Bartrum. — The Distribution of Igneous Rocks in New Zealand. 423 



to outcrop in that creek, which has not yet been investigated by the 

 writer. 



One specimen is a quartz diorite with abundant greenish-brown horn- 

 blende. The plagioclase shows bent twinning lamellae, and its margins 

 have been markedly granulated, as the photomicrograph (Plate XXVIII, 

 fig. 1) shows. The margins of the hornblende are less perfectly granu- 

 lated, but the fact that granulation is present indicates the probable 

 primarv character of the amphibole. Typical fine-grained dioritic gneisses 

 are represented by a rock consisting of ragged green or bluish-green horn- 

 blende and plagioclase with much gramdated material, and possessing a 

 banded structure. 



A most interesting discovery was made later when small pebbles of 

 several diorites were found in the alluvium laid bare by wave-attack in a 

 small flood-plain on the west shore of Shoal Bay, Auckland Harbour. They 

 were greatly weathered, but their character was readily decipherable. One 

 diorite shows abundant greenish-yellow hornblende which has been largely 

 chloritized, and very prominent and typical twinned ilmenite. The pre- 

 sumption is that these pebbles, like the Albany boulders, come from the 

 Tertiary beds. - 



In June, 1916, Professor Marshall showed the writer a dioritic rock 

 obtained from the Tertiary sequence near Komiti Point, Kaipara Harbour, 

 so that diorites seem to have formed an important element in the pre- 

 Tertiary terrain of the Auckland district. 



It is interesting to note that similar gneissic rocks (hornblende 

 gneiss, &c.) are present in a mid-Tertiary conglomerate in the gorge of 

 the Waipaoa River, Poverty Bay.* 



• 



Gabbroitic and Dioritic Rocks, Baton and Graham Rivers, Nelson. 



There are several specimens of coarse basic rocks collected some years 

 ago by Professor A. P. W. Thomas from the gravels of the Baton and 

 Graham Rivers, Nelson, in the collections of Auckland University College, 

 and a note upon them may be of interest, though full descriptions can 

 serve no purpose. 



They are essentially very coarse-grained hornblende rocks, with abund- 

 ant coarse iron-ore (mainly ilmenite), usually secondary epidote, quartz in 

 varying amounts, coarse apatite, a few flakes of biotite, and finally sphene. 

 Pale pyroxene is generally present in large amount, and perhaps gives the 

 clue to the origin of much of the hornblende. 



There are several types of rock represented. In the majority, deeply 

 pieochroic brown and often strongly schillerized hornblende is the pre- 

 dominant mineral, feldspar being comparatively unimportant. The amphi- 

 bole usually encloses pale augite poecilitically, and the sharp outlines of 

 this latter often show that much of the brown hornblende is not, as was 

 suspected, uralite. There is, however, undoubted pale-green uralite present 

 in most sections, which sometimes shows distinctly a transition or altera- 

 tion into the deep-brown hornblende. 



In two sections with very coarse, sharply idiomorphic hornblende a pieo- 

 chroic deep-green to brownish-yellow hornblende borders the deep-brown 

 variety, and is occasionally intergrown with plagioclase. In one of these 



*W- J. Sollas and A. McKay, Rocks of Cape Colville Peninsula, vol. 2, 1906, 

 pp. 175 et seq. 



