REPORT OF TEE CHIEF ASTRONOMER 



413 



SESSIONAL PAPER No. 25a 



To describe the phenoerysts and most of the constituents of the ground- 

 mass would be merely to repeat the description of the corresponding minerals in 

 the normal rhomb-porphyry. A principal difference between the two rocks is, 

 however found in the fact that here biotite is only a very rare accessory and 

 does not occur in the ground-mass. The phenocrystic ' anorthoclase ' retains all 

 its peculiarities ; the same mineral is the more abundant feldspar of the ground- 

 mass, the other being glass-clear (probably highly sodiferous) orthoclase, giving 

 the familiar rectangular sections as in many alkaline porphyries. Plagioclase 

 feldspar seems to be entirely absent from the rock. 



FlGUltE 26. — Partly diagrammatic drawing from thin section of 

 ground-mass of "shackanite" (neither phenoerysts nor the rare" pyrox- 

 ene granules of ground-mass shown). The roundish, often polygonal 

 crystals are analcite, with and without minute inclusions. The rec- 

 tangular, trapezohedral, and lath-shaped crystals are alkali feldspar. 

 Magnetite grains represented in actual proportion. All these crystals 

 lie in a matrix of brown glass and serpentine (dotted). Diameter of 

 circle, 1 mm. 



The quite isotropic analcite forms sharply marked polygonal or round, 

 colourless crystals varying from 0*02 mm. to 0*2 mm. in diameter (Fig. 26). 

 Some of them seem to be quite devoid of inclusions but the majority are charged 



