462 



MB. H. H. READ ON THE 



[vol. lxxix, 



showing that occasionally it is as basic as andesine. Very fine 

 albite and pericline twinning is seen. The second type of felspar 

 is one which seems to merit the name of soda-microcline. 

 This occurs in grains and crystals of late formation, which in most 

 cases show a hazy lattice-twinning, best seen when the crystal is 

 almost in the position of extinction. Extinctions on cleavage- 

 Fig. 4. — Contaminated rocks. 

 A JS 



A & B, Arnage Type. Co. = cordierite; PI. ■= oligoclase-andesine ; Qz. = quartz; 

 Bi. = biotite; Ga. = garnet; the solid black is iron-oxide; X 20 (see 

 p. 463). 

 [The rock of B has been analysed : see Analysis VII, Table III, p. 464.] 



C, Kinharrachie Type. PL = andesine : Qz. = quartz; Ho. = hornblende ; X 30 



(see p. 466). 



D, Ardlethen Type. PI. = oligoclase ; Or. = orthoclase ; Mic.=microcline ; 



Qz. = quartz ; Bi.=biotite ; X 20 (see p. 472). 



flakes are always low ; the sign is negative. Its refractive index 

 is distinctly higher than that of the orthoclase present, and has a 

 mean value of 1-526. The optic axial angle is small, 2 V being 

 about 40°. This last observation, taken in conjunction with the 

 other optical properties, indicates that this felspar is a microcline 



