The western descent, facing a moisture-laden, aërial current from the Pacific, condenses on its higher portions a great amount of water, which has piled upon the summits in the form of snow, and is absorbed upon the upper plateau by an exuberant growth of forest. This prevalent wind, which during most undisturbed periods blows continuously from the ocean, strikes first upon the western slope of the Coast Range, and there discharges, both as fog and rain, a very great sum of moisture; but, being ever reinforced, it blows over their crest, and, hurrying eastward, strikes the Sierras at about four thousand feet above sea-level. Below this line the foothills are oppressed by an habitual dryness, which produces a rusty olive tone throughout nearly all the large conspicuous vegetation, scorches the red soil, and, during the long summer, overlays the whole region with a cloud of dust.
Dull and monotonous in color, there are, however, certain elements of picturesqueness in this lower zone. Its oak-clad hills wander out into the great, plain-like coast promontories, enclosing yellow or, in spring-time, green bays of prairie. The hill-forms are rounded, or stretch in long, longitudinal ridges, broken across by the river cañons. Above this zone of red earth, softly modelled undulations, and dull, grayish groves, with a chain of mining towns, dotted ranches and vineyards, rise the swelling middle heights of the Sierras, a broad, billowy plateau cut by sharp, sudden cañons, and sweeping up, with its dark, superb growth of coniferous forest to the feet of the summit-peaks.
For a breadth of forty miles, all along the chain, is spread this continuous belt of pines. From Walker’s Pass to Sitka one may ride through an unbroken forest, and will find its character and aspect vary constantly in strict accordance with the laws of altitude and moisture, each of the several species of coniferous trees taking its position with an almost mathematical precision. Where low gaps in the Coast Range give free access to the western wind, there the forest sweeps downward and encamps upon the foot-hills, and, continuing northward, it advances toward the coast, securing for itself over this whole distance about the same physical conditions; so that a tree which finds itself at home on the shore of Puget’s Sound, in the latitude of Middle California has climbed the Sierras to a height of six thousand feet, finding there its normal requirements of damp, cool air. As if to economize the whole surface of the Sierra, the forest is mainly made up of twelve species of coniferæ, each having its own definitely circumscribed limits of temperature, and yet being able successively to occupy the whole middle Sierra up to the foot of the perpetual snow. The average range in altitude of each species is about twenty-five hundred feet, so that you pass imperceptibly from the zone of one species into that of the next. Frequently three or four are commingled, their varied habit, characteristic foliage, and richly colored trunks uniting to make the most stately of forests.