Thomson. — Rocks from Parapara, Bluff Hill, and Waikaiva. 37 



compact hornblende crystals, often white in the centre, and smaller anhedral 

 feldspars. In section the hornblende * presents rhomboidal shapes with 

 ragged outlines, and is seen to be a strongly pleochroic variety of common 

 hornblende, with occasional tremolitic cores. Not seldom some crystallo- 

 graphically discontinuous hornblende crosses the main phenocryst, a feature 

 often observed in uralite. The feldspars give more rounded outlines, and are 

 turbid, and filled with calcite. They include also hornblende, magnetite, 

 and epidote. The groundmass consists of elongated, often needle-like 

 hornblende prisms, with associated biotite flakes, slightly elongate feldspars, 

 probably albite, showing Carlsbad twinning, magnetite grains, and occa- 

 sionally a green epidote. The rock is thus a porphyroblastic hornblende- 

 schist derived from a basic igneous dyke-rock. 



The next three specimens to be described come from a small headland 

 about half a mile round the coast to the south-west, just beyond the mouth 

 of the harbour. There is here an interesting complex of coarse holocrystalline 

 rocks. Three elements may be distinguished, a dark dioritic rock (No. 1), 

 which appears to vein a much lighter dioritic rock (No. 2), occasionally 

 enclosing both the latter and a coarse hornblendic rock (No. 3) as xenoliths. 

 The dark diorite (No. 1) is often gneissose, while the walls of light diorite 

 (No. 2) which surround it are quite massive. 



In section, all three rocks present a similar assemblage of minerals, but 

 the relative amounts of iron-ores, hornblende, and feldspar vary very 

 considerably. Hornblende forms the predominate mineral of No. 3, and 

 is a brown-green variety with a fine schiller-structure in the centre, sur- 

 rounded by a margin of green hornblende. Besides the hornblende, there is 

 a limited amount of a basic feldspar and iron-ores. The hornblende shows 

 a fair approach to idiomorphism, but the larger iron-ores, probably ilmenite, 

 are moulded both on hornblende and feldspars. Minute octohedra of magne- 

 tite are abimdantly included in the two last-named minerals, but are probably 

 of secondary formation. 



In No. 1, iron-ores and hornblende of the same nature as above described 

 are abundant, but feldspar predominates, and besides these minerals a 

 little apatite and pyrite are found. A very peculiar zoning is observable 

 in the feldspar ; there are only two zones, separated by a boundary of the 

 most irregular nature. The cores are in some instances as basic as labra- 

 dorite, but the exteriors are so acid that there is a marked difference of relief 

 between the two on lowering the condenser of the microscope, and the Becke 

 effect may be easily observed. The Carlsbad and albite twinning of the cores 

 does not persist in the exteriors, as in the case of albitization recently de- 

 scribed by Bailey and Grabham* in the quartz dolerites of the central valley 

 of Scotland, but it appears probable that something of similar nature has 

 taken place here. 



The structure differs from that of No. 3 in that the hornblende is dis- 

 tinctly ophitic to the feldspar. This observation is in accord with recent 

 theories of the dependence of structure on eutectic relations, f since in the 

 former rock the hornblende is in excess, and in the latter the feldspar pre- 

 dominates. But caution must be used in describing rocks as much altered 

 as these, for, though in No. 2 the hornblende is moulded on the feldspar. 



* Bailey, E. B., and Grabham, G. W. " Albitization of Basic Plagioclase Feld- 

 spars." Geol. ]Mag. Dec. V, vol. vi, p. 250. 1909. 



t Vogt, J. H. L. " Physikalisohe-cliemisohe Gesetze der Kristallisationfolge in 

 Eruptiogesteine." Isoh. min. u. petr., Mitt, xxiv, p. 437. 1905. 



