FOSSIL FORESTS OF THE YELLOWSTONE. 303 



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 hori- 

 zon 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/. 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 com-, 

 mon 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 diame- 

 ter from five to seven feet, but none of this size were seen exposing 

 the rings throughout the entire section. Judging from the close- 

 ness 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. Tak- 

 ing 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 



