The traveller is never long allowed to forget, in the silent beauty of the supreme wilderness, the park's uncanny nature. Suddenly encountered columns of steam rising from innocent meadows; occasional half-acres of dead and discolored brush emerging from hot and yellow mud-holes within the glowing forest heart; an unexpected roaring hillside running with smoking water; irregular agitated pools of gray, pink, or yellow mud, spitting, like a pot of porridge, explosive puffs of steam; the warm vaporing of a shallow in a cold forest-bound lake; a continuous violent bellowing from the depths of a ragged roadside hole which at intervals vomits noisily quantities of thick brown and purple liquid; occasional groups of richly colored hot springs in an acre or more of dull yellows, the whole steaming vehemently and interchanging the pinks and blues of its hot waters as the passing traveller changes his angle of vision--these and other uncouth phenomena in wide variety and frequent repetition enliven the tourist's way. They are more numerous in geyser neighborhoods, but some of them are met singly, always with a little shock of surprise, in every part of the park. The terrace-building springs in the north of the park engulf trees. The bulky growing mounds of white and gray deposit are edged with minutely carven basins mounted upon elaborately fluted supports of ornate design, over whose many-colored edges flows a shimmer of hot water. Basin rises upon basin, tier upon tier, each in turn destined to clog and dry and merge into the mass while new basins and new tiers form and grow and glow awhile upon their outer flank. The material, of course, is precipitated by the water when it emerges from the earth's hot interior. The vivid yellows and pinks and blues in which these terraces clothe themselves upon warm days result from minute vegetable algae which thrive in the hot saturated lime-water but quickly die and fade to gray and shining white on drying. The height of some of these shapeless masses of terrace-built structures is surprising. But more surprising yet is the vividness of color assumed by the limpid springs in certain lights and at certain angles. Climbing the terraces at the expense of wet feet, one stands upon broad, white, and occasionally very damp plateaus which steam vigorously in spots. These spots are irregularly circular and very shallow pools of hot water, some of which bubble industriously with a low, pleasant hum. They are not boiling springs; the bubbling is caused by escaping gases; but their waters are extremely hot. The intense color of some of these pools varies or disappears with the changing angle of vision; the water itself is limpid.