The trees are found to occur in different planes or horizons of growth, one above another, until the whole series represents a thickness of many hundreds, and possibly thousands, of feet. Going back to the first of these growths, it is found to have been destroyed by an outpouring of volcanic material, which partially or wholly submerged it. After the flow had ceased, the ordinary atmospheric and aqueous agencies began work, eroding the surface in some places and depositing the products of erosion in others, while vegetation rapidly covered the newly-formed soil. A subsequent flow destroyed this second growth and gave a new horizon, on which the same process was repeated. This continued until there were at least nine, and probably twelve, of these consecutive growths. [Illustration: _Terry Engr. Co._ _U. S. Geological Survey of the Territories._ Section of Amethyst Mountains.] The lava flows in this particular section do not seem to have been characterized by great heat. They were composed of volcanic agglomerate, in which there was a large admixture of mud and water, with sufficient heat to destroy life, but not to char or consume its products. The percolation of siliceous waters gradually turned the arboreal vegetation into stone by the process of substitution, and thus preserved in these silent monuments a record of the events which once transpired there. When the last of the eruptions had ceased, there existed in this locality a vast depth of volcanic _ejectamenta_, composed of many layers, on each of which was standing, buried in the layer next above, the trunks of extinct forest growths. [Illustration: _Terry Engr. Co._ _Haynes, Photo., St. Paul._ Petrified Trees near Yancey's.] After the cessation of volcanic activity, the eroding agencies of the Quaternary Period carved out the valley of the Lamar River through these accumulated flows, and laid bare the remains of their vegetable growths. To-day the tourist may see upon the slopes of Specimen Ridge, side by side, the living and the dead, the little conifers of present growth and the gigantic trunks of unknown species which flourished there eons ago. Some of the petrifactions are very perfect. Roots, bark, parts showing incipient decay, worm holes, leaves--all are preserved with absolute fidelity. The rings of annual growth may be counted, and these indicate for the larger trees an age of not less than five hundred years. Some of the stumps are fully ten feet in diameter. Here and there the ponderous roots stand imbedded in the rock face of the cliff, where erosion has not yet undermined them. In one case, a large tree that had fallen before petrifaction lies partly exposed, both ends being still imbedded in the rock. Some hollow trees show interiors beautifully lined with holocrystalline quartz.