This uplift brought with it the activity of erosion which has continued through the ages until finally almost all of the 3,000 feet of upper layers of material have been washed away, and the many logs, that had once been so deeply buried, have again been exposed on the surface; but now as hard, colorful stone. Erosion continued to carry the soil away from the petrified logs, exposing more and more of them. As forces of erosion lowered the surface of the ground little by little, the petrified logs, too hard to be affected, settled with it, eventually accumulating with sections of other logs that had been buried on a lower level. Thus, the present surface of the ground is rather thickly covered, in many spots, with wood that was originally scattered through approximately the upper 100 feet of this Chinle Formation. In the vicinity of the Rainbow and Third Forest, at least, about 300 feet more of this formation still remains. So far as we know, wood is to be found throughout this entire layer. Therefore, theoretically at least, it may be said that 25 per cent of the petrified wood that is here is visible on the surface, the rest still remaining buried below. Three species of trees have been found here in petrified form. The most common one is an Araucarian Pine (_Araucarioxylon Arizonicum_), a primitive member of the pine family. This species became extinct long ago, but there are still several species of modern Araucaria native to South America, Australia, New Zealand, and other South Pacific islands which are apparently very similar to this ancient form. Some of the modern types have been imported to this country and are used for ornamental purposes in certain locations in Florida and along the Pacific coast. The most common ones are known as the “Monkey Puzzle Tree” and “Norfolk Island Pine.” Claims made by roadside stand operators along Highway 66 that the petrified wood offered for sale is “beach walnut,” “cactus,” etc., have no basis in fact. Two other types of petrified wood are found here in smaller amounts. These are the _Woodworthia Arizonica_, a cone-bearing tree somewhat similar to the Araucaria and the _Schilderia Adamanica_, a tree with peculiar radiating rays in the wood. Paleontologists are not sure where this latter kind belongs in systematic plant classification. What happened during the millenniums that the logs lay buried in their Chinle tombs? How did these trees turn to stone? Most of our text books tell us that the petrification of wood is a replacement process. Bit by bit, water removed wood tissue and in its place left a mineral deposit in exactly the same form as the original, so that when the process had been completed there was no wood left but in its place an exact stone duplicate. This theory was accepted for a very long time, but recently some scientists were not satisfied with it because certain chemical actions that would have to occur during such a process were difficult to explain.