(4) The siliceous sinter at XII.
A deposit of sinter and a calcareous sandstone, known as the Kalahari Kalk, considered by Dr Passarge to be of Miocene age, overlies a sandstone and curious breccia (Botletle Schnichten).
The water of most of the springs and geysers holds silica in solution in considerable quantities, so that as it cools and evaporates it deposits a dazzling white sinter which has covered many square miles of the valleys and contrasts strongly with the dark green of the surrounding forests.
The water of many of the springs contains sulphur, iron, alum and other materials in solution, which in places stain the pure white sinter with bright bands of colour.
Weed, "Formation of Travertine and Siliceous Sinter by the Vegetation of Hot Springs," in the 9th Annual Report of the Director of the United States Geological Survey (Washington, 1889); descriptions in the 5th, 6th and 12th Reports of the Hayden Geological and Geographical Survey of the Territories (ibid., 1871, 1872 and 1878); J.