C.—2c

12

therefore evident that the geology of the Murehison district is by no means simple, and only by means of a careful and detailed survey made by experienced geologists can the true succession of the rocks ■and their structure be made out. The following remarks should be read in conjunction with Dr. J. Henderson's report published in volume 1 of the Journal of Science and Technology. (See also sketch-map in 11th Ann. Rep. Geol. Surv.) The pre-Tertiary rocks of the Murehison district and surrounding areas consist mainly of granite, •greywacke, and argillite of Palaeozoic age. These rocks are of no interest in connection with the oil possibilities, and need not be described. Resting upon them is a belt of Lower and Middle Tertiary strata, in plan shaped like an elongated diamond, extending northward from the Sheriff River, thirty miles south of Murehison, to the upper Wangapeka Valley, nearly twenty miles north of Murehison. In its middle part this belt is over twenty miles wide. It therefore covers a large area. The Tertiary rocks are mainly conglomerates, sandstones, mudstones, and limestones. The total thickness of these strata is great, possibly over 10,000 ft. According to all the geologists who have reported on the district, the limestone part of the succession is the youngest, but opinions differ as to whether conglomerates or arkositic sandstones form the lowest part. The management of the "Murehison Oil Company, however, believes that the limestones are the oldest beds, and that they are succeeded by calcareous sandstones, and these by the conglomerates. Probably the exact opposite is the true order ; but in the section from Doughboy Stream eastward through Murehison to the Mangles River the apparent succession is certainly from limestone upward to sandstone and to •conglomerate, as stated by the Murehison Oil Company. The rocks dip at high angles eastward from Doughboy Creek to the Mangles-Buller junction, and are much disturbed by strike-faults. If the faults are reverse faults, then the apparent order of the strata could be opposite to the true order, and this is my present opinion. From a little above the mouth of the Mangles to Blue Duck Creek the rocks seen are conglomerates (with interbedded sandstone, &c.) that clip at high angles to the north of west. East of Blue Duck Creek sandstones dipping in the same direction appear. Towards the mouth of Blackwater Stream the sandstones flatten in dip, and half a mile to the east, at the oil-seepages, are said to be dipping steeply eastward. Sandstones continue for perhaps two miles to the east. Their strike as observed by me is north-north-east (32°), and their dip 40° to 45° to the south of east. These observations, taken as they stand, would indicate that between Doughboy Creek (west of Murehison) and Blue Duck Creek (several miles east of Murehison) there is a huge syncline, which is followed eastward by an anticline, with its crest near the mouth of Blackwater Stream. The complicating effects of faults on the structure, however, have to be taken into account. When this is done the syncline disappears, and the anticline, as a possible oil-bearing structure, is shorn of much ■of its significance. Petroleum Prospects, etc. In the bed of the Mangles River, less than half a mile east of the mouth of Blackwater Stream, :small amounts of oil and inflammable gas are constantly escaping. Gas also rises from the bed of Blackwater Stream, just below the road-bridge. Elsewhere in the Murehison district several gasescapes are known. Near "Warwick Saddle, many miles south of Murehison, four oil-seepages have been found. Inflammable gas is stated to exude from mudstones in the Owen River district. These facts indicate clearly enough that the Tertiary rocks of the Murehison district are petroliferous, but they are far from indicating that petroleum is present in quantity sufficient to repay the oil-driller.' Encouraged by the oil and gas seepages, and by what seems to be an anticlinal structure, the Murehison Oil Company has begun to bore close to the Mangles River, about a quarter of a mile above the mouth of Blackwater Stream. At the time of my visit this bore was 376 ft. deep, and was partly lined with 10 in. casing. Work had been suspended for a few days on account of several lengths of the casing on hand being defective, and therefore requiring reconditioning, work that had to be done at Nelson. The rocks penetrated in the bore consisted mainly of somewhat calcareous sandstones, with bands of sandy limestone, shale, &c. ■9. PROPOSED TUNNEL-SITE, PEGLEG GULLY, AND DAM-SITE, MANUHERIKIA FALLS, CENTRAL OTAGO. (Summary of reports by H. T. Ferrar.) On the 23rd April, at the request of the District Engineer, Public Works Department, Dunedin, a geological examination was made of the site and surroundings of a proposed irrigation-water tunnel through a ridge near Pegleg Gully, at the upper end of Manuherikia Valley. The object of the examination was to ascertain if any unforeseen difficulties would be encountered when tunnelling is undertaken, and to determine how far slipping of the hillside above the tunnel-entrance would endanger the tunnel when excavated. It was found that the proposed tunnel would pierce schistose greywacke for the greater part of its length. This rock is much jointed, and will probably require timbering during excavation, and parts at least of the bottom and sides will probably need to be lined with concrete to prevent leakage of water. The slip material above the proposed tunnel-entrance on north side of the ridge will need to be supported, and it was recommended that trial drives be made at two points in order to gain .some idea of its dimensions. Advice was also sought by the Public Works Department with regard to the structure (faults, joints, &c.) of the rocks on which the proposed Manuherikia Falls Dam is to be placed, and the rigidity of its foundation. The rock is jointed greywacke, the structure lines being directed approximately west-north-west and east-south-east and at right angles to the river-gorge. The joint-planes of the greywacke dip at high angles towards the north. Hence the foundations of the dam wheu completed will be seated on, and the wings will be supported by, one and the same hard band of rock.

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