So vast is the region occupied by the Pacific forest and so varied the conditions dependent upon climate, soil, and elevation which influence its growth, that great variations in the genera and species of trees composing it are to be expected. This prediction is soon verified when one travels through the forest. The extremes may be indicated briefly by referring to the fact that at the north the trees are mainly spruces, firs, and cedars, and at the south include the giant cactus, arboreal yucca, and the fan-leafed palm. In its medial division are the great forests of western Washington and Oregon, composed mainly of firs and cedars, and the no less magnificent forests of redwood-trees on the Coast Ranges of northern California and the west slope of the Sierra Nevada. Like the boreal forest, the one under consideration is largely composed of coniferous trees, although in the valley, and especially along the borders of streams in southern Canada, Washington, etc., a few species of broad-leaved trees, such as the maple, cottonwood, ash, and alder, thrive in close association with dark conifers; while in similar situations farther south oaks growing in scattered groves give a park-like character to the land, as in the southern portion of California. In contrast with the Atlantic region, the western portion of the continental forest belt is singularly lacking in broad-leaved trees, and such as are found are usually of small size and but little economic importance. This lack, however, is perhaps more than counterbalanced by the number both of individuals and of species and the great size and magnificence of the conifers. One of the densest and in many ways most thoroughly representative portions of the Pacific forest where it occupies an excessively humid region occurs on the west side of the Cascade Mountains in Washington, inclusive of the Puget Sound basin and the region to the westward from which rise the Olympic Mountains.