BY H. I. JENSEN. 875 



Associated with such pheiiocr3^sts in tlie same and other rocks 

 occur Carlsbad twins of simihir habifc, in which it was determined 

 with the selenite plate that 



Bx, == a. 

 These crystals are therefore positive and must be looked upon as 

 a variety of anorthoclase closely related to albite. 



Hence we have in general two types of felspar phenocrysts. 



Optically positive, usually prismatic, crystals of the sanidine 

 habits with a refractive index less than Canada balsam, Carlsbad 

 or Baveno twinning, cleavage parallel to c and almost at right 

 angles to b, and extinction angle from 0°-12° on edge 6 c. 

 This felspar is a variety of anorthoclase in which the albite mole- 

 cule predominates. 



Optically negative crystals either prismatic with square 

 sections or tabular parallel to a, showing Carlsbad or Baveno 

 twinning. Their extinction angles vary from 0°-10'^ on the edge 

 b c. They are clear and furnished with crosscraeks like sanidine. 

 This felspar is assignable to the species of orthoclase (sanidine) 

 and anorthoclase sanidine. Albite inclusions are common. 



These two forms may occur together in the same rock or inter- 

 grown in the same crystal, and numerous intermediate types 

 exist. 



The felspars of the base are referable in different rocks to 

 different species varying from pure sanidine to albite. Usually 

 the predominating felspar of the phenocrysts is abundant or 

 predominating also in the base. 



Hornblende occurs sometimes in rods, sometimes in minute 

 grains clumped together in dendritic aggregates forming a 

 poikilitic intergrowth with felspar. It is frequently replaced by 

 hsematite and occasionally in weathered rocks by limonite. It 

 has the usual appearance of the arfvedsonite already described 

 in the paper dealing with the Warrumbungle Mountains. 



In N.46, a rock rich in blue amphibole and very poor in 

 segirine, it was ascertained that in the blue amphibole Bx^ = r, 

 hence the mineral is positive. The extinction was nearly 

 straight and the absorption scheme proved to be 



f (derp blue-black) .>> b (lavender) •>- n (briglit light green). 



