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Show 178 rupts the evenness of the flat. A specimen of this clayey soil gave, on analysis, the following result: Clay with fine silt 64.70 Calcium carbonate 3.36 Calcium sulphate 0.43 Sodium chloride 10.52 Sodium sulphate 3.90 Magnesium sulphate Trace. Calcium phosphate Trace. Potassa, lithia Traces. Chemically- bound water 15.68 96.59 This soil is unfit for agriculture, containing too much clay and salts. If, on the one hand, desiccation of former lakes proves that the amount of evaporation exceeded that of the aqueous precipitates, there exist, on the other hand, facts tending to prove that the dryness of the climate is still on the increase, namely, the disappearance of forests within the last three centuries and the drying up of springs within the last fifty years.* These phenomena recall our observations in New Mexico and Arizona, whore indications of increasing dryness are numerous; to mention only the forests of dead cedar-trees standing mummy- like, the occurreuce of shells of land- snails ( Planorbis) in localities where not a single snail is found at the present day, the deserted ant- hills, the dry arroyos, and the ruined towns of now barren tracts. t In close connection with the decrease of aqueous precipitates in New Mexico, Arizona, and Eastern California, appears to stand the increase in Utah, the Great Salt Lake having risen 15 feet in the last twenty- five years, and will, if this accumulation continues, submerge the capital of the Latter Day Saints at no very remote period. The most satisfactory explanation of these phenomena appears to me can be given by the assumption of changes in the country- level. A gradually rising country will experience a decrease in the annual mean of temperature, consequently, also, in the amount of evaporation ; while, on the other hand, the aqueous precipitates will increase, the distance of the clouds becoming smaller. As the attraction grows with the square of proximity, a lifting of 100 feet of au extensive mountainous country will suffice to change the climate perceptibly. Two causes, therefore, co- operate to increase number and volume of springs and to swell creeks and lakes. My hypothesis is this, that New Mexico, Arizona, and Eastern California are undergoing a gradual subsidence, while Utah is, like the coast of California, slowly upheaved. A characteristic feature of the Mohave Desert is the black coating of rocks and of the bowlders and gravel! that cover the barren plains, adding perceptibly to the dismal impression of the scene, the blackening appearing like a mourning garb for departed flora. Miuers call these rocks " suuburnt," and as curious as this expression sounds to the ear of the naturalist, there is a grain of truth in it, the black coloration being of the deepest shade where the surface of the bowlder is exposed to the direct sunlight, while at the under side it is much less developed and sometimes replaced by a reddish color. Analogous phenomena were observed by Littel in the Libyan Desert, by others at Syene, at the Congo River, and by Humboldt on the Orinoco. Berzelius, who examined the coating from the latter locality, upon the request of Humboldt, declared it due to mixture of oxide of iron and manganese. A chemist can hardly be long in doubt in regard to it, as bydrochlorio acid dissolves the coating with liberation of chlorine, and manganese easily can be shown in the solution thus obtained. It appeared to me a matter of some interest to ascertain the quantities necessary for the production of the black coating, and for this purpose a blackened small bowlder, weighing 80 grams, was treated with hydrochloric acid until the natural color of the rock ( granite) developed. The acid poured off" gave: Sesquioxide of iron 0.078 gram. Binoxide of manganese 0.038 gram. Oxide of nickel traces. * The miners of El Dorado Cafion, in the Black Canon range, stated to me the ceasing of a large spring in the vicinity within the past fifteen years. tSee annual report U. S. Geographical Surveys West of the Oue Hundredth Meridian. 1875. t On the eastern slopes of the Payute range, some 20 miles east of Cottonwood Island, a bed of coarse conglomerate, 10 feet in thickness, was observed, of which each pebble was provided with the black coating. |