ON THE GREAT ALKALI PLAIN. In the central portion of the great North American Continent there lies an arid and repulsive desert, which for many a long year served as a barrier against the advance of civilisation. From the Sierra Nevada to Nebraska, and from the Yellowstone River in the north to the Colorado upon the south, is a region of desolation and silence. Nor is Nature always in one mood throughout this grim district. It comprises snow-capped and lofty mountains, and dark and gloomy valleys. There are swift-f…