[21] See Part III: [page 189].
[22] See Part III: [page 165].
Taluses and Boulder Heaps.—Taluses and broken debris, and in general boulder heaps of all sorts, have interstices and openings between the boulders, and in these it occasionally happens that ice is found. This is most common among the taluses at the base of cliffs, but in some cases ice is found among broken rocks on the sides of gently sloping hills, or even on the plateaus of their summits. Sometimes the ice and snow on the bottom of rock gorges all melts away, while further down, in the hollows of the boulders forming the floor, ice still remains. The rocks of which these ice bearing taluses are formed are generally gneiss, granite, limestone, sandstone, basalt or porphyry. Among such boulder taluses the phenomenon designated as Windröhren or Ventarolen, that is, windholes, is frequently found. Sometimes the air among such boulder formations is quiet, but as a general thing draughts pour out at the lower openings during the hot months, and blow into them during the cold ones.
Freezing Wells, Mines and Tunnels.—Subterranean ice is also found in certain places in connection with man’s handiwork. In a few wells in the United States, the temperature in winter becomes abnormally low, and for four or five months these wells freeze up and become useless. A case of a freezing well was recently observed near la Ferté Milon in Central France.[23] Ice is reported also as forming in various mines in Europe, Asia and America; in fact, it is not an uncommon occurrence. Occasionally, also, ice forms in tunnels.
[23] See Part I.: pages [74], [79], [89]. Part III.: [page 206].
Cold Caves.—Caves with abnormally low temperatures may be divided into two classes. First, caves where the temperatures are lower than the normal, without becoming low enough for ice to form; and second, caves where the temperatures sink so low, that ice forms.
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.