AVALANCHES. BY B. E. FERNOW, WASHINGTON, D. C. (Colorado Meeting, June, 1889.) MINING interests in the Western mountains are very seriously affected by the danger to property and life from destructive snowslides and avalanches. This is a danger which the miner has largely brought upon himself by an improvident stripping of the timber from the mountain slopes, and, therefore, he might be expected to take an interest in the reforestation of the areas thus devastated. A canvass was lately undertaken by the writer to ascertain approximately the extent of the damage suffered from this source, but the difficulty of obtaining reliable data has proved too great to permit, as yet, even an approximation. So much, however, is certain that the loss of life is considerable, and that much property is destroyed almost every winter. True, neither the number of avalanches, nor their destructiveness, nor the amount of loss compares with what Switzerland experiences. This is due not only to the greater precipitousness of the Alps and to different climatic conditions, but also to the fact that in Switzerland a larger number of settlements is exposed to these dangers. A very complete and interesting official report on the very numerous and destructive avalanches occurring in Switzerland during the season of 1887-1888 has just been published as a contribution to the inquiry, whether and how these dangers can be averted. Peculiar weather-conditions, in addition to heavy falls of snow, were the reasons for the frequency of the phenomenon during that winter. This report records 1094 avalanches, by which 850 houses were destroyed or severely damaged; 84 persons and 752 cattle were buried; traffic on railroads and on highways was greatly interrupted, and the damage to personal property alone was valued at over $100,000. One of these snow-masses covered about half an acre, with a cone 250 feet in height. During the same year in Tyrol the number of avalanches was 2647. Of these 1355 run over permanent beds; that is, they recur annually, so that they are named, like rivers, as it were; and 765 are expected to recur periodically. The loss that winter was estimated at over $300,000. Some localities are more liable to snow-slides than others; geological, topographical and climatic conditions, and the greater or less extent of forest-destruction, produce this difference in the frequency of their occurrence. In Switzerland necessity has long ago stimulated the study of the nature of avalanches and the adoption of protective measures; but only lately has a systematic plan for such measures been adopted. All the protective works that have been built during the last few years under an energetic direction of the forestadministration did their duty well. In the valley of the Leventina torrent, where numerous avalanches fell yearly, even the largest, which was of annual occurrence, was kept out in spite of the large masses of snow; and the success achieved in preventing the formation of these dangers in every case has established beyond doubt the importance of reforestation in connection with other measures of protection. It is probable that measures are frequently adopted in our mountain districts to protect property against this danger. Only one case, however that of the Madonna Mine, at Monarch, Colorado; Superintendent, H. D. Mitchell—has come to the writer's notice, where systematic protection is attempted by a system of stakes and old stumps, connected by old wire-cables. The snow-sheds of railroads crossing mountain passes are built, to some extent, as protection against avalanches. Notably is this the case with the Canadian Pacific railroad, the experiences of which with avalanches are interestingly described in a paper read before the American Society of Civil Engineers, by Mr. Thomas C. Keefer, who says the cost of protection against snow-which, it is true, includes measures to keep the track free from snow in general-has reached within two years the handsome sum of $2,900,000. Besides specially constructed" avalanche-sheds," other methods of protection, to be described further on, are used; but they are all directed against the damaging effects, not against the formation, of avalanches. With increased permanency of the establishments and settlements to be guarded, greater permanency in the protective works will also be needed, and a closer study of the requirements of such problems will be called for. The object of this paper is to call attention to one of the prevent able causes of avalanches, namely, indiscriminate forest-destruction, and to state the principles which underlie the formation and action of snow-slides, and which must be kept in view in the construction of protective works. CONDITIONS OF AVALANCHES. The formation of avalanches or snow-slides depends (1) on the nature of the snow; (2) on conditions of the weather; (3) on geological, topographical and surface-conditions of the ground. Snow falling at low temperatures is dry and composed of small loose flakes, without coherence, because the dry crystals do not attach themselves to each other. Such snow does not hang on or ball easily; it lies loose, is easily moved by the wind, like sand, and causes snow-drifts; while the wet snow, falling at higher temperatures (snow may fall at 40° Fahr.), which has begun to thaw while falling and contains much enclosed air, packs tightly and shrinks quickly. On the plain the snow moves only by settling and by drifting. On an incline there is a movement, though it may be but slight, within the mass-a settling which at the same time takes a downward direction, following the law of gravity and depending in its amount on the angle of inclination, configuration, and especially cover of the ground; and also on the nature and amount of the snow. If the snow is of small amount and frozen to the soil, of course it cannot move. Otherwise, even if the conditions are not favorable to motion, and the movement may be hardly noticeable, yet it exists within the mass and becomes apparent by the formation of horizontal rifts on the surface of the snow. This movement, with the pressure due to it, is capable of displacing buildings without much injuring them. The more favorable the conditions for motion, the greater becomes this movement, until at last a slide occurs, moving, in larger or smaller masses, more or less rapidly over the incline. If a large amount of dry snow falls on a steep deforested slope it behaves like a sand-heap, the particles getting into motion for lack of stability and cohesion in the mass; and, moving forward in the direction of the slope, it sets in motion other masses of snow, and presently the slide "pulls out." The heavier particles gradually accumulate near the ground, while the finer particles form a cloud of snow, bursting into the air like dust and settling down only gradually. This falling cloud, which obscures the mountain view for some time, compresses the air to such an extent that it precedes the avalanche like a tornado, followed and pressed on by the latter in hasty flight and exerting an enormous pressure, often over great distances. Sometimes it is felt two or three miles beyond the actual course of the avalanche. This so-called "flurry" whirls upward to the height of 100 feet or more above the descending snow, preceding the slide and giving to this class of slides their destructive character. The destruction which is due to the snow-masses of such a slide is insignificant in comparison to that of the compressed-air current. The snow is light, and by the fall dispersed and scattered, and may even pass through a thinly-stocked forest without doing much damage. But the "flurry" will snap off and uproot big trees and carry them with rocks, stones and ice over the mountain slope; and the whirlwind produced in front of it has been known to be of such terrific force as to pick up a man and break and dislocate every bone before he touched the ground again, a limp mass, without a bruise or break in skin or clothing. One such air-current preceding an avalanche is reported to have carried a full-grown larch tree bodily over the tower of a prison, lodging it 300 yards beyond and to have laid low timber 200 to 300 feet beyond the avalanche. The momentum which the loose mass of snow acquires, if not checked, is sufficient to carry it over and across a valley and up the opposite slope 200 to 300 feet. By the impact at the bottom, where the snow accumulates, there occurs, to be sure, a compacting of the mass; but this, on account of the dry and loose condition of the snow particles, is not very great. These slides, which seem to be the most common in the Rocky mountains, occur during heavy falls of snow in cold weather, mostly in the early part of the winter. They may be called dust- or powder-avalanches, or perhaps avalanches proper, in contradistinction to the snow-slides, described further on, and are due simply to the inability of the snow to keep stable in such masses.* In rare snow snow * The dictionaries and encyclopædias do not seem to know the word slide," which is the term used in the Rocky Mountain region. The word " slip" is used to denote “a large mass of snow which slips down the side of a mountain and sometimes buries houses" (Webster), while "avalanche " is defined as "a large mass of snow, earth and ice, sliding or rolling down a mountain" (Webster), or "falling down a precipice" (Ogilvie). (To avale-to fall, descend, be lowered: old French aval-towards the valley.) There exists, therefore, no definite, distinct idea that might belong to the one word or the other exclusively; and the words have been used as synonyms. instances they are occasioned by winds after the storm is over, if, perchance, the snow has been able to preserve its loose and dry condition and to keep in position long enough. If the snow falls when the temperature is not low, it is wet, heavy and compact, and hangs to the soil closely. If not very deep, it remains undisturbed, provided the soil be not wet or slippery and the ground not very steep. The larger the mass of snow and the warmer the temperature, the greater is the danger of a movement in the mass itself and a consequent slide. In such a slide the snow remains compact and does not disperse, except when it falls over precipices. It does not, therefore, exert much pressure upon the air. It flows more like a snow-stream, now sliding, now rolling over and balling together. In spite of its great weight, its velocity is, by reason of its friction against all obstacles on its road, much smaller than that of a dust-slide, and its effects reach over a smaller space. These may be called ground-slides or true slides; for the motion is really a sliding one only. Such slides of wet snow are apt to occur more often in late winter or spring, especially if the snow has fallen on top of a frozen crust of old snow, or when thawing weather occurs with or soon after the snow-fall. The packing of the snow is such that any persons buried in it are rarely rescued alive; it becomes as hard as ice and will yield only to the pick or powder. In fact, specially designed icechisels are used on the Canadian Pacific railroad in removing such snow masses. Lastly, a third kind of avalanche, with which we have nothing to do in this country, except, perhaps, in Alaska and in Canada, is the glacier-avalanche, which is formed when large masses of the glacier disconnect themselves from the main flow, and, falling over a precipice, break up into something like a dust-avalanche. Apart from the nature of the snow, the following conditions are of moment in the formation of avalanches : Compact rock-formations are less favorable to the formation of avalanches-especially ground-slides-than stratified rocks. The latter offer more opportunity on the side of the dip than on the side of the outcropping (head). Granite and granitic gneiss mountains are therefore less liable to avalanches than slates (including the calcareous clay-slates known in Switzerland and Italy as flysch), limestone, etc. Often, however, the angle of inclination on the dip is so much less than on the head as to reverse the conditions above |