456 Transactions. 



With regard to its structure, the rock belongs to the holocrystalHnc 

 class of basalts, for, while there is a fair amount of isotropic material 

 present, this is due to alteration and not to the presence of glassy matter. 

 Further, the order of crystallization of the various constituents is not well 

 marked, the mutual relations between the augite and feldspar with regard 

 to priority varying considerably. No ophitic structure has been observed. 



Turning now to the granite, we see that, in hand -specimens, it is of a 

 white colour, rather fine-grained, with pronounced orthoclase difiering 

 very little from white in colour, and pronounced quartz crystals. The 

 ferro-magnesian mineral is biotite, but is unusually rare in occurrence ; 

 the rock, therefore, resembles in appearance some specimens of ditroite 

 Avithout the blue colour of the sodalite. 



In section, the rock is seen to be made up mainly of quartz, orthoclase, 

 and biotite. The quartz occurs in large plates without crystal outUne, en- 

 closing in some cases fluid pores, which are arranged in lines. Occasion- 

 ally the quartz occurs fixed into the interstices between feldspar crystals, 

 showing probably that the mineral crystallized after the feldspar. 



The orthoclase also occurs in large plates, but, unlike the quartz, is usually 

 semi-decomposed to form kaolin. Karlsbad twinning is usually to be seen. 

 In addition to the orthoclase, several crystals of microcline and oligoclase 

 are to be seen. The microcline shows the peculiar cross-hatched appear- 

 ance under crossed nicols, and the oligoclase, of which a considerable 

 amount occurs in the rock, shows both Karlsbad and albite twinning 

 lamellation. A very well-marked zonary banding has also been noticed 

 in the orthoclase. 



The hiotite occurs in small plates with ragged outlines and no distinct 

 terminations. It exhibits strong pleochroism from light yellow to deep 

 brown, and has almost straight extinction, and is very susceptible of altera- 

 tion. The latter produces a green coloration, and in places a green chlo- 

 ritic pseudomorph with pieces of magnetite. Its inclusions are rare, and 

 consist mainly of magnetite and pieces of apatite. No accessory minerals 

 have been seen. 



The structure is that characteristic of all plutonic rocks — i.e.. hypidio- 

 morphic — only a minor proportion of the constituents developing their 

 external forms freely. The normal order of the crystallization has also been 

 obeyed : the biotite and magnetite have first crystallized, then the feldspar, 

 and lastly the quartz with microcline, though the oligoclase has probably 

 ]ireceded the quartz. 



The next geological formation we must discuss belongs to the Miocene 

 period in New Zealand, and, more particularly, to the series called by Captain 

 Hutton the Oamaru and Pareora series. The strata belonging to this series 

 Hutton, in the report on this district above referred to, has seen fit to sub- 

 divide as follows : (a) Brown-coal formation ; {b) Oamaru formation proper. 

 We shall deal with these separately. 



(a.) Brown-coal Formation. — The main body of this occurs skirting round 

 the Wairaki Hills, but another small patch is also found on the south-west 

 side of the Longv/ood Range at Orepuki, and for a short distance on both 

 sides of the Waimeamea River. As Hutton remarks, it never rises to great 

 altitudes, but occupies portions of old valleys scooped out of Secondary and 

 Palaeozoic rocks. The Orepuki occurrence has roughly the shape of a tri- 

 angle, with an area of about one and a half square miles. The seam is 

 exposed only in water-races, in which some difficulty was experienced in 



