Page:Popular Science Monthly Volume 43.djvu/316

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302
THE POPULAR SCIENCE MONTHLY.

The fossil trees exposed at this point are along the upper slope of the southern wall of Lamar Valley. The slope here makes an angle of about thirty-three degrees with the horizon, and is about nine hundred feet long. The petrifactions are standing all the way up this slope, interspersed with the living conifers of to-day, represented at Fig. 1. At first sight it appears that either these ancient trees grew upon the slope now exposed, and that there had been no change in the slope from that day to this, or that the present had brought back exactly the same surface conditions as existed when the now silicified trees were alive. Such an apparently simple conclusion would, however, involve more remarkable phenomena than are yielded by the true explanation.

A little consideration, taken in connection with the formation of the bluffs that connect Lamar Valley with the higher lands to

PSM V43 D316 Ancient and living tree line.jpg
Fig. 2.

the south and west, shows clearly the action that has placed the living and petrified trees upon the same slope at this and at many other points in the region.

A series of forests has grown upon successive levels, each level having been produced by an accumulation of volcanic material which destroyed the then existing forest. This explanation will be readily understood from Fig. 2. The level upon which the first forest grew is indicated by 1. The level of the volcanic accumulation which destroyed this growth of trees is shown at 2. Upon this second level came another growth of trees, which in turn was destroyed by the accumulation extending to the level 3. Still another forest grew upon 3, which in course of time was destroyed. This alternate growth and destruction was repeated