I have already said that I believe that the cold of winter is the cause of the ice in caves. To make this clearer, I may say that I look on glaciÈres as the last outcrop, the outside edge, so to speak, of the area of low temperatures, which has its culminating point in the Northern Hemisphere in the Arctic Ocean, Greenland and Siberia, and in the Southern Hemisphere in the Antarctic; and which is manifested to us in the snows of mountain peaks, and immediately round us in frozen ponds and rivers and snowy blizzards; and which, as it disappears each summer, leaves its last traces in our latitudes in sequestered gorges and convenient caverns. In every case, it seems to me, glaciÈres are simply refrigerators, which preserve the ice and snow accumulated in them during the winter. They all follow the same general laws as to the origin of their contents, modified only in slight degree according to the varying natural local conditions, such as the water supply, or the protection from sun and wind, or the thickness of the overhead rock, or the altitude or latitude. I cannot see that there is anything remarkable about the fact that the cold of winter is able to penetrate and make itself felt sometimes for a slight depth in the earth’s crust; a depth, so far as yet known, never exceeding one hundred and fifty meters. It seems to me that glaciÈres only emphasize a law of nature, which has doubtless been formulated many times in connection with springs and phreatic waters, and that is, that where we find cold waters underground, we may be sure that they have penetrated from the outside.

If we look first at the mode of formation of overground perennial ice, that is, of the ice of glaciers and of rock gorges; and then at the evidences of the mode of formation of underground perennial ice, in boulder heaps, wells and caves; we will soon see that the transitions between them are gentle in character and that there is nothing unnatural about the formation of the ice in glaciÈres.

Glaciers.—Everyone now knows the main characteristics of glaciers. They are formed in parts of the earth where the land or the mountains reach to the region of perpetual snow. The snows fall from the sky, and accumulate into a snow cap, which by its own weight and by melting and regelation, gradually changes to ice. This, by the laws of gravitation, descends to lower levels, and in mountain valleys extends sometimes far below the snow line into the region of cultivated fields. These valley prolongations of the perpetual snow caps are the glaciers. The important point to notice here, is that the formation of glaciers is originally entirely due to the precipitation of moisture by cold in the upper portions; while the destruction of glaciers is due to the action of heat melting the ice in the lower portions, where they disappear in the shape of streams of running water. It is, therefore, not surprising that the greatest glaciers are found in the Arctic and Antarctic regions and in the highest mountain ranges; and that in the tropics glaciers are either wanting or exceedingly small.

Gorges and Troughs.—Gorges and gullies, where ice remains over, are a transitional form between glaciers and glaciÈres. In many mountain ravines or canyons, the enduring snow consists principally of the avalanches which have fallen from the heights above during the winter and solidified in the bottom of the ravines. Freezing gorges proper, however, are not dependent on avalanches for their supply, but they receive the accretions to their ice directly from the winter snows. These fall into the gorge itself and by melting and regelation gradually solidify into a mass of ice which, when well sheltered against sun and wind, remains over sometimes till the following winter. By their mode of formation, therefore, it is evident that the ice in these gorges has some of the characteristics of glaciers; that it is due to the same prime causes as the ice of glaciers or the ice on ponds and rivers, namely the cold of winter; and in fact, it is not far wrong to consider these gorges as miniature glaciers.

Freezing gorges, however, show, also, certain degrees of kinship to freezing caverns and taluses, principally in the protection afforded to the ice against external destructive influences. The ice is almost always found in positions where it receives little, if any, of the direct rays of the sun and, also, where it is scarcely, if at all, exposed to any winds. The sides of the fissures and surrounding trees generally afford the necessary protection. Some of the forms which the ice assumes in gorges, such as long pendent icicles, are also more characteristic of underground than of overground ice.

The freezing troughs or basins found in Siberia are evidently closely related to gorges, and the fact that the ice is found in less sheltered places may be explained by the high northerly latitudes of these troughs, in general between fifty-seven and sixty degrees.

The Winter’s Cold Theory.—The places where ice is found underground differ in one important respect from gullies and troughs, and that is, in the fact that above the ice there is rock or soil, which, in true caves, takes the form of a roof. This causes some important distinctions between overground and underground perennial ice. It means that the ice is formed directly in the caves, and that it is genuine subterranean ice, and not, except perhaps near the entrance, solidified snow. The roof, while not admitting the winter snows, is, however, a protection against warm summer rains, and, of course, entirely cuts off radiation from the sky. If, therefore, it keeps out some cold, it also acts as a protector against heat.

That the cold of winter is the source of the cold which produces the ice which forms underground, and that it is through its influence, with the assistance of certain secondary causes, that some caves are converted into what are practically natural ice houses, seems to me the true explanation of the phenomenon of subterranean ice, not only since it is the simple and obvious explanation, but also because all the facts, so far as I have myself observed, are in accord with this theory.[56]

To form subterranean ice, just as to form any other ice, two things are necessary: the first is cold, the second is water. Cold is supplied by the cold air of winter, and water must in some manner find its way into the cave while the cold air is there.

The process is as follows: The cold air of winter sinks into and permeates the cave, and in course of time freezes up all the water which, in the shape of melting snow or cold winter rain or spring water, finds its way in; and once ice is formed it remains long after ice in the surrounding open country has melted away, because heat penetrates with difficulty into the cave. The only effect of the heat of summer is to melt the ice.

The proofs, to my mind, of the truth of this view are: 1—GlaciÈres are always found in parts of the world where, during part of the year at least, the temperatures of the surrounding country fall below freezing point. 2—All observations by reliable observers show that the temperatures of glaciÈre caves vary, but in a much narrower thermometric scale, with those of the outside air: that the temperatures are lowest, and as a rule below freezing point, during the winter months; and that the temperatures are highest, and as a rule above freezing point, during the summer months. 3—Ice is never found far from the mouth of caves, but always near enough for the cold air to get in. 4—Evaporation, according to my observations, is, as in all other forms of ice in nature, connected mainly with the melting, not the freezing of the ice.

Geographical Distribution and Altitudes.—GlaciÈre caves proper are found in various parts of Europe, Asia, and America, mostly in the smaller mountain ranges or in the outliers of the snowy mountain chains; generally in limestone and occasionally in basaltic formations. There are a good many in the Jura; a few in the Swiss and the Italian Alps; a number in the Eastern Alps of Tyrol and Carinthia. There are some in Hungary, several in Russia, one in Iceland, one on the Peak of Teneriffe, a number in Siberia, one in Kondooz in Central Asia, one in the HimÁlaya, one in Japan, and one in Korea. I have heard so far of over fifty glaciÈres in North America, several of which are in Pennsylvania. From all over the world there are some three hundred places reported where subterranean ice is said to occur. This includes gorges, boulder heaps and freezing mines and wells, all of which exist in much the same localities as glaciÈre caves.

All the glaciÈres which I know of, are situated in a latitude or at an altitude where ice and snow forms for part of the year in the surrounding open country. None are reported from India or Africa, or in fact from any low-lying places in tropical latitudes. Most of them are found in middle latitudes, and only where during part of the year, at least, there is a cold season, that is, where for some time the thermometer stands below freezing point.

GlaciÈres are, in general, at fairly high altitudes. The Schafloch is at 1780 meters; Skerisora in Transylvania at 1127 meters; DÓbsina at 1100 meters; the GlaciÈre de Saint-Georges at 1208 meters. It is true that there is one freezing cavern in the sub-tropical latitude of Teneriffe, La Cueva de la Nieve; but it is at an altitude of 3300 meters, and where snow falls every year in the open on the Peak. Unless some freezing cave is hereafter discovered in a region where there is no ice in the open in winter, I do not see how the imperative necessity of the cold air of winter for forming the supply of ice can be controverted.

Thermometric Observations.—That the cold air of winter is the important factor in the production of cold is proved, also, by the thermometric observations recorded in various caves by different observers. They all tell the same tale: that the temperatures vary with those of the outside air, that they are lowest in winter and highest in summer. I quote in the “List of GlaciÈres”[57] a few of those published; but there are many more, and they all show the same general characteristics.

A comparison of all the figures recorded proves that, as a rule—inside of glaciÈre caves—from about the first of November to the first of July, there are winter temperatures, that is temperatures below freezing point; and from about the first of July to the first of November, there are summer temperatures, that is temperatures above freezing point.

The observations prove also that the inner temperatures vary less than the outer, that is that they range within narrower limits. They also show that the inner air is but slowly affected by the outer air when the latter is above freezing point, the inner temperature rising then only gradually. Per contra, when the outside temperature drops quickly much below freezing point, the inside temperature generally drops correspondingly at once, proving that the cold air has sunk by its weight into the cave. The observations also prove that the old idea that the temperature of caves is the same throughout, can no longer be considered correct. The observations also appear to show, that the temperature of a cave does not necessarily represent the mean annual temperature of a surrounding district. Observation is still entirely lacking on the mean annual temperature of glaciÈres, so that one cannot speak definitely about the matter; but it seems likely that the mean annual temperature of a glaciÈre cave is lower than the isotherm of its locality; and it seems more than probable that on the same isotherm different glaciÈre caves may have different mean annual temperatures, varying with the elements of size, quantity of ice, position of body of cave and of entrance, water supply and other factors.

Ice near the Entrance of Caves and the Surface of the Soil.—An important proof that it is the cold air of winter which forms the ice is the fact that the latter is always found near the entrance of caves or near the surface of the soil. It never extends far within. To the best of my knowledge, ice has never been found two hundred meters from the entrance nor at any depth beyond one hundred and fifty meters. In all caves of great extent, the temperature far in is about the same as that of the surrounding rock, and in all deep borings the temperature increases with the depth and at great depths the temperature becomes high. This nearness of subterranean ice to the outside air is one of the best proofs, that, paradoxical as the whole phenomenon appears at first, yet in reality it is an extremely simple matter.

The position of the entrance of a cave in relation to the body of the cave is an important factor in permitting the cold air to permeate and remain in the cave. In all the caves or gullies I have examined myself, the main mass of ice is well below the level of the entrance, and even if the latter is sheltered against the wind, it is not sheltered against the cold air of winter. This is heavy, and by its own weight sinks well down to the bottom, freezing up in course of time all the moisture that may drip from the roof, or that may come into the cave in the shape of melted snow or cold winter rain. The summer air, which is warm and, therefore, light, can only enter the cave with great difficulty; and, as a rule, before it dislodges the winter air and destroys the ice, another winter’s freeze reverses once more the conditions. These principles seem to hold of every known glaciÈre. It is true, that at the Frauenmauer, the floor of the cavern rises somewhat from the entrance; but the highest point of the floor is still below the level of the top of the entrance, so that the cold air can flow over the highest point without difficulty. The same appears to be the case at the PosselthÖhle; while at Amarnath in Kashmere, where the floor is said to rise to the back wall, the entrance is about as large as the area of the floor, so that the ice must also be below the level of the top of the entrance.

The position or situation of the entrance is important. In almost all cases it has a northerly exposure, and is sheltered against entering winds. If these two conditions do not exist the ice supply surely suffers. Sometimes the entrance is more or less tortuous. In some cases it is protected by a fringe of trees. Still, there is no absolute rule about entrances. The FriedrichsteinerhÖhle faces about due south, and at midday in summer, the sun shines all the way down to the ice floor, causing mists to form. In the KolowratshÖhle, the entrance is badly sheltered against the wind and this undoubtedly affects the supply in summer and causes more rapid melting there than in some other cases.[58]

Freezing boulder taluses invariably have the ice near the surface, and probably it is never a dozen meters distant from the open air. These taluses are one of the strongest links in the chain of evidence proving the winter’s cold theory. The snow and ice on the surface of the taluses and on the surface of the boulders in gullies melts away, while it still lingers underneath the boulders. It seems self-evident that the melting snow water has run to the lowest level and there congealed, and then remained because it was better sheltered than the ice outside.

The subsoil ice of the tundras of Siberia and Alaska is almost identical with the ice of boulder formations, except that it extends under larger areas. It is the product of a climate where there is a long, rigorous winter and it is not surprising that the ice is found at greater depths than in more southerly latitudes.[59] The depth to which the ice extends is, of course, determined by the depth to which the winter’s cold can penetrate the soil. There is no doubt that the causes of this ice are local, that is, that it is due to the long prevailing low temperatures.

The freezing wells of which the most conspicuous examples are at Brandon, Owego, Decorah, and in Montana, seem also due to local causes and the ice is never far from the surface, that is, not over twenty meters; and apparently also it forms above the water horizons which supply the wells.

The ice sheet on Mount Etna[60] does not seem to be at any great depth. It apparently had a different origin from most subterranean ice masses, in that the snow probably fell first and was then covered by a flow of lava. It is, therefore, almost sui generis in its mode of formation, unless there are similar sheets on other volcanoes, which is probable in a country like Iceland, and which is said to be the case in Tierra del Fuego,[61] But the original cause of the ice sheet on Etna was the same as all other subterranean ice masses, namely the cold of winter.

Evaporation and Movements of Air.—The formation of subterranean ice is sometimes assigned partly to evaporation or to expansion of the air. The theory is an old one, and both scientific and non-scientific men have advanced it.[62]

I have already said that I believe it is the movements of the air which cause a cave to be a glaciÈre cave or a normal cave. When they act in such a way as to permit the cold air of winter to permeate a cave, we find low temperatures and ice; when they do not, we find the temperature about the same as that of the ground and no ice.

That the ice is not formed in apparently static caves, by movements of air producing evaporation, seems to me proved by what I have noticed in regard to the atmosphere. The dryness or moistness of the air within a glaciÈre cave is coincident with the state of freezing or thawing of the cave. When I have visited a large cave in June, everything was frozen tight, there were no drips nor mushy ice, the air was relatively dry and the sensation of cold not unpleasant. When I have visited a cave in August, the ice was soft and mushy, water was dripping from the roof, the atmosphere was moist, and the cold penetrating. It seems to me that the facts go to show that it is not evaporation which forms the ice, but the melting of the ice which fills the cave with moisture. If there are any draughts or movements of the atmosphere when above freezing point, then their tendency is to vaporize the ice.

The process of the formation of ice in relation to the atmosphere is as follows: the cold air permeates the cave and freezes up all the drip: the atmosphere becomes dry: gradually warmer air gets in and the ice begins to melt: then the atmosphere gets charged slowly with the vapor of the melting ice. This process is the exact opposite of the formation of ice by evaporation; it is the atmosphere which is made humid by the vaporizing of the ice, and by the drip. When the air is thoroughly saturated with the vapor, being scarcely renewed from outside and but a few degrees above freezing point, it undoubtedly retards evaporation, acts like a blanket and lessens the rate of melting of the ice.[63]

Everything I have personally observed in freezing windholes shows that in them also the cold of winter and not evaporation is the cause of the ice. They answer to the same tests as other glaciÈres, of geographical distribution and altitude, nearness of ice to the outside, thermometric observations, and dampness of the air when the ice melts. Equally with other glaciÈres, the movements of air in windholes do not depend on the presence of ice, but the ice does depend on the movements of air and a water supply at the proper time. A proof that it is the cold of winter which makes the ice in windholes, is that the ice is always found at the lower extremity, for the reason that it is at that end that the cold air enters and to that end that the water gravitates. The reason that ice is more rarely found in windholes than in apparently static caves, is due to the movements of air. Unlike the caves where the heavy cold air preserves the ice by remaining pent up, as soon as the outside temperature rises the heavy cold air in windholes tumbles out at the lower opening and is replaced gradually by air at a higher temperature. This also flows out and when it is above freezing point, it naturally melts the ice and becomes humid: in fact, it vaporizes the ice as it passes, and dissipates the moisture into the outer air.

It is, however, certain, that in caves with a temperature some degrees above freezing point, when there is either running water or strong drips, evaporation may be, and sometimes undoubtedly is, a factor in lowering the temperature somewhat.[64] As in some windholes there is occasionally moisture on the rock surfaces where the air current passes, the evaporation from these surfaces doubtless lowers the temperature of the draughts, and it may be, also those of the rock surfaces, a little.

Further observations, however, will be necessary in regard to evaporation underground, as the data are still insufficient to make absolutely positive statements.[65] I fail to see any evidence to show that evaporation ever lowers the temperature of draughts underground below freezing point, only that it may help to lower them to something less than they would otherwise be. Taking all the facts which I have myself observed, and all I have read of in the reports of others, my own conclusion is that we have no proof that evaporation underground is ever strong enough to produce ice.

Time of Formation of Ice.—Everything I have seen points to the fact that ice begins to form in a cave as soon as the temperature of the cave has sunk below freezing point, whenever, from any cause, water gets into the cave. The cold may begin to penetrate caves as soon as outside frosts have occurred, that is in the fall months, about November; and as soon as the temperature inside sinks below freezing point, ice will begin to form, provided also that water gets into the cave, from rains or springs or any other source.

In the mid-winter months, although there is then plenty of cold, the water supply is generally lacking, as the outside moisture is mostly frozen up and the result is that the winter months are not those when the ice is mainly formed. Some is undoubtedly formed in certain caves whenever during the course of the winter a surface thaw outside furnishes water to the cave,[66] but in other cases this is not so and the ice does not appear before the spring. In all cases it is in the spring, before the cave has parted with its store of cold, and when both the air and the rock walls are chilled below freezing point[67] that the ice forms fastest. Then plenty of water is furnished by the melting of the snows and the unlocking of the brooks, and also by early spring rains. All this surface water runs through the fissures into the still freezing cave and there becomes ice. Not only the air, but also the rock walls are chilled below freezing point, and as the rocks part slowly with the cold stored in them, this cold helps to freeze the water pouring in.

The natural law in relation to time seems to be this: Ice may be formed in caves as soon as the outside temperature sinks below freezing point. In some caves it forms intermittently all through the cold months because there is a water supply. In other caves it only forms in the spring, because there is no water supply in the winter months. In all cases, however, the end of winter is the time when most of the ice is formed.

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