Higher and more stately the tree grew, and in 1729 it attained the age of two thousand years. At the age of two thousand and eighty-eight years the fourth fire attacked it. This fire burned an eighteen-foot scar upon the trunk of the old tree. In 1900, after the lapse of almost a century, only a small part of this wound was overgrown. This year, 1900, came the reaper, the axeman, who laid low this aged and monumental tree! What starts forest fires? Some are started by lightning; others are kindled by meteors that are flung from the sky, or by fire that is hurled or poured from a volcano; a few are caused by spontaneous combustion; and many are set by man. Down through the ages primitive and civilized men have frequently set fire to the forest. These fires are set sometimes accidentally, sometimes intentionally. The forest has been fired to drive out game, to improve pasturage, to bewilder the enemy during war, and to clear the land for the plow. During one of my Colorado camping-trips a high October wind brought me the information that spruce wood was burning near by. While I was searching for the fire in the thick needle carpet of the forest floor, a spark from above settled before me. A fire was sputtering and starting in a tree top about thirty feet above the earth. This fire was starting where a dead leaning tree-trunk was rasping and rubbing against an upright one. The bark of the standing tree was powdered and tufted with wood-dust which had been ground by friction from the trunks as they swayed and rotated in the wind. This inflammable wood-dust, together with accumulated bark-bits and needles, had been set on fire from the heat generated by these two big sticks rubbing together. Plainly this was a friction fire. The incessant swaying of treetops in the tireless wind occasionally causes a smoke from friction at points where overlapping limbs or entangled trees are rubbing. Within a few minutes after my discovery, this fire was roaring eagerly through the treetops. Friction fires are rare, but my old notebooks tell of numerous fires that were set by lightning. Before this fire, which was in the Sangre de Cristo Mountains, had died out, a lightning-set fire in the mountains of central Colorado had attracted my attention with massive, magnificent smoke-clouds, which were two or three thousand feet above the mountain-tops. Though thirty miles distant, these clouds occasionally took on the bossy white splendor of big cumuli assembling for summer rain. I resolved to see the fire at close range.