This simple arrangement in zones and sections brings the forest, as a whole, within the comprehension of every observer.  The different species are ever found occupying the same relative positions to one another, as controlled by soil, climate, and the comparative vigor of each species in taking and holding the ground; and so appreciable are these relations, one need never be at a loss in determining, within a few hundred feet, the elevation above sea-level by the trees alone; for, notwithstanding some of the species range upward for several thousand feet, and all pass one another more or less, yet even those possessing the greatest vertical range are available in this connection, in as much as they take on new forms corresponding with the variations in altitude.

Crossing the treeless plains of the Sacramento and San Joaquin from the west and reaching the Sierra foot-hills, you enter the lower fringe of the forest, composed of small oaks and pines, growing so far apart that not one twentieth of the surface of the ground is in shade at clear noonday.  After advancing fifteen or twenty miles, and making an ascent of from two to three thousand feet, you reach the lower margin of the main pine belt, composed of the gigantic Sugar Pine, Yellow Pine, Incense Cedar, and Sequoia.  Next you come to the magnificent Silver Fir belt, and lastly to the upper pine belt, which sweeps up the rocky acclivities of the summit peaks in a dwarfed, wavering fringe to a height of from ten to twelve thousand feet.

[Illustration:  EDGE OF THE TIMBER LINE ON MOUNT SHASTA.]

This general order of distribution, with reference to climate dependent on elevation, is perceived at once, but there are other harmonies, as far-reaching in this connection, that become manifest only after patient observation and study.  Perhaps the most interesting of these is the arrangement of the forests in long, curving bands, braided together into lace-like patterns, and outspread in charming variety.  The key to this beautiful harmony is the ancient glaciers; where they flowed the trees followed, tracing their wavering courses along canons, over ridges, and over high, rolling plateaus.  The Cedars of Lebanon, says Hooker, are growing upon one of the moraines of an ancient glacier.  All the forests of the Sierra are growing upon moraines.  But moraines vanish like the glaciers that make them.  Every storm that falls upon them wastes them, cutting gaps, disintegrating boulders, and carrying away their decaying material into new formations, until at length they are no longer recognizable by any save students, who trace their transitional forms down from the fresh moraines still in process of formation, through those that are more and more ancient, and more and more obscured by vegetation and all kinds of post-glacial weathering.

Had the ice-sheet that once covered all the range been melted simultaneously from the foot-hills to the summits, the flanks would, of course, have been left almost bare of soil, and these noble forests would be wanting.  Many groves and thickets would undoubtedly have grown up on lake and avalanche beds, and many a fair flower and shrub would have found food and a dwelling-place in weathered nooks and crevices, but the Sierra as a whole would have been a bare, rocky desert.