H.—34.

4. RECONNAISSANCE SOIL SURVEY OF THE CENTRAL PART OF THE NORTH ISLAND. (By L. I. Grange and N. H. Taylor.) From the 14th November, 1930, to the 24th May, 1931, the writers were engaged on a reconnaissance soil survey of the volcanic-ash showers of the central part of the North Island. This work is under the supervision of Mr. T. Rigg, Assistant Director and Chief of the Agricultural Department of the Cawthron Institute. In January an excursion was made to Te Papa Parish, south of Tauranga, to advise whether certain unimproved sections were likely to be " bush sick." The month of May was largely devoted to a detailed survey of abandoned farms in Ngaroma district, south-east of Te Awamutu. The broad belt of country extending south from Hamilton to National Park Railway-station, east to Mamaku, and west to a north-and-south line through Mairoa was selected for the season's work. In a short paper one of the writers* described the following volcanic showers —Ngauruhoe, Taupo, Rotorua, Mairoa, and Tongariro —occurring in this area. Data, mostly additional to that contained in that paper, are set out below for each of these deposits, and two more, the Egmont and Hamilton showers, are also described. Ngauruhoe Shower. Ngauruhoe ash, the youngest of the deposits mentioned, extends farther to the west and northwest of the volcano than was at first thought. The sandy soil, at National Park Railway-station, to a depth of 3 in. to 4 in. from the surface, is formed from this ash, and the top 2 in. of the soil at Oio, Owhango, and Taumarunui contains fragments of Ngauruhoe andesite. A fusion analysis of the top 3 in. of Ihe soil from Piriako Hill shows that there is a considerable admixture of andesitic ash. Stock grazed in the area covered by Ngauruhoe ash does not suffer from malnutrition. The ash covers a considerable area of the Taupo pumice which is known to be unhealthy. Taupo Shower. The Taupo shower, rhyolitic in composition, was blown from vents active on the site occupied by Lake Taupo. During the present survey the boundary where the ash is 6 in. thick when deposited from the air was accurately mapped in the districts to the north and west of the source. This was easily done, as the shower has a coarse texture, that of a sandy silt and sand, whereas the deposit it covers is a sandy or silty loam, and its colours —grey, light brown, and creamy yellow—are distinctive. The 6 in. boundary-line is two miles south of Putaruru Township, whence it extends westward to a point on the Te Awamutu - Arohena Road about twenty miles from Te Awamutu. South-west it crosses the Main Trunk Railway near Puketutu and runs southward to a mile or so east of Tuhua (west of Okahukura) and to the Wanganui River seven miles west of Taumarunui. The deposit thins rapidly near its outskirts. On the Tokoroa-Putaruru Road it decreases from 18 in. to 6 in. in a distance of two miles and a half. At a mile or so beyond the 6 in. boundary one cannot detect any Taupo ash in the soil. At three miles from the 6 in. boundary in the Taumarunui district the top soil contains practically none of the hypersthene crystals that are so abundant in the Taupo deposit. There is an unexpected irregularity in the thickness of the Taupo shower in the Wharepuhunga and western Ngaroma districts. At Wharepuhunga the 6 in. boundary-line bends sharply towards Lake Taupo and deviates to the south-east at least three miles inward from an otherwise mooth curve. For eight miles still farther south-east from this re-entrant, and over a width of about four miles, the Taupo shower is at most 7 in. or 8 in. thick, whereas on either side it is 18 in. or more. On rollmg country in the western .portion of the Ngaroma district at the south-east end of this second re-entrant the ash is in many places between 2 in. and 3 in. thick. The reason for this irregularity is not definitely known. Except where covered with Ngauruhoe ash or removed by erosion or slumping, the Taupo pumice is the parent material of the soil over the area lying towards Lake Taupo from the 6 in. boundary-line. The soil profile of the upland forested areas, where the rainfall is heavy, is different from that on the lowlands. The upland type is found at Mamaku, Ngaroma, and east of Kopaki. A profile at Ngaroma is : — 2 in. dark sandy silt. (Top.) 3 in. grey sandy silt. 5 in. dark-brown cemented sand, sandy silt, and creamy-yellow sand. An example of a lowland profile is that at Lichfield : — 4 in. black silt. 4| in. mottled light-brown and creamy-yellow compacted silt. 4 in. creamy-yellow sand. In places a poorly developed grey layer, apparently due to more active leaching, is present in the lowland soil profiles. Sheep and, to a less extent, cattle become " bush sick " when grazed on soils derived from Taupo pumice. The disease appears to be most acute in the Mamaku, Ngaroma, and Kopaki districts, where the grey layer is well developed, and is not acute in western Ngaroma, where the pumice is between 2 in. and 3 in. thick. In the last-mentioned locality grass roots readily penetrate the underlying sandy loam, and perhaps there obtain substances that cannot be obtained from the Taupo pumice alone.

*L. I. Gbanqe : Volcanio-ash Showers. N.Z. Jour. Sci. & Tech., Vol. 12, No. 4, pp. 228-40 ; 1931.

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