136 EEPOBT— 1891. 



3x2 mm. and over. They have mostly sharp, straight out- 

 Imes, formmg obtuse-triangular, rhomboidal, trapezoidal, and 

 polygonal figures with unequal angles, and in cases figures 

 closely imitating those of crystals (Fig. 7). I have much 

 hesitated in describing these and the particles mentioned as 

 occurring in the coarse porphyry as glass, but the following 

 considerations have decided me to do so : — First : Amongst the 

 considerable number observed in many thin sections there is 

 not a single one with outlines indicating a mineral of the 

 isometric system (for example, nosean, which is hardly ever 

 absent in phonolites), and to consider them all as sections of 

 ill-formed crystals of that system seemed to me unreasonable. 

 Second : They cannot belong to the tetragonal or hexagonal 

 systems under a similar unreasonable supposition of all being 

 ill-formed crystals cut at right angles to the optic axis, because 

 they show no traces of an interference-figure in convergent 

 polarised light. It must also be mentioned that there is not 

 the least indication of their being casts or negative crystals of 

 some pre-existing mineral removed. 



The specific gravity of the Purakanui rock is 2 -68-2 -73 ; 

 and quantitative analyses of it made by Messrs. F. B. Allen, 

 M.A., B.Sc, and P. Fitzgerald, students of the Dunedin 

 University School of Mines, gave the following results: On 

 treatment of the rock-powder with cold hydrochloric acid, 

 the solution quickly becomes stiff with gelatinous silica, and 

 there are dissolved altogether 54-8 per cent. 



Analysis of Sol. Portion 

 Bulk Analysis of (54-8 per cent.), 



the Rock. calculated to 100. 



44-4 

 AUO3 29-6 



5-8 



6-0 



1-8 



10-0 



2-4 



100-21 100-0 



On comparmg these results with those recorded of Euro- 

 pean phonolites,''' a tolerably close agreement is found, and, 

 as the rock under notice also conforms in mineral constitution 



* Petrographisclic Stuclien an den Phonolithgesteinen Bohmen's, von 

 Dr. Emanuel Boricky. Prag, 1873. 



Beitriige zur Petrographie der Plutonischen Gesteine, von Justus 

 Rath. Berlin, 1873. 



