The formation consists of syenitic granite, with veins of basaltic rock, hornblende, and quartz, while, in some parts, half-decomposed laterite underlies the soil. The plateau is not a flat table-land, but a succession of undulating hills and intervening grassy valleys, with ravines thickly wooded, numerous streams, and occasional rocky ridges running up into fine mountain-peaks. The streams all go to swell the great river Cauvery, by its tributaries the Moyaar and Bowany; the Moyaar descending from the hills by a fine waterfall at Neddiwuttum, on the northern slope; and the Bowany flowing down between the Koondahs and Neilgherries to the south. The soil of the plateau is very rich, being formed by the decomposition of basaltic and hornblende rocks, mixed with the clayey products of the granite, and much decomposed vegetable matter. The latter consists of the grass killed down to the roots by the frost, washed in by the succeeding rains, and mixed with the subsoil, increasing its richness and depth season after season. The richest land is on the lower slopes, where there are accumulations of soil washed from the hills above:[400] and there are extensive deposits of peat in the valleys, which afford supplies of fuel. The chief defect in the soil is the absence of lime. The temperature and amount of humidity vary according to the locality. At Ootacamund, 7300 feet above the sea, the means of the thermometer range from 42° to 68°, while in the two other lower and warmer stations of Coonoor and Kotergherry, about 6000 feet above the sea, the range is from 52° to 71°. The annual rainfall at Ootacamund is sixty inches, at Coonoor fifty-five inches, and at Kotergherry fifty inches. During the south-west monsoon, from May to September, the rain comes down in torrents at Sispara, and in the western parts of the Neilgherries, but their force is somewhat exhausted before reaching Ootacamund, in the centre of the plateau. At that station the rainfall, during the south-west monsoon, is about thirty-four inches; and the range of Dodabetta, which rises up like a wall, immediately to the eastward of Ootacamund, almost entirely screens the eastern part of the hills from the rains of the south-west monsoon, and there the rainfall is only twelve inches from May to September. During the portion of the year from October to April the western parts of the hills are comparatively dry, the prevalent winds are from the north-east, and the rains which they bring with them from the Madras coast do not extend farther west than the neighbourhood of Ootacamund. Kotergherry, and the eastern parts of the hills, receive the full benefit of the rains from the north-east monsoon, but they are not heavy, and the rainfall at Kotergherry, in that season, is thirty-eight inches. Ootacamund also gets some of the rain of the north-east monsoon (thirty-six inches), so that, in that central part of the plateau, there is a belt which