The rainfall in Borneo is so great, the rainy season lasting from October to April,[8] that the rivers are very numerous and copious, rolling down large volumes of water. Severe droughts are, however, not uncommon during the fine season of the S.W. monsoon. Between Kuching and Bruni are the Sadong, Batang Lupar, Saribas, Kalaka, Rejang, Bintulu, and the Baram rivers, all available as waterways for trade with the interior. For fifteen miles only from its mouth is the Batang Lupar navigable by steamers, above that, though a fine broad river, it is obstructed by dangerous shoals. The Rejang is navigable by steamers for 170 miles, nearly as far as the first rapids. This noble river descends many stages by as many plunges from terraces. Between the rapids the river is deep, sluggish and broad for many miles. Boats that can be hauled up past the rapids can ascend a distance of 650 miles from the mouth. The Baram river is navigable by steamers for some twenty miles above Claude Town, that is, eighty miles from the mouth, but owing to the exposed position of the bar and to the heavy seas breaking over it, and also to the silting up of the mouth during the N.E. monsoon, only very small craft can then enter, but during the S.W. monsoon it can be entered by steamers of light draught. In Dutch Borneo as well there are magnificent rivers. The same cause that has made some of the rivers so uncertain in their mouths has produced vast stretches of morass, overgrown with the nipah palm and mangrove, and infested with mosquito swarms; but the beach is almost everywhere of beautiful white sand, reaching to where the graceful casuarina tree grows as a belt above the reach of the tide. The tropical heat, added to the great rainfall, makes Borneo a vegetable paradise; indeed, it presents the appearance of one vast surface of sombre evergreen forest, starred with flowering orchids, and wreathed with creepers, of a richness perhaps unsurpassed even in South America. The hills and ranges of upland consist of blue metamorphic limestone on which is superposed a thick series of sandstones, conglomerates, and clay-shales. Piercing these beds are granite and a variety of plutonic rocks, as diorite, porphyrite, etc. These latter are developed in greatest abundance in the antimony districts, where they are in immediate contact with the limestone that has been fissured and tortured by upheaval. The sandstone shales have also been tilted and distorted; nevertheless in places they retain their original horizontal position. They are usually found to be impregnated with peroxide of iron. It is in this formation that the cinnabar deposits occur.