Picture a wide, gently undulating expanse of land covered with tall grass, over which, as it bends to the breeze, a gleam of light ever and anon flashes brightly. It is a rolling prairie in North America, midway between the Atlantic and Pacific Oceans. On either hand the earth and sky seem to unite, without an object to break the line of the horizon, except in the far distance, where some tall trees, by a river's side, shoot up out of the plain, but appear no higher than a garden hedge-row. It is truly a wilderness, which no wise man would attempt to traverse without a guide.