140 GREAT SERPENTINE BELT OF NEW SOUTH WALES, iv., 



that all the felspar in the rocks is albite or acid oligoclase. The 

 excess of anorthite molecule must be present in the pyroxene, 

 which is abundant in the first three, but rarer in the last : the 

 presence of some epidote and chlorite also removes some of the 

 alumina that is reckoned in the calculation as if it formed anor- 

 thite. The felspars are often quite clear, and their position in 

 the acid group is nearly always determinable from their positive 

 biaxial character, and their refractive index which is lower than 

 that of Canada balsam. More exact determinations are difficult. 

 The Carlsbad twinning law has not been seen in combination 

 with the albite law; extinction-angles measured on the albite 

 lamellae perpendicular to (010) give maxima between 8° and 16", 

 indicating a composition between acid oligoclase and albite. Prof. 

 Becke's methods of investigation in convergent light are rarely 

 applicable(l*). A measurement made of the angle between the 

 points of emergence of the optic axes of adjacent lamellae of a 

 pericline twin, indicates a composition of about Abg g An^ 5, which 

 again is within the limits expected. Occasionally, the felspars 

 are slightly zoned, albite is at the margin, and oligoclase within. 

 In the earlier account of these rocks, it was stated that andesine 

 predominated among the quartz-dolerites. This is not confirmed 

 by a more extended investigation. Andesine occurs but rarely, 

 and the crystals of which it forms the central parts are strongly 

 zoned. The determination of andesine in some slides remains 

 doubtful owing to the dusty character of the felspar, and in other 

 cases a testing of the Canada balsam shows that it has been 

 insufficiently cooked, and has therefore a refractive index less 

 than that of albite, leading to the above-mentioned error. It 

 was reported, also, that spongy felspar occurred in one instance, 

 though the mineral is usually compact; further search has not 

 brought to light any other instance of spongy felspar than one 

 described. In the porphyritic rocks, no difference can be found 

 between the composition of the microlites and of the phenocrysts. 

 Apart from its unusual composition^ there is little to suggest 

 that the acid felspar is not of primary origin. In one rock only 

 has labradorite been found, namely, 1040, the specimen illustrated 

 in Fig.l. The felspar forms clear fresh laths, and is associated 



