Something similar takes place on the Martian mainland in its flat river basins, which correspond to the salt lakes in our deserts. On account of the bitter cold and the consumption of the water in the process of disintegration (the carbon dioxide has been largely used up in the same manner), precipitation has almost ceased on Mars and most of the water in circulation emerges from the interior of the planet along the fissures. As it contains hydrochloric acid and carbon dioxide it extracts from the soil salts, such as the chlorides of sodium (common salt), of calcium and of magnesium, all present in common sea-water to which it was brought by the rivers. The compounds of calcium and magnesium are not precipitated as carbonated salts through the medium of crustacea as is the case on earth. The strong solar radiation during the summer partly evaporates the water into the thin air, leaving the salts behind. On account of the low temperature, this vaporization on Mars is probably slower than on the Earth. Along the cracks in the crust, a kind of dry salt-lakes are formed similar to the generally shallow and occasionally dry lakes common in the deserts of Central Asia as described by Hedin. We know that Mars possesses a pronounced desert climate. There finally remains in the lowest sections of the water courses a concentrated salt solution, which parts with its water more and more reluctantly, so that the salts which most strongly hold the water crystallize at the deepest points. If the winter’s cold is sufficiently severe (below -55° C. or -67° F.) ice is extracted even from the most concentrated solutions, which mainly contain chloride of calcium. In spite of such extreme temperatures, evaporation into the rare atmosphere is not negligible and the ice crystals partly vanish, to reappear in the coldest regions of the planet, that is, around the pole which at the time is turned away from the sun. On the ocean, now frozen solid throughout, a polar-cap of snow and hoar-frost is formed which finally reaches as far as the 38th parallel on the southern hemisphere (see Figs. 18 and 19), where winter occurs when Mars is most removed from the sun, and to the 58th parallel (see Fig. 19) on the northern hemisphere where winter reigns while Mars is nearest to the Sun and consequently not quite so cold. Similar conditions obtain on the Earth although not to such a marked degree. [Illustration: Fig. 18. Photograph of Mars, enlarged and retouched; taken by Lampland ] [Illustration: Fig. 19. Appearance of Mars April 8, 1907, as observed by Quénisset. Below one pole a dark line is plainly visible. ] [Illustration: Fig. 20. The south pole spot on Mars observed by Jarry-Desloges July 10, 1909. ] [Illustration: Fig. 21. The appearance of Mars during an observation by Antoniadi in 1909. The whole disk is somewhat hazy; below, the details are hidden by clouds of sand. ]