75

C.—9

the different lodes in Martha Hill, while the lodes worked by the Waihi Company show no signs of being exhausted for many years to come. Over the Silverton Hills mining is being also vigorously prosecuted. Over the Waitekauri field, including the Komata and Maratoto divisions-of the same, vigorous work is being prosecuted in all directions, and within these a large mining population is located. Concluding Remarks. The foregoing report embodies the results of the past year's work within the boundaries of the Cape Colville Peninsula. Necessarily, over so extensive an area, difficult of exploration, and of which the geology was not only difficult but practically over the greater part unknown, not much of close and detailed geology was to be expected. Nor under the circumstances was this attempted. In such a case the collection of rock-specimens necessary to illustrate the rocks of different formations and special localities would have proved such an impediment that but a small part of the whole could have been examined during the past season. The collecting of evidence from mines had also in a great measure to be abandoned, other than so far as this was necessary to a knowledge of the formation within which these might chance to lie. The broader questions relating to the age, character, and limits of the older sedimentary formations and the volcanic rocks of the Peninsula, the relations of these to each other, and the modification of the surface due to the greater movements of the Peninsula, &c, though seemingly not directly connected with an investigation of which mining and the advancement of mining was the principal object, yet proved of such essential consequence that without this knowledge the main object could not be effected.

REPORT ON THE SILVER-BEARING LODES OF THE NEIGHBOURHOOD OF BLIND BAY, GREAT BARRIER ISLAND, AUCKLAND, BY ALEXANDER McKAY, F.R.G.S., GOVERNMENT GEOLOGIST. Mr. A. McKay to the Under-Secretary for Mines. Sir,— 15th June, 1897. In accordance with your instructions, under date of January 19th, 1897, I left Auckland for the Great Barrier Island on the 24th February, where till the end of the month I was engaged in making the examination required to enable me to report on the silver-mining now being carried on in the neighbourhood of Blind Bay, on which I have the honour to submit the following report: — General Sketch of the Geology of the Great Barrier Island. My examinations were confined to the immediate neighbourhood of where the silver-mining lodes are found, and, except as given by Professor Hutton, who in January, 1868, reported on the geology of the island, there is no other description of the whole of the Great Barrier Island on record. I therefore extract from Professor Hutton's report his remarks on the general geological structure of the island. He says,— " General Geological Structure. — The greater part of the island is composed of volcanic (Trachytic) rocks resting on and partly enveloping sub-metamorphic slates and sandstones of Upper Palaeozoic age. These Palaeozoic rocks come to the surface only in the northern and central parts of the island, and form steep mountain-ranges, for the most part thickly covered with bush. Over the greater portion of the island the older rocks have been worn down to or below the present sea-level, and here they are covered by thick masses of trachytic tufa and agglomerate of late Tertiary age. Speaking generally, this land, except the bottoms of the valleys, is poor, and bears more fern and manuka scrub than bush. The trachytic tufa seems to be too purely siliceous to form a good soil, and when charged with oxide of iron decomposes into a pink or brick-red ' laterite,' which is exceedingly barren. Where, however, it mixed with streams of lava of trachyte, or, still better, the dark-coloured trachy-dolentes, they add to the soil, on decomposition, the alkalies and alkaline earths that are wanting in the fine-grained tufa, and so improve it. The same thing happens when blocks of lava are collected together in the tufa to form an agglomerate; and, as these lavas and agglomerates are considerably harder than the fine-grained tufa, they resist the action of the rain and atmosphere much better, and so generally form the summits of the hills. This is the reason why here, a.s in many volcanic countries, the soil at the tops of the hills is often more fertile than that of the slopes or valleys. " Palaozoic Bocks. —The whole of the north part of the island, from the Needles to a line drawn from Maori Bay to Waikaro, is composed of dark-blue siliceous slates and diorite sandstone, the slates often containing rounded stones of a fine-grained dark-grey sandstone. These rocks, especially at Mine Bay, are cut by numerous dykes of quartz-porphyry, felstone, and diorite (granite of the miners), and the dip is very obscure. I am, however, inclined to think that a synclinal axis runs from Mine Bay to Rangiwhakea, rising towards the east, as the dip in Needles Bay is 70° S.S.W., and at Waikaro 45° N.W. To the southward the continuity of the strata is interrupted by a mass of tufa, but a little north of Harataonga, Palaeozoic rocks apparently similar to those in the north part of the island, are again seen; and in Harataonga Bay they appear to rest on dark-grey thin-bedded sandstones interstratified with laminated and indurated shales, dipping 20° N. " An anticlinal axis probably passes a little south of Hirakimata, for Mount Young (1,260 ft.) is composed of pink-and-white siliceous slates, with small veins and nodules of chert, clipping 70° S. I was not able to ascertain the relation that these slates bear to the other Palaeozoic rocks further north. The strike of the whole of the Palaeozoic strata is nearly east and west over the whole of the north and central parts of the island."