88 B. KOTO 



the expense of quartz. The felspar is usually larger in size than 

 the quartz-grains, and never shows any indication of crystallographic 

 faces ; on the contrary, the periphery is exceedingly irregular having 

 deep indentations and sharp projecting points. 



It appears to the writer that the felspar grew externally by 

 secondary enlargement, consequently it has tendency to assume more 

 or less a porphyritic habitus. While the exterior of a larger felspar 

 was still undergoing the process of accretion, the imbibed silica, or 

 in part a silicate of sodium or potassium which had served as a 

 solvent, seems to have just begun hardening. Such being the case, 

 the margin of the porphyritic felspar assumes a ^?<rtSi-granophyritic 

 or micropegmatic structure, though by no means a perfect one, when 

 compared with the typical development in granophyres, fig. 2, FL II. 



The felspars often shoAv a corroded appearance. They are of a 

 remarkably vitreous aspect, quite fresh, and often bear evident traces 

 of the basal cleavage ; parallel stripes are few, and there is no poly- 

 synthetic lamellar structure. The oblique extinction takes place at 

 about 27° with reference to the vertical stripes. It may be inferred 

 from these facts that the felspars are mainly orthoclase, while some 

 may probably belong to an acidic plagioclase. 



The most prominent feature in the felspar is the interp(3sition of 

 opaque, highly lustrous iron-glance ; the latter l)y reflected light shows 

 a slight bluish tinge, especially on the surface of thick lamellae into 

 which, according to Miigge,^) the mineral is said to be easily parted, 

 the plane of the lamelhe corresponding to 11 = ir (lOll) of iron-glance. 

 Thin plates of blood-red iron-mica together with epidote-grains and 

 rutile-needles are also mixed up with the o})aque iron-glance in the 

 felspar- substance as in fig. 3, PL II. All these are localized in the 

 centre, and the periphery of the felspar is fringed with greenish 



1) Neues Jahrbuch f. Miu. etc. 18Si, I Baud p. 216. 



