Page:Popular Science Monthly Volume 48.djvu/677

Rh of Egypt been ascribed to the mud carried down by the flood waters of the Blue Nile from the uplands of Abyssinia during the season when torrential rains prevail there.

Without denying a certain efficiency of this cause, a closer examination easily proves it to be inadequate to account for the millennially undiminished fertility of the Nile Delta. The average annual mud deposit of the Nile floods amounts to less than the thickness of a common pasteboard. Were this the best of stable manure well worked in, it could not produce the effect claimed. But examination proves it to be simply a rich soil, such as thousands of farmers could haul and spread upon their lands if it could produce the effect ascribed to this Nile sediment. Besides, perpetual fertility belongs equally to the lands of the neighboring Fayoom, which, being irrigated only with the clear water of Lake Mœris, do not receive the benefit of any sediment. But the analysis of that water, or of the clear Nile water itself, does not show it to contain any unusual amount of fertilizing matter in solution.

There are other examples, too, of lands perpetually productive for thousands of years without fertilization. One is the "regur" lands region of the Deccan, forming part of the plateau of south central India; another is the "loess" region of China, drained by the head waters of the Yellow River, and for ages the granary of that empire. In both these cases no alluvial fertilizing deposits come into play, and there is little or no irrigation. But in both cases we have a semiarid climate, the rainfall being close to the limit of actual deficiency; in the case of Egypt the deficiency is extreme during nine months out of the twelve.

What, then, is the effect of a deficient rainfall upon the nature of soils formed under its influence? And can these effects serve to explain, in any measurable degree, the choice of the ancient civilizations?

In the course of his investigations of the soils of the United States, the writer has had occasion to make extensive comparisons of the soils of the Atlantic humid region with those of the arid and semiarid West. A summary of the results of these comparisons was given in Bulletin No. 3 of the United States Weather Bureau in 1892. They led to important conclusions of a general nature, some of which could have been readily foreseen on general principles. Continued investigations made since have given additional confirmation, and have developed new facts having important bearings upon the possible utilization and productive