Wild.— Geology of the Bluff. 321 



Under the Microscope. — Examination of thin sections shows the rock 

 to be composed essentially of feldspar, augite, hypersthene, and horn- 

 blende, with magnetite as an accessory constituent. The feldspar, which 

 on an average forms half the rock, occurs usually in plates, ranging in 

 size up to 1*4 mm. long by 1 mm. broad. It also is found in irregular- 

 pieces enclosed ophitically by the feno-magnesian minerals. It is a 

 triclinic variety, showing both coarse and fine albite lamellation. The 

 extinction-angle ranges up to 27°, this angle being the one recorded most 

 frequently in sections as nearly perpendicular as possible to the albite 

 lamellae parallel to the brachipinacoid. 



According to the statistical method of Michel Levy for determining 

 the feldspars, this angle denotes labradorite. In some cases, however, 

 an angle of 16° in found on each side of the trace of the twinning-plane. 

 In such a basic rock this figure indicates andesine, and Thomson thinks 

 this is the prevailing species, though he affirms that " probably more 

 than one variety of feldspar is present." Undulose extinction and the 

 bending of twin lamellae in a number of the crystals give evidence of 

 considerable crushing. 



Of the ferro-magnesian minerals hornblende is the most prominent. 

 It frequently occurs as a fringe of varying breadth bordering the crystals 

 and masses of pyroxene. In these cases it is a pale-green colour, and 

 rather feebly pleochroic on the inner margin, but in the outer portion of 

 the fringe it becomes more compact and denser in colour and pleochroism, 

 changing from yellow-green to browny green. 



Often, again, the hornblende occurs in masses apparently independent 

 of the pyroxenes. Under these circumstances it is compact, brownish- 

 green in colour, and strongly pleochroic. Thomson* mentions that " the 

 cores of the hornblende crystals generally consist of a paler variety in 

 optical continuity with the green exteriors, so that the former pre- 

 sence of pyroxene is suggested.'' This point will be further discussed 

 (pp. 331-2). 



Both monoclinic and orthorhombic pyroxenes are to be seen. Augite 

 occurs in rounded grains and in irregularly shaped crystals. It is 

 colourless and non-pleochroic, and, where fresh, shows brilliant polariza- 

 tion colours of the second order. But often it is cloudy, and shows signs 

 of decomposition, which, no doubt, ultimately gives rise to the particles 

 of chlorite recorded by Huttonf and Thomson.* 



Diallage also occurs, though somewhat sparingly. It encloses minute 

 tabular scales of a reddish-brown colour, arranged parallel to the basal 

 plane, giving it the schiller structure, which distinguishes it from augite. 



The orthorhombic pyroxene is hypersthene. In some cases it exhibits 

 schiller structure like the diallage, but it may be distinguished from the 

 latter mineral by its pleochroism and by the fact that it extinguishes 

 straight. These characters also distinguish it from augite. The pleo 

 chroisrn showed a or a brownish red, b or b reddish yellow, c or c 

 green, pale watery colours in each case. To determine definitely that 

 the mineral was not enstatite, many sections were examined in convergent 

 light to secure an optic axial interference figure, and thus find its optical 

 character, but these attempts were unsuccessful. However, the pleo- 



* Thomson, J. A., " Notes on some Rocks from Parapara, Bluff Hill, and Waikawa," 

 Trans. N.Z. Inst., vol. 42 (1910), p. 33. 



t Hutton, F. W., " Notes on the Eruptive Rocks of Bluff Peninsula," Trans. N.Z. 

 Inst., vol. 23 (1891), p. 353. 



11— Trans. 



