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



(Becke's method). These facts indicate an acid felspar. These 

 felspar fragments are much broken up through mechanical and 

 chemical agencies, and polarize as a mosaic. Extensive corrosion 

 has gone on, many of the grains having been almost entirely 

 dissolved, but no difference in the character of the rock can be 

 seen in the neighbourhood of the enclosure. One idiomorphic 

 crystal was found having the outline of a clinopinacoidal section 

 of felspar with [001] extensively developed, and [110] and [201] 

 less so. Owing to the extent of decomposition and alteration no 

 exact determinations of this felspar are possible. These inclusions 

 are almost certainly derived from the granite through which the 

 dyke has been intruded. If this is so, the occurrence of such 

 felspathic material in the larger nephelines is additional evidence 

 against the intratelluric origin of the latter. In the large amount 

 of enclosed granitic material the Kosciusko nepheline rock 

 resembles some of the other dyke rocks of this district, the olivine 

 basalts being particularly rich in granite enclosures. 



No. 4 does not differ essentially from No. 1, except that it is 

 considerably more decomposed. As a result of this decomposition 

 several new minerals have made their appearance, namely, analcite, 

 calcite, and natrolite. They possess a pseudo-amygdaloidal 

 arrangement. The analcite is slightly greenish in colour, owing 

 to chloritic stains, but is perfectly isotropic and characteristic in 

 every other particular. The calcite also calls for no particular 

 remark. The mineral we have called natrolite occurs in the form 

 of little tufty aggregates with a tendency towards spherulitic 

 arrangement, and is included in the analcite. These fibres possess 

 a refractive index not distinguishable from that of the analcite. 

 The double refraction is quite noticeable, even though the fibres do 

 not extend through the whole thickness of the section, the colours 

 being greyish-white. The extinction is parallel and at right angles 

 to the length of the needles. 



The felspathic inclusions are, if anything, more abundant here 

 than in No. 1, but possess similar characters slightly modified 

 by the greater amount of decomposition. In addition to the 



