Beyond this to the northward the extension of the range is quite difficult to definitely assign, for, geologically speaking, the Sierra Nevada system occupies a broad area in Oregon, consisting of several prominent mountain groups, while in a physical sense the chain ceases with Shasta; the Cascades, which are the apparent topographical continuation, being a tertiary structure formed chiefly of lavas which have been outpoured long subsequent to the main upheaval of the Sierra.
It is not easy to point out the actual southern limit either, because where the mountain mass descends into the Colorado desert it comes in contact with a number of lesser groups of hills, which ramify in many directions, all losing themselves beneath the tertiary and quartenary beds of the desert.
For four hundred miles the Sierras are a definite ridge, broad and high, and having the form of a sea-wave. Buttresses of sombre-hued rock, jutting at intervals from a steep wall, form the abrupt eastern slopes; irregular forests, in scattered growth, huddle together near the snow. The lower declivities are barren spurs, sinking into the sterile flats of the Great Basin.
Long ridges of comparatively gentle outline characterize the western side, but this sloping table is scored from summit to base by a system of parallel transverse cañons, distant from one another often less than twenty-five miles. They are ordinarily two or three thousand feet deep, falling at times in sheer, smooth-fronted cliffs, again in sweeping curves like the hull of a ship, again in rugged, V-shaped gorges, or with irregular, hilly flanks opening at last through gateways of low, rounded foot-hills out upon the horizontal plain of the San Joaquin and Sacramento.
Every cañon carries a river, derived from constant melting of the perpetual snow, which threads its way down the mountain—a feeble type of those vast ice-streams and torrents that formerly discharged the summit accumulation of ice and snow while carving the cañons out from solid rock. Nowhere on the continent of America is there more positive evidence of the cutting power of rapid streams than in these very cañons. Although much is due to this cause, the most impressive passages of the Sierra valleys are actual ruptures of the rock; either the engulfment of masses of great size, as Professor Whitney supposes in explanation of the peculiar form of the Yosemite, or a splitting asunder in yawning cracks. From the summits down half the distance to the plains, the cañons are also carved out in broad, round curves by glacial action. The summit-gorges themselves are altogether the result of frost and ice. Here, even yet, may be studied the mode of blocking out mountain peaks; the cracks riven by unequal contraction and expansion of the rock; the slow leverage of ice, the storm, the avalanche.
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.
In the centre of the coniferous belt is assembled the most remarkable family of trees. Those which approach the perpetual snow are imperfect, gnarled, storm-bent; full of character and suggestion, but lacking the symmetry, the rich, living green, and the great size of their lower neighbors. In the other extreme of the pine-belt, growing side by side with foothill oaks, is an equally imperfect species, which, although attaining a very great size, still has the air of an abnormal tree. The conditions of drought on the one hand, and rigorous storms on the other, injure and blast alike, while the more verdant centre, furnishing the finest conditions, produces a forest whose profusion and grandeur fill the traveller with the liveliest admiration.
Toward the south the growth of the forest is more open and grove-like, the individual trees becoming proportionally larger and reaching their highest development. Northward its density increases, to the injury of individual pines, until the branches finally interlock, and at last on the shores of British Columbia the trunks are so densely assembled that a dead tree is held in its upright position by the arms of its fellows.