Caves where the temperatures sink below the normal, but in which ice does not form either in winter or in summer, are found in several places in different parts of the world. They are termed in French _cavernes froides_ and in German _kalte höhlen_. There are but few data from reliable observers about such cold caves. Some descriptions are given without thermometric measurements, and the statements that the caves are cold, mean nothing beyond the fact that they feel colder than the outside air. It is, however, conclusively proved that cold caves exist, and that while they are not freezing caverns, yet that they have a temperature lower than the mean annual temperature of their district. In fact, the assumption, which had passed into an axiom, that caves always have the same temperatures as the mean annual temperature of the district, must certainly be given up. Cold caves are generally in one of two shapes: 1, in the shape of a sand glass,--two cones above each other meeting at the narrowest point--where the upper cone lets the heavy cold air descend easily, while the lower bell shaped cone prevents its escape; and 2, where two sink holes open into one pit, which is in the shape of a bell.[24] [24] _Les Abimes_, page 563. _Glacière Caves or Freezing Caves._--Caves where the temperatures sink so low that ice is able to form, are found in many different rock formations and in various positions, shapes and sizes. The rock formation of freezing caverns is generally limestone, but sometimes it is marble, lava, basalt, gneiss or granite. In all cases, however, the rock is either porous or else it is broken and fissured, as otherwise the water supply necessary to the formation of ice could not find its way in. Glacière caverns may, for the sake of convenience, be classified into several classes, according to their position or to their form. The lines of transition between them, however, are so indefinite in nature, that it is often difficult to specify a cavern as belonging to any special type. The most important factor in classifying glacière caves is their position. Under this head there are two main divisions: first, pit caves; second, cliff caves.