3. In a number of places, there are abnormally low temperatures underground either for the whole or only for part of the year. Although commoner than hot caves, yet the underground places with low temperatures are also rare and abnormal. They may be divided into two groups: 1, Those where the temperatures are lower than the normal, without becoming low enough for ice to form; and 2, Those where the temperature sinks so low, that ice forms. It is difficult to make definite divisions among the various forms of natural refrigerators, but it is correct, probably, to classify them under five heads, in accordance with the different kinds of formations of the hollows in the rocks: 1. Gullies, gorges, and troughs where ice and snow remain. 2. Soil or rocks overlaying ice sheets. 3. Taluses and boulder heaps retaining ice. 4. Wells, mines and tunnels in which ice sometimes forms. 5. Caves with abnormally low temperatures, and often containing ice. 1. _Gorges and Troughs._--Gullies, gorges and basins which retain snow and ice are fairly numerous in mountain districts. They are generally ravines, or rock fissures, or hollows, in positions below the snow line where snow and ice are sufficiently protected, from sun and wind, to remain long after snow in the surrounding open country, at the same altitude, has melted away. Some of these gorges are small, some big. As a rule, they are deep and narrow. In north-eastern Siberia, a form of permanent surface ice is found, which the Tungusses speak of as _tarinnen_, which means "ice troughs" or "ice valleys."[15] These _tarinnen_ are broad valleys, with either a horizontal floor or one sloping gently in the form of a trough, over which the ice is spread in the form of a sheet. The Tungusses assert that the ice in some of these troughs never wholly melts away, although it lessens in quantity from the beginning of May till the end of August, after which it once more increases. [15] _Bulletin de la classe physico-mathématique de l'Académie Impériale des Sciences de St. Pétersbourg._ 1853. Vol. XI, pages 305-316. _Subterranean Ice Sheets._--In several places in different parts of the world there are underground ice sheets which extend over large spaces; they are common under the tundras of Alaska; and there are fine examples on Kotzebue Sound,[16] on the Kowak River,[17] and along the Yukon River.[18] The "Ice Spring" in Oregon seems to be a formation of the same kind. Several examples of these subterranean ice sheets are reported also from different parts of the Russian Empire. [16] See Part III: page 167. [17] See Part III: page 167. [18] See Part III: page 166.