Thomson. — Rocks from Parapara, Bluff Hill, and Waikawa. 35 



linearly arranged granules of magnetite, giving rise to a rude schiller-like 

 striation oblique to the prismatic cleavages. This latter alteration is very 

 common in the hornblende of rocks that have suffered shearing, and seems 

 to lie due to an attempt on the part of magmatically formed brown horn- 

 blende, rich in iron and titanium, to adapt itself to the altered conditions 

 according to the volume law. From the presence of these striations and the 

 strongly green colour of the hornblende, it appears original, and not a uralitic 

 pseudomorph of pyroxene, and the rock is therefore an altered hornblende 

 dolerite or gabbro. 



The other four rocks come from the contact aureole of the granite, one 

 being an amphibolite intercalated among the stratified rocks. It is almost 

 completely recrystallized, and in description it is desirable to make use of 

 the terms introduced by Becke.* These are based on the belief that in 

 crystalline schists the formation of the different minerals has proceeded 

 simultaneously, since each may be found enclosed in the others, in opposition 

 to the sequence of growth amongst minerals of igneous rocks. Owing, how- 

 ever, to the different powers of crystal-growth exhibited by the minerals, 

 they differ widely in their development of crystal-faces, so that some appear 

 moulded on others. The resulting structure is called " krystalloblastisch," 

 or, anglice, " crystalloblastic," and the apparent order of separation is termed 

 " the crystalloblastic order." Well-shaped (euhedral) crystals are termed 

 " idioblastic," those with in-egular boundaries (anhedral) " xenoblastic," 

 a'nd other structural terms used in the descriptions of igneous rocks are 

 similarly adapted with the suffix " blastic." In this amphibolite there are 

 evidences of former igneous structure in the presence of original phenocrysts 

 of feldspar. The rock is therefore " porphyro-blastic." The minerals, 

 especially the sphene, are frequently honeycombed with inclusions, and 

 the crystalloblastic order appears to differ from that established by Becke, 

 in that feldspar is idioblastic to sphene and hornblende ; but this apparent 

 difference may be due to the incomplete recrystallization of the feldspar. 

 These three minerals are the most abundant constituents of the rock, but 

 epiclote, apatite, and pyrite are also present, and secondary sericite is forming 

 in the feldspar, which is near andesine in composition. The mineralogical 

 composition of the rock shows that it is an ortho-amphibolite, the large 

 crystals of plagioclase that it was porphyritic, but to arrive at the nature 

 of the original rock an analysis would be necessary. 



The three stratified ]'ocks are interesting as containing contact minerals, 

 a class which must be common but has not yet been widely observed in 

 New Zealand. One rock is a biotite-hornfels, containing biotite, quartz, 

 plagioclase, feldspar, chlorite, and magnetite. The quartz forms a fine 

 mosaic of polygonal grains, between the interstices of which are inserted 

 abundant flakes of biotite, locally decomposed to chlorite. 



The other two rocks are marbles with contact minerals. In one — the 

 " complex-carbonate rock " of the Survey — there is little besides the large 

 polygonal carbonate grains. White mica and sphene can be recognised, 

 but small needles of a highly refringent and bi-refringent mineral lying in the 

 carbonate defy determination in section, although they can scarcely be 

 other than rutile. The other must have been an impure dolomitic rock, 

 for, besides calcite, a colourless augite occurs very abundantly both in separate 

 grains and in small aggregates. Octagonal cross-sections may occasionally 



* Becke, F. Ueber JSIiueralbestand und Struktur der krystallinischen Sohiefer." 

 Comptes Reiidus ix Sess. Cong. Geol. Inter. Vienna, 1904, pp. 553-70. 



2*— Trans. 



