The dangers of these solitudes are increased by the great mobility of the soil. When a strong wind blows, huge sand-columns, rising like water-spouts to a height of eighty or a hundred feet, advance whirling through the desert, and suddenly encompass the traveller, who can only save himself by a rapid flight. Such is the instability of the soil, that in a few hours a plain will be covered with hillocks or _Medanos_, and recover after a few days its former level. The most experienced muleteers are thus constantly deceived in their knowledge of the road, and are the first to give way to despair, while seeking to extricate themselves from a labyrinth of newly-formed medanos. These constant transformations and shiftings in the desert, which Tschudi graphically calls ‘a life in death,’ take place more particularly in the hot season, when the least pressure of the atmosphere suffices to disturb the dried-up sands, whose weight increases during the winter by the absorption of moisture. The single grains then unite to larger masses, and more easily withstand the pressure of the wind. The summer, or dry season, begins in November. The rays of the vertical sun strike upon the light-coloured sands, and are reflected with suffocating power. No plant except the cactuses and tillandsias, which manage to thrive where nothing else exists, takes root in the glowing soil; no animal finds food on the lifeless plain; no bird, no insect, hovers or buzzes in the stifling atmosphere. Only in the highest regions the condor, the monarch of the air, is seen sailing along in lonely majesty. In May, which in these southern latitudes corresponds to our October, the scene changes. A thin, misty veil extends over the sea and the coast, and, increasing in density during the following months, only begins to diminish in October. At the beginning and the end of this damp season the mist generally ascends between nine and ten in the morning, and falls again at about three in the afternoon; but in August and September, when it is most dense, it rests for weeks immovably over the earth, never dissolving in rain, but merely descending in a fine, penetrating drizzle, called ‘garua’ by the inhabitants. In many parts rain has not been known to fall for centuries, except only after very severe earthquakes, and even then the phenomenon is not of constant occurrence. The mist seldom ascends to a vertical height of more than 1,200 feet, when it is replaced by violent showers of rain; and, remarkably enough, the limits between both can be determined with almost mathematical precision, as there are plantations, one half of whose surface is invariably moistened by garuas and the other by rain.