a—3

48

(1.) The Nature of the Drift in which the Gold occurs. The auriferous drifts of Otago now worked with dredges may be divided into five classes, each possessing distinctive features of their own. They aire : (a) The banks and channels of rivers traversing gravel-filled valleys; (6) the beds of rivers running through rock-bound gorges; (c) valley deposits ; (d) filled-in lake-beds ; and (c) marine littoral deposits. The (a) section is the most important. It includes the Clutha where its course is through the Moa, Dunstan, Earnscleugh, and other flats; the Kawarau near Arrowtown and Cromwell; the Upper Clutha above Cromwell; the Manuherikia, the Mararoa, and other rivers. In the channels there is a constant concentration and reassortment of all detrital matter brought into the river by its tributaries, and by the erosion of its banks. Where the flats are wide there is often evidence to show the river has not always run in the same channel. Dredging is carried on under the most favourable conditions in this class of country, the tendency of the dredge-designers being to constantly increase the lifting-power both as regards depth and capacity of the machines, and to look for profitable returns from the quantity of material treated rather than from the close extraction of the gold. (b.) The Beds of Bivers running through Bock-bound Gorges. —These gorges occur where the rivers have cut into the schistose rocks which form the surface of the greater part of Northern Otago. Some examples are the bed of the Kawarau from Victoria Bridge to the Gorge Township; the Clutha from Cromwell to Clyde, and again from Alexandra to Coal Creek. The conditions generally are unfavourable for dredging. Where the direction of the gorge is at right angles to the strike of the schists the bed of the stream is comparatively free from heavy rocks, but where it corresponds to the strike, and the dip of the rock is at a high angle one of the walls becomes what is locally known as a slip-sideling, the surface-rocks sliding upon their bedding-planes from the higher to the lower levels, and depositing masses of rock in the channel. The rapid flow of water through these gorges has a tendency to disintegrate the rock-bottom so quickly that what is known as a soft-schist bottom, formed by the decomposition of the rocks in situ, rarely occurs. Apart from the hard nature of the bottom, the dip of the mica-schists in some of the gorges is at a considerable angle from the horizontal: in some parts of the Kawarau Gorge it is 40°, and in the Dunstan Gorge from 25° to 28°.* Like all other natural agents, water attacks the line of least resistance. In these rocks it is the mica and clay between the quartz foliations which yield first to erosion, thus forming corrugations. Where the quartz plates are thin the lip of a dredge-bucket will break them down, and may reach some of the gold in the hollows; but where the quartz occurs in lenticular masses the gold, so far as bucket-dredging is concerned, is safe for all time. (c.) Valley Deposits. —In this paper this term is used with reference to those deposits formed by the breaking-down of the auriferous-quartz drifts, cements, and conglomerates, and their redeposition in shallow depressions. The original formation may have entirely disappeared, leaving only the reassorted gravels in the valley-bottoms. The creeks traversing these valleys are usually small, the work of denudation and distribution having taken place at a period when the rainfall was greater than now. The Waikaka, Charlton, and Waimumu flats are examples of this class of ground. Occasionally bands of clay are found interbedded with the gravels, and sometimes, if in the vicinity of volcanic rocks, the wash is more or less cemented with the iron set free by the decomposition of the basic lavas. When sufficient water is available these deposits can be dredged without difficulty : the gold is distributed throughout the wash. A variety of . bottom is met with—pipeclay, lignite—and frequently where the enclosing elevations are gravelhills the deposited wash rests upon the undisturbed gravels. (d.) F%lled-in Lake-beds. —Auriferous deposits consisting of angular rock fragments are found forming the surface of what at one time were lakes and mountain-tarns. If the lake were a large one, the gravels are only auriferous near the old margins, and then only in the vicinity of the outlets of streams draining gold-bearing country. In ground of this decription the dredging-bottom, representing the old lake-bottom, consists of clay and fine sand, originally carried in suspension into and deposited from still water. The wash is very little water-worn, and always derived from the schists of the neighbourhood. The broken rock has evidently been swept into the lake by sudden floods in the adjacent creeks, caused by the temporary damming of the creek by slides, and the subsequent bursting of the dam. The gold was carried with the rock, and is evenly scattered through the wash, the bottom layers being little richer than the top. In most instances these deposits form excellent dredging areas. The Upper Ida Valley and Speargrass Flat are examples of this class of ground. (c.) Littoral Deposits —The sea-beaches of Otago have never been successfully dredged. Hitherto attempts have been made with dredges constructed on the suction principle to deal with the sands, but, as the problem is one of concentration and subsequent extraction only, they have so far failed. These auriferous beaches occur in many parts of Otago ; the most extensive are between the Clutha mouth and Waipapa Point. This portion of the coast is within an area of elevation. Upon some of the beaches the sand extends some distance inland, and is covered with bush. The gold is associated with layers of magnetic and titaniferous ironsand interstratified with siliceous beach-sand. The highest layer where I examined the deposits is 4 ft. above high-water mark in the ridge of sand-dunes that faced the ocean, but inland they occur 8 ft. above the same level. The layers of black sand continue with more or less regularity to a proved vertical depth of 15 ft. below high-water level, and will probably be found to live down to greater depths. There must be many square miles of these deposits, and in the near future their exploitation by dredging will be undertaken in earnest. The richness of the deposits is undoubted. The watershed of the Clutha (upper and lower), with its tributaries, is, down to the Beaumont, almost entirely included in the Wanaka and Kakanui geological formations. The rocks of these formations consist of mica, chlorite, hornblende, and argillaceous schists. It would be natural to expect that the gravels forming the river-drift of this system would be composed almost exclusively

* Hutton and Ulrioh.