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



A distinct feature of this rock is the large amount of derived 

 fragments, chiefly quartz felspar and mica, which the rock contains 

 and which it has caught up from the gneissic granite forming the 

 walls of the dyke. In this respect the nepheline rock of Kosciusko 

 closely resembles the basalt dykes of the same neighbourhood. 

 The latter contain a vast number of enclosures of granite, individual 

 fragments being in many cases several inches in diameter. The 

 fact of occasional crystals of sanidine being enclosed in the 

 nepheline crystals is, we think, an argument against the intra- 

 teiluric origin, in this case, of the larger crystals of nepheline, 

 which we therefore consider pseudo-porphyritic. The Kosciusko 

 rock is also remarkable for the large proportion, 64%, soluble in 

 HC1. The fact that 2-81% of the soluble portion is ferric oxide 

 shows that part of the aegirine must have been attacked by the HC1. 



On comparing the nepheline rock of Kosciusko with the soda- 

 series of Port Cygnet, Tasmania, one is at once struck with the 

 fact that melanite garnet, so abundant in the Port Cygnet rocks, 

 is absent from those of Kosciusko. A possible explanation of 

 this is afforded by the experiments of MM. F. Fouque and Michel 

 Levy. 1 They state that, in their experiments in the artificial pro- 

 duction of rocks, a mixture of 3 parts of nepheline and of 1 . 3 of 

 augite after fusion and cooling extended over two days, (following 

 their system No. 4) produced microlites of nepheline and augite. 

 The augite crystallised out just after the nepheline. When the 

 proportion of pyroxene was diminished, pyroxene no longer formed. 

 For example, a mixture of 10 parts of nepheline and 1 of augite 

 yielded beautiful crystals of nepheline, small octahedra of spinel, 

 and brownish isotropic rhombic-dodecahedrons of melanite garnet. 

 Evidence is as yet insufficient as to whether or not the nepheline 

 dyke rocks of Kosciusko are complementary to the basic dyke 

 rocks of the same neighbourhood. 



As regards age, the Kosciusko basic rocks are probably Tertiary, 

 whereas the nepheline rocks are perhaps somewhat older, Cretaceo- 



1 Synthese des Mineraux et des Roches. Paris, 1882, pp. 63-64. 



