easy to 

 distinguish, when both are found togeth(-r unchanged. Tlie feIdH|):u' 

 appears üomctr/r-pohjßonal in outlines, owing to the dispo.^ition of 

 the hi otite and quartz ; and ] latches of it, widely separated from each 

 other, behave optically ahke, showing that they are all one mineral, 

 just as in the case of jiegmatite. Its j)olariz;ition-colours are weak, and 

 zonings are frequently oliserxed. 



There occurs also another feldxpur which (k^^iates in habitus 

 irom the preceding in having a granular form, jiolysynthetic twin- 

 ning lamellation, and comparatively high polarization-color.rs. This 

 plagioclase is idiomorjjhic in comparison with the above-mentioned 

 orthoclase, and appears less altered than it. The plagioclase as well as 

 orthoclase often contains round grains of quartz/ around which the 

 feldspars show more or less signs of decomposition. The angle of 

 extinction of the plagioclase is small, making nearly 2° only with 

 the sutures of twins, thus pointing out theaiidesine-oli<j-oclastic nature 

 of the present feldspar. 



The hiottte occurs in irregular folia?, and is of a reddish-liî-owii 

 colour ; nitile disposed as in sagenitc is found in great abundance ; the 

 pleochroi<^ lialos seen around thick but short cr\'sta]s of zircon may be 

 frequently observed. l)esides large lamellœ of biotite there (jccur 

 minute scal<;s of the same mineral. They are round or sometimes 

 rudely six-sided, non-pleochroic lamellge <^f microscopic dimension.s 

 with dee})-reddish-brown coh^ur. A spindle-shaped, lamellated, 

 highly pleochroic crystal is only a side-\ iew of the same mica,. These 



1 Kalkowsky, Die Giieiss/ormation des Eulengebirges. p. 30. 



