The spread of Quercus macrocarpa, or bur oak, in Kansas is shown in figure 1, which copies Gates' map of its distribution according to counties (2) based on collections in the Kansas State Herbarium, superposed on a photographic adaptation of Throckmorton's soil map lacking the differentiating colors of the original (7), gives the mean annual precipitation of each county up to 1930 and demarks the Shelterbelt Zone in Kansas. Fourteen major soil regions are shown on the map, classified according to the basic materials from which the soils are derived and the agents of their deposition (wind, water, glaciers, decay of rock already in position forming residual soil). Throckmorton calls attention to the fact that there are many minor varieties of soil in each region which cannot be shown because of the small scale of the map. He tells us that, while the soil in some places is barren, as a whole it originally was, and still is, in various degrees, fertile. Kansas has 105 counties. The bur oak occurs in at least fifty-six of these. Scanning the map we observe that if a line were drawn from its northwest to its southeast corner, the bur oak would be found in but nine counties of the southwest half while occurring in forty-seven counties of the northeast half. The average of the mean annual rainfall of the counties of the upper half is slightly over thirty-one inches, while that of the lower half is close to twenty-four inches, a difference of more than seven inches. Undoubtedly this difference must be an important factor causing the disparity in the spread of the bur oak in the two halves, but we may conclude it is not always the prevailing factor, for when we compare the counties individually we see that some counties with the bur oak have less rainfall than some others without it. Besides precipitation, evaporation, topography, and soil characters are influences of primary importance. As to the soil, figure 1 shows that the bur oak is found in all of our principal kinds, excepting three in the southwest corner, where extremely low rainfall would seem enough to exclude it. The farthest west the bur oak occurs in Kansas is in Sheridan county, a county of the shelterbelt zone, with a mean annual precipitation of 2024 inches, and it is found in no other county with a mean precipitation as low as that. The bur oak occurs nowhere in the Kansas section of the drainage basin of the Cimarron excepting in Barber and Harper counties, where a broken topography affords relatively moist areas due to seepage and protection from wind in depressions, draws and ravines; and, suprisingly, it occurs spontaneously nowhere in the vast drainage basin of the Arkansas river, excepting in the easternmost section in Sedgwick and Cowley counties. However, north of this basin the bur oak makes a westward advance along the valleys of the Smoky Hill and Saline rivers and the north and south forks of the Solomon river, and the creeks, ravines, and draws that drain into these rivers. Fastening our attention on the shelterbelt zone we find there twenty-nine whole or fractional counties, and the bur oak occurring in but eight of these
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