77

a—9

On its south side, and east of the upper part of Blind Bay, the rocks are coarse breccias and solid floes of grey andesite belonging to the Beeson's Island group of Miocene volcanic rocks, as developed at many places on the Cape Colville Peninsula. On the northern side of the bay the rocks are mainly andesic lava floes that, due probably to hydrothermal action in or near them, have been much decomposed and leached of their soluble contents, till they now present the general appearance of the older series of volcanic rocks on the Cape Colville Peninsula. Andesic rocks much or but little decomposed appear on the south-east and north-east slopes of the mountain Te Ahumata and towards its northern end, under a lesser thickness of superincumbent sinter deposit than appears on the middle and higher part of the range. The same rocks appear on the southwest and southern slopes of the mountain, up to heights varying from 500 ft. to 700 ft. above the sea, and form the hills towards the coast-line west and north-west of Sanderson's Creek. Sanderson's Creek rises on the higher grounds of the northern part of the range, and flows south-west and south to the point where it falls into Blind Bay. On the ridge between the upper part of Sanderson's Creek and the waters falling into the Whangaparapara Harbour andesitic rocks reach to a height of fully 700 ft. above the sea, but above that and even lesser heights the north-eastern end of this spur range is formed of siliceous sinter that, solid or in huge slips, covers to a low level the slope towards Whangaparapara Harbour. The sinter deposits of Te Ahumata cross the valley of Sanderson's Creek at the gorge above the Proprietary Mine, and _are there seen at a lower level than elsewhere where the rocks occur in situ. On all sides except the northern, the middle and lower slopes of Te Ahumata are encumbered, and the andesite rocks often obscured, by enormous slips of sinter material that have broken away from the higher part of the mountain. One of these slides reaches the sea at the head of Blind Bay, and another, a little further to the west, reaches the shore-line in the valley of Sanderson's Creek. Mount Te Ahumata, as seen from Blind Bay, lies to the north of the upper part of the bay, and on the south side the line of cliffs forming its higher part stretch nearly east and west. All the higher part of the mountain is formed of light-grey or white siliceous sinter, which outwards form vertical cliffs devoid of vegetation, and vary from 300 ft. to 500 ft. in height. It thus appears as though Te Ahumata had been suddenly and recently transplanted from the heart of the Southern Alps, in the South Island, and, still retaining a thick covering of snow and neve field, has till now succeeded in resisting at the same time the stronger sunbeam and milder climate of the Great Barrier Island. The great slips of sinter rock that in broad streams cover the southern sides of the mountain strongly resemble some of the less rock-encumbered glaciers of the South, and tend, if not to complete the illusion, at least to perfect the comparison that has been here made. On the northeast side of the mountain from the south and south-east sides the line of cliffs is continuous, without alteration of direction, to the culminating point at the trig, station 1,292 ft. above sea-level. From the trig, station the line of high cliffs trends more to the westward, and lowers considerably. The northern part of the range does not exceed I,oooft. in height. The andesic rocks are found at higher levels up to 900 ft. over this part, but sinter deposits still form the water-divide and two somewhat prominent heights at the northern extremity of the range. As seen from the outer part of Blind Bay the top of the mountain seems to be a narrow ridge, but from the south-east it is seen to be a sloping uneven tableland which in the line of north-west and south-east extension is slightly lower towards the south. The mountain and immediately adjacent areas have been in past times the scene of intense and long-continued hydrothermal action, which has deposited at the surface the immense beds of siliceous sinter that have already been mentioned. But, extensive and thick as these deposits now are, they are much less than formerly they have been. What the former limits of the deposit were might be hard to determine, but it seems probable that it extended over the whole area within which silver-mining is at the present time being carried on, and it is likely that the quartz lodes bearing silver have been formed since the commencement of hydrothermal action; and there are examples of quartz reefs clearly subsequent to the formation of the lower part of the sinter deposit. The andesite rocks on which the sinter deposits rest are probably of Miocene date, and therefore to Younger Miocene or Older Pliocene times must be referred the period of hydrothermal action, and not to latter times, as there is some evidence that it must have taken place and closed before the period of rhyolitic outburst which made such display of volcanic action along the east side of the Cape Colville Peninsula. The lowest and first-formed deposits of sinter consist of fine-grained flinty or evenly-bedded strata that betray no evidence of the presence of vegetable matter, but in the middle and higher parts the sinter deposit is one vast repository of broken plant-remains so thickly crowded together as seemingly to constitute its whole mass. Chiefly these remains belong to plants of no great size, but fragments apparently of small trees are sometimes to be met with. It would thus seem that a continuous growth over the same area was contemporaneous with the accumulation of by far the greater part of the sinter deposit, and that it thus was not of very rapid accumulation. The slow accumulation of the sinter deposits, their great thickness and vast extent, their loss in thickness, and the circumscription of their area due to denudation indicate the lapse of a long period of time since hydrothermal action began and since it ceased. There are now no hot springs in or about any part of the mountain, though further to the east, in the low grounds of the Kaitoke Valley, thermal waters are still escaping at the surface. On the higher part of the mountain considerable quantities of obsidian are met with, and at times it occurs in considerable masses, varying from 12 in. to 15 in. in diameter. The whence of this is not easily determinable. It is not to be met with on the hills formed of andesic rocks, solid or brecciated, away from the mountain, and it never occurs on surfaces where sinter forms hills overlooking the hollows and shallow valleys of the mountain-top, and is never included in the