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National Geographic : 1894 Jun 22
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The Afforestation of the desert Areas. everywhere. While a vast territory on the Atlantic side and a narrower belt on the Pacific coast, connected by a broad belt through the northern latitudes, was almost entirely under its undisputed sway, and while the back-bone of the continent, the crest and slopes of the Rocky mountains, was more or less in its possession, there still remained a vast empire in the interior unconquered. Of parts of this territory we feel reasonably certain from strong evidences that the forest once occupied them, but has been driven off by aboriginal man, the firebrand taking sides with the grasses, and the buffalo probably being a potent element in preventing reestablishment. In other parts it is questionable whether the lines along the river courses, the straggling trees on the plateaus and slopes, are remnants of a vanquished army or outposts of an advancing one. In some parts, like the dry mesas, plateaus and arroyos of the interior basin, and th.e desert-like valleys toward the southern frontiers, it may reasonably be doubted whether arborescent flora has more than begun its slow advance from the outskirts of the established territory. Certain it is that climatic conditions in these forestless regions are most unfavorable to tree-growth, and it may well be ques tioned whether in some parts the odds are not entirely against the progress of the forest. Temperature and moisture conditions of air and soil, determine ultimately the character of vegetation, and these are dependent not only on latitude, but largely on configuration of the land, and especially on the direction of moisture-bearing winds with reference to the trend of mountains. The winds from the Pacific ocean striking against the Coast range are forced by the compression and subsequent cooling to give up much of their moisture on the windward side; a second impact and further condensation of the moisture takes place on the Cascade range and Sierra Nevada. On descending, with consequent expansion, the wind becomes warmer and drier, so that the interior basin, without additional sources of moisture and no additional cause for condensation, is left without much rainfall and with a very low relative humidity, namely, below 50 per cent. The Rocky mountains finally squeeze out whatever moisture remains in the air currents, which arrive proportionally drier on the eastern slope. This dry condition extends over the plains until the moist currents from the gulf of Mexico modify 137
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