65

G.—9

and weathering of the rock. Frequently this rock contains fragments of dark andesite, which when cut and polished is seen to be holoerystalline. These rocks extend eastward to Cook's Bay on the south side of Mercury Bay, and on the west side of that form Shakespeare Cliff. They weather into fantastic shapes, some remarkable cliffs of this kind appearing opposite Mercury Bay Township. These rocks are followed by solid grey rhyolites that form the hills to the south and east as far as the waterfall on the road from Mercury Bay to the lower Tairua. In the valleys of the creeks draining into the swamp at the head of Cook's Bay fragmental rocks again appear, and before reaching the saddle by which the road passes into the valley of the Purangi Stream considerable quantities of obsidian are met with in the higher beds. At one place a somewhat greater than ordinary quantity of obsidian appears as loose fragments alongside the track, and at first sight it seemed as though this had been collected by the Maoris for the purpose of obtaining cuttingflakes and other instruments ; but as on closer examination it was seen that this had not been the case, it became important to trace the loose fragments on the surface to the beds from whence they had been derived. After some search the source of these was found to immediately underlie a thick bed or floe of coarse, spongy, or highly vesicular rhyolite that forms the higher part of the hill west of the saddle, and descends from this into the valley of the Purangi. This particular stratum containing much obsidian stretches westward through the hills to the shores of Whitianga Harbour, and is seen to have a low dip to the south-east. On the east side of Cook Bay there are considerable deposits of light-stone-coloured or yellowish-brown rocks, some parts of which consist wholly of spherulite, from which many rock-slides have been prepared by Mr. Bradshaw, of Mercury Bay. Mr. Cheeseman, of the Auckland Museum, informs me that perlite also has been obtained from this locality, but no sample of this rock is in the collections made by me from the neighbourhood of Mercury Bay. Round the sources of the Purangi the spongy coarse vesicular rhyolite forms a prominent band dipping through the hills to the south-east in the direction of the coast at Whigmore's. To the westward, overlying beds of decomposed rhyolite or rhyolitic tuff of various colours, forms the hills for a considerable distance in the direction of Whenuakite ; and to the south-east of the broad swampy plain that lies to the left of the road to the Whenuakite the hills towards the coast-line are composed of like materials. Towards the south-east from the Whenuakite Station the more coherent tuff rhyolites again make their appearance, and there appears to be a rude synclinal arrangement of the beds between a point to the north of Boat Harbour and the mouth of the Whitianga River, at Mercury Bay. The lowest beds seen in this area are solid, somewhat splintery, brown and green rhyolites, that form the range between the middle part of Stony Creek and the upper part of the Whenuakite. These rocks rest on augite andesites developed to the south-east, and constitute the limit of the rhyolite formation, in this direction within the area north of the lower Tairua River. From the upper Whenuakite the eastern boundary of the rhyolite trends to the south-west outside a tongue-like projection of andesites and ash-beds belonging to the Kapanga group, which passed it again runs south-east to the north head of the Tairua River. Over this area of the rhyolites no special examination of the rhyolite has been made, except in the mountain forming the north head of the entrance to the Tairua River. Here the most beautiful examples of fluxion rhyolite and supherulite may be obtained. On the north side of the mountain, where exposed to the full force of the open sea, very fine sections of the beds are displayed, and here the structure of the rock is seen to the best advantage. Fluxion-structure is exhibited on a gigantic and also in the most minute scale; and at the same time the rock is often spherulitic, a peculiar modification of spherulitic subcrystallization being here common. This consists of a solid nucleus of spherulitic substance from which spring radiating spikes of the same material, the lateral margins and terminations of which are often encrusted with crystals of quartz. Through the kindness of Mr. Bradshaw, of Mercury Bay, microscopic slides of this echinal spherulite were prepared from specimens collected by me, and these constitute objects of the most wonderful beauty and scientific interest. Later in the season were forwarded to me specimens collected by H. D. M. Haszard, Government surveyor at the Thames, which shows this structure in specimens of large size, but apparently without the attaching quartz crystals. On the shore of the tidal estuary of the Tairua River rhyolites appear at the mill and township, but a little beyond this andesites appear on the left bank of the river, and, by way of a small island in the channel, these cross to the south-eastern side of the valley. Higher up the river, to a point between the Upper Landing and Broken Hills, the rhyolites are on both banks of the river, and the rhyolite area to the north and west is thus connected with an extensive development of the same rocks to the east, south of the Tairua. Returning for the time being to the district around Mercury Bay, the rhyolites are found on each shore of the different arms of Whitianga Harbour, and along the valley of the Rangihau to Gumtown. The boundary between the Acidic group and the rocks of the Beeson's Island group reaches a short distance up the Rangahau River from where it is joined by the Waiwawa, and thence the boundary runs along the south-east slopes of the range between the two rivers to Dirty Camp, lying to the east of the upper valley of the Rangihau. To the south and east the whole country between the sources of Stony Creek and the waters running south-east into the Tairua River is formed of rhyolite and its allied rocks, and on the height of. land between the Rangihau and the sources of the Kauaeranga Rivers the rocks of this group attain an elevation of fully 2,500 ft. above the sea. They are well displayed along the banks of the Rangihau and in the " Big Slip" at the source of one of its lesser branches. On Bull's Run the rocks are partly sedimentary, partly andesic, but mainly rhyolite. From the sedimentary beds of this part were collected a number of well-preserved plant-remains, which, however, owing to their not having been critically examined by a competent authority and determined specifically, cannot be made use of in fixing the age of the deposit in which they were found, or of this group generally. Fluxion-structure is the chief characteristic of the rhyolites in this part of the district. 9—C, 9.