364 T. W. E. DAVID, F. B. GUTHRIE, AND W. G. WOOLNOUGH. 



tive of a very marked change in the composition of the magma 

 after their formation. The change must have taken place after 

 the crystallization of the segirine, but prior to that of most of the 

 sanidine and before any nepheline had started to form. The 

 evidence as to the nature and date of this change is we think, 

 quite conclusive. 



A short description is given here of the eruptive rocks associated 



with the phonolitic nephelinite: — 



Granite — Three miles from Jindabyne towards Cooma, on road. Typical 

 hypidiomorphic texture rather coarsely crystalline. Evidence of 

 dynamical metamorphism in the undulose extinction of many of the 

 minerals, bending of biotite flakes, the bending and faulting of twin 

 lamellae of felspar and the peripheral shattering of all the minerals. 

 Quartz is abundant in irregular grains, generally much shattered at the 

 edges. It contains plentiful liquid and gaseous inclusions arranged in 

 very definite planes, which, at any rate in some cases, pass from grain to 

 grain without interruption. In addition to enclosures of the older segre- 

 gations which occur abundantly as rock constituents, the quartz also 

 contains straight and curved trichites which are apparently opaque. 

 These are in some cases at any rate arranged very regularly. In one 

 section nearly perpendicular to the optic axis, they lie in lines making 

 angles of 60° with one another, and therefore in all probability parallel 

 to the faces of the primary rhombohedron. The largest of these trichites 

 reach the dimensions of very small acicular crystals. Some of these are 

 certainly apatite, others are probably rutile. The quartz is considerably 

 cracked, the cracks being roughly parallel throughout the rock. 



The Felspar, whieh in some cases exhibits traces of crystalline form, is 

 apparently most of it plagioclase. It is twinned after the albite law, 

 and in most cases after the Carlsbad law too. The peripheral shattering 

 and decomposition render E.I. determinations difficult, but apparently 

 the R.I. is higher than that of quartz, indicating a rather basic variety of 

 plagioclase. The sections are not suitable for measurement of extinction 

 angles. One whose shape and the absence of albite twinning indicate a 

 plane parallel to M (010) gives an extinction angle of 22° in a positive 

 sense from the trace of the cleavage parallel to P 001. Another section 

 nearly in the zone _L M (010) gave extinction angles for the albite lamellae 

 16° and 18° on opposite sides of the plane of composition of the twin. 

 The felspar probably lies between a basic andesine and an acid labradorite 

 probably the latter, as the measurements in the section parallel to M 

 (010) are the more reliable. A small quantity of a second felspar is 

 present, which from its low R.I. (less than that for balsam) and untwinned 

 character is probably orthoclase. The enclosures in the felspar are 



