588 THE GEOLOGY OF THE WARRUMBUNGLE MOUNTAINS, 



t^^pe seems to predominate in the dykes, but occurs also in other 

 rocks. 



Orthophyric Fehpars.— The felspar in the Scabby Rock speci- 

 mens has the orthophyric tendency, although it is not quite 

 isometric. The crystals seem to be usually enclosed by three 

 sets of facesc(OOl), 2/(201) and 6(010). Sometimes a^( 101) and 

 a (100) are present. In some crystals the angles, as measured, 

 show that the faces m(llO) have developed to the exclusion of 

 b and y. 



Carlsbad twinning is not frequent, but Baveno twins in 

 doublets and fourlings are common. Manebach twinning is rare. 



At first sight this orthophyric felspar seems to be a water-clear 

 sanidine, but on closer examination with high power a line 

 striation is noticed, in some sections both lengthwise and cross- 

 wise, f'iving a fine meshed appearance. Some crystals have an 

 isometric core of unstriated felspar, probably true sanidine. The 

 striations are due to microscopic twinning on the albite and 

 pericline laws. The outermost zone is frequently pure albite. 

 The felspars are therefore composed essentially of a cryptopertbitic 

 intergrowth of orthoclase and albite or soda-microcline, occasion- 

 ally with a core of sanidine and a rim of albite; the main bulk 

 is therefore anorthoclase or microcline cryptoperthite. 



The refractive index of all the crystals is lower than that of 

 Canada balsam. The optic sign is negative (Bx^^ a); the inter- 

 ference figure is, however, nearly uniaxial, and occasionally 

 crystals occur which are positive in sign (albite): a lies near a 

 and b = ^'- The crystals are somewhat elongated along the 

 a axis, so that sections parallel to a are square, and sections 

 parallel to b and c are somewhat elongated parallelograms. 

 Extinction is usually straight or nearly so in the direction of 

 elongation, but of a shadowy nature in most cases, due to ultra- 

 microscopic twinning. The extinction of the border zone usually 

 varies slightly from that of the interior, the angle increasing 

 from the interior outwards. 



Prismatic Felspars. — The prismatic (lath-shaped) felspars 

 so abundant in the other soda-trachytes of the area are much 



