2. In a few hollows close to the surface, there are temperatures much above the normal temperature of the ground. Such places are rare and abnormal.[13] The heat is generally due to the presence of hot springs or to some volcanic action in the immediate neighborhood. In the case of one cave close to the surface, the heat is due to some limekilns which are situated immediately overhead.[14] Where these warm hollows are genuine caves it seems proper to call them “hot caves.”
[13] Kraus. Höhlenkunde, page 86.
[14] Grotte du Jaur. Les Abimes, page 160.
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.