This desiccation, however, is a mere trifle when contrasted with the process by which the mighty salt deposits in Germany were formed. It took place, we believe, in a shallow bay extending southward from the Arctic Sea. As the salts, first gypsum, then common salt, and lastly more soluble potassium and magnesium compounds, crystallized by degrees new water masses entered from the sea. At the same time, the bottom of the gulf slowly receded, giving room for fresh evaporating floods. The salt layers deposited in this manner sometimes reach a depth of more than 1000 m. (about one half a mile). We can thus gain a conception of the immense quantities of water evaporated and the enormous time required therefor. The deposits would long ago have been carried away from their original place were it not for the fact that they finally were covered with a layer of slime nearly impervious to water. The most soluble salts, such as chloride of magnesium, have nevertheless been extracted to a great extent. The extremes of aridity or humidity have of course not occurred during the brief time known to history. Of a special interest is the question in what direction the climate at present is tending. In this connection, Huntington has aroused great attention by propounding the theory that the earth is now in a period of rapid desiccation. Judging by the testimony of geology, it seems beyond doubt that an age of humidity prevailed simultaneously with the ice-period in northern Europe, over several parts of the globe, in fact everywhere as far as we know except in Australia. This is clearly borne out by the higher levels of the lakes and their consequent greater extension in earlier days. So far as Tibet and Central Asia are concerned we have already mentioned this fact. But in America and Africa the humid period was even more patent. The Great Salt Lake has covered an area many times greater than at present as testify the picturesque terraces in its surroundings (compare Fig. 10). According to the researches by Passage, this period was also strongly pronounced in Africa. A large fresh-water body occupied the Congo basin, the Tsad lake had a far greater expansion than now, and mighty rivers traversed Sahara. [Illustration: Fig. 10. Extension of the great Lake Bonneville in Utah, of which the Great Salt Lake is a remnant.]