_Temperatures._--The phenomena of glacières are so closely connected with temperatures that it seems necessary at this point to mention some general facts in connection with subterranean temperatures, even if these still form a subject of some uncertainty, and one about which further observation is desirable. Subterranean temperatures may be grouped under three heads: 1, Ordinary or normal temperatures; 2, Temperatures above the normal or super-normal temperatures; 3, Temperatures below the normal or sub-normal temperatures. 1. In the great majority of caves, cellars and subterranean places of all descriptions, the temperature of the air is about the same, all the year round, as that of the ground. The frost of winter and the heat of summer penetrate the earth for some trivial distance, a few meters perhaps, and lower or raise the temperature of the ground temporarily. Below this there is a stratum where the temperature is found to vary but little the entire year and which, in a majority of cases, approximates the mean annual temperature of the district. Below this invariable stratum, the temperature generally rises slowly, not at exactly the same rate everywhere, but in a regular increase. This increase of temperature averages 1° C. for every 32 meters. As most caves and cellars are of small depth and as they take their temperatures from that of the ground, it follows that as a rule their temperatures are moderate and pleasant. And as the air of the majority of caves and subterranean hollows is about the same as the temperature of the surrounding rock, it is correct to call subterranean air temperatures closely approximating the ordinary temperature of the ground, ordinary or normal temperatures. As already stated, with an increase of depth, there is, in almost all cases, a regular increase of temperature. For this reason, mines, which are much the deepest hollows reached by man in the surface of the earth, are, as a rule, warmer in the lower levels: if deep, they are also hot. And this is so generally the case that warmer temperatures at the bottom of mines may be considered as normal. 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.