496 Transactions. — Geology. 



slaty shales. So far as I could ascertain, however, the igneous 

 rocks on the summit do not appear to be intrusive, and in this 

 view I am confirmed by the fact that the section of the rock 

 under the microscope conclusively proves that it is not, and 

 never has been, holocrystalline. In other respects my obser- 

 vations tend to confirm Mr. McKay's views, more especially 

 as to the height to which the slates reach. Mr. Park, on the 

 other hand, shows * a section across Te Moehau in which he 

 represents the mass of the mountain to consist of solid 

 andesites, the andesites reaching nearly to sea-level. My 

 examination, so far as it went, of the spurs and ridges on the 

 slopes of the mountain does not, however, tend to support 

 this view.! 



From stratigraphical and petrological considerations, there- 

 fore, I am disposed to consider these volcanic rocks as forming 

 part of an old andesitic flow which formerly extended far to 

 the east, and which had its origin in the dykes that are now 

 found on the eastern slopes. One of these dykes is 50 ft. in 

 width, approximately vertical, and strikes north and south, 

 parallel with the main axis of elevation of the peninsula. The 

 microscopical examination, as will be seen from the detailed 

 description of the sections, does not discourage this view. 

 The feldspars are identical, and the augite of the andesite is 

 represented in the dyke rock by its plutonic congener, horn- 

 blende. In both, the feldspars are highly corroded, and, in 

 fact, everything points to a common* origin, the difference in 

 texture, in size, and nature of the ferro-magnesian silicate 

 being clearly due to different conditions of cooling. 



Appended are detailed descriptions of the rocks mentioned 

 above, together with notes on a hornblende andesite which 

 overlies the slate spur from Moehau on the low saddle between 

 Cabbage Bay and Waikawau. 



Summit of Moehau. 



Augite Andesite. — A compact, dark, greenish-grey, non- 

 porphyritic rock. Specific gravity, 2-63. Base abundant and 

 much decomposed. Microhtes and laths of feldspar up to 

 0-05 mm. in length. Feldspars much kaolinized, showing 

 poly synthetic twmning. From their extinction angle (about 

 30°) they must be grouped with labradorite. Phenocrysts 

 •much corroded. PjToxenes monoclinic, faintly zoned, ranging 

 up to 0'8 mm. in length. Purplish and very slightly pleo- 



* " Geology and Veins of the Hauraki Peninsula " : James Park, 

 ■ F.G.S. 



t Mr. Park has since informed me that I am in error in supposing 

 that his section as shown is across Te Moehau, it being in reality a 

 section six miles to the north of the mountain. This at once explains the 

 apparent discrepancy. 



