A medium-weight truck, unloaded, was being driven northward along one of the secondary highways in the Sacramento Valley. On its floor was lying a piece of two-by-four lumber about two feet long, recently used to block a wheel. The pavement on the secondary highway was not very smooth, and the truck bounced considerably. The two-by-four worked back a little at every jolt until it balanced at the edge. The truck then crossed a badly designed culvert. The two-by-four toppled off, rolled over and slid by the force of its own momentum, and then lay near the edge of the pavement.

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In the year 1579 Sir Francis Drake landed on the California coast, and in the same year a cedar tree was sprouting on the lip of a ravine far up in the Sierra Nevada. Where thousands of contemporary seedlings succumbed to the pitiless competition of the mature forest, this particular sprout survived; it grew for more than two centuries under moderately favorable conditions, and attained a thickness of nearly three feet. It was, however, rooted somewhat insecurely, and in the year 1789 a windstorm overthrew it. The long trunk toppled across the ravine and downhill; with a shattering crash it struck a ledge of rock, and broke into three pieces.

Decay is slow in the Sierra; nevertheless, the base and top of the tree, having fallen into somewhat moist spots, disappeared before the first emigrant train passed close by, letting their wagons down the canyon-side with ropes. The central section of the tree, a jagged-ended bole about twenty feet long and two feet in diameter, lay upon the rocky ledge, as if upon a prepared foundation, almost insulated from contact with moist earth.

The railroad was put through not a hundred yards uphill from where the bole was lying. Below, along the river, men constructed the highway and the pole lines for telephone and telegraph and electric power. The planes hummed high overhead, and upon winter week-ends an up-ski operated nearby.

The bole had lost its bark, but otherwise still seemed sound timber. Inevitably, however, the slow fire of decay burned within it, and chiefly in the lower parts, which were in contact with the rock. Since even partially rotted wood is lighter than sound wood, this process of decay caused the gradual rise of that theoretical point called the center of gravity, until the log was in unstable equilibrium. During the last autumn, as decay weakened the fibers along its lower side, the bole had begun to settle downhill, sometimes as much as a hundredth of an inch a day. Every movement in this direction, however small, brought closer by a month or two the moment at which the bole would have to readjust its position.

Moreover, during the past summer, a chipmunk had burrowed beneath, and now lay hibernating comfortably. By removing a pound or two of gravel he had lessened the capacity of the foundation to resist the slowly shifting weight.

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