Page:Popular Science Monthly Volume 43.djvu/317

From Wikisource
Jump to navigation Jump to search
This page has been proofread, but needs to be validated.
303
FOSSIL FORESTS OF THE YELLOWSTONE.

until at this place (Specimen Ridge) there grew and were destroyed certainly nine successive forests and very probably twelve. This is all indicated in Fig. 2.

The number of growths was determined in two ways: first, where the roots of the petrified trees are shown at different heights in the same vertical plane the horizons of growth may be counted directly; second, when the roots do not show, a sufficient vertical distance must be allowed between horizons to insure that the projecting body at one level does not have its roots in the horizon next below.

In the second method it was sometimes possible to settle the point by following the volcanic ledges to the right or left until a petrifaction with roots exposed, decided the question.

In later times, when the volcanic accumulations had ceased and the agents of denudation began their work, the layers of lava and the great sheets of volcanic conglomerate were gradually eaten away, and a valley formed extending in the figure from e to f. Along the southern slope of this valley are growing the conifers of to-day, and on the same slope also stand the petrified stumps, the relics of many successive forest growths. Thus, though the living and the petrified trees now stand on a common slope, the latter did not, like the former, all grow at the same time, but succeeded each other at intervals of considerable length.

These standing silicified stumps and fallen trees were found varying in diameter from one to seven feet. Two sections of trees were found so perfect that the rings of annual growth throughout could be counted, except a few, perhaps fifteen or twenty, near the heart and bark. One tree, measuring three feet in diameter, had two hundred and twenty-two rings of growth; and another, of three feet five inches diameter, had two hundred and forty-three—this without any allowance for a few missing rings at the center and toward the bark. The larger of these trees was only about half the size of the largest seen. Many were found varying in diameter from five to seven feet, but none of this size were seen exposing the rings throughout the entire section. Judging from the closeness of the rings in certain well-preserved portions of these larger trees, many of them must have been at least five hundred years in attaining their growth, if the rings were truly annual. Taking one half this number, two hundred and fifty years, as the more probable age of the successive forests at this point, it is seen that the earliest of these trees were living more than two thousand years before the latest, during which time there were alternating conditions of growth and accumulation of volcanic material.

This estimate makes no allowance for the time necessary for the formation of a soil upon the volcanic material, which at first