The nearest approach to desert conditions to be found in North America occur in the valley of Utah and Nevada and the southern portion of the Great Basin region in Arizona and Mexico. The bottoms of these valleys are, in some instances, occupied by shallow lakes in winter, when scanty rains occur, but during the long, hot summers they become completely desiccated, and are then broad expanses of hard mud, cracked by drying so as to resemble a tessellated pavement of cream-coloured marble. These mud-flats or _playas_ are frequently absolutely without plant life. Excepting the playas, however, and, in numerous instances, a narrow belt of ground encircling them, which is white with efflorescent salts, the valleys of even the most arid portion of the Great Basin region are generally plant-covered. The most common and most widely spread of the shrubs on these shadeless plains is the sage-brush. So characteristic is this plant of countless valleys from Canada to Mexico within the general region of the Pacific mountains that to one familiar with the country the term "sage-brush land" brings to mind the leading features of the region designated. The sage-brush lands are far from being desert areas, however, for in early spring a profusion of low, sweet-scented flowers bloom beneath the gray-green _Artemisia_, and sufficient bunch-grass to sustain considerable herds may be expected in the same localities. The vast, irregular belt of forest encircling the central treeless portion of the continent also dies out on its northern border, where the subarctic forest is succeeded northward by the Barren Grounds and tundra plains. Clearly the explanation of the absence of trees in the prairie region and the adjacent plateaus cannot be applied to scarcely less extensive treeless plains at the far north, where rain falls in summer and the soil is always abundantly charged with moisture. It needs no argument to show that the control among the conditions governing tree growth at the north passes to the temperate element of climate, and that the timber-line is there determined, as it is on high mountains, by the severity of the winters' storms and frosts and the shortness of the summers. THE TREELESS MOUNTAIN TOPS