Part 2

Above this cliff fossil trunks appear in great numbers and in regular succession. As they are all perfectly silicified, they are more resistant than the surrounding matrix and consequently stand above it. Most of them are only a few inches above the surface, but occasionally one rises as high as 5 or 6 feet. The largest trunk observed in the park is found in this locality. It is a little over 10 feet in diameter, a measurement that includes a part of the bark. It is very much broken down, especially in the interior, probably having been so disintegrated before it was fossilized. It projects about 6 feet above the surface.

At many places about Amethyst Mountain there are numerous fragments of fossil wood and many hollow trunks. The material in which they had been embedded has been eroded away, and they lie around in somewhat the same attitudes that are shown by all the trunks in the Arizona fossil forests, but there is little doubt that they were originally erect and have simply fallen by their own weight because of the removal of the material around them.

Many of the trunks here, as well as elsewhere in the park, had decayed in the center before they were fossilized, and some of the hollow interiors are filled with clusters and rosettes of beautiful crystals of amethyst, which doubtless suggested the name given to the adjacent mountain. Much of this finely preserved wood, as well as the trunks containing the crystals of amethyst, was broken up and carried away by collectors of minerals and curiosities before the Government control in the park was made sufficiently rigid to insure proper protection.

SPECIMEN RIDGE.

In many respects the most remarkable of the fossil forests is on the northwest end of Specimen Ridge, about a mile southeast of Junction Butte and about opposite the mouth of Slough Creek. So far as known, this forest was first brought to scientific attention by Mr. E. C. Alderson, of Bozeman, Mont., and the writer, who discovered it in August, 1887. It is found on the higher part of the ridge, and covers several acres. The trees are exposed at various heights on the very steep hillsides, and one remarkable feature of the forest is that most of them project well above the surface.

One of the largest and best preserved trees stands at the very summit of the slope (see title page). This trunk, which is that of a giant redwood, is 26½ feet in circumference without the bark and about 12 feet in height. The portion of this huge trunk preserved is the base, and it exhibits to a considerable degree the swelling or buttressing so well known in the living redwood. The roots, which are as large as the trunks of ordinary trees, are now embedded in solid rock.

On the steep hillside a short distance below the big tree just mentioned are the two trunks shown in figure 5. They are about 2 feet in diameter and 25 feet high, and stand some 20 feet apart, and we may imagine them to have formed the doorposts of the “ancient temple” of which Holmes speaks. Both these trunks are without the bark. On the left of the figure is one of the huge irregular masses of rock that has been carved out by erosion.

In figure 6 is shown another trunk about 3 feet in diameter and nearly 30 feet high. In several places along the trunk the thick bark may be noted. This tree is a pine, as are the two last described, and slightly below and behind it are two living pine trees, which are about the size it must have been when living. Another trunk, some 12 feet in height, is shown in figure 7, and in figure 8 there may be noted a standing trunk and above it another that has recently fallen.

The height attained by the trees of this fossil forest can not be ascertained with certainty, since the tallest trunk now standing is only about 30 feet high, but every one observed is obviously broken off, and does not show even the presence of limbs. Perhaps the nearest approach to a measure of the height is afforded by a trunk (shown in fig. 10) that happened to have been prostrated before fossilization. This trunk, which is 4 feet in diameter, is exposed for a length of about 40 feet, and as it shows no apparent diminution in size within this distance it is safe to assume that the tree could hardly have been less than 100 feet high and very probably may have been higher. This trunk is wonderfully preserved. As may be seen from the illustration, it has broken up by splitting along the grain of the wood into great numbers of little pieces, which closely resemble pieces of “kindling wood” split from a clear-grained block. In fact, at a distance of a few yards it would be impossible to distinguish this fossil “kindling wood” from that split from a living tree.

The large redwood trunk already mentioned (title-page) as being nearly 10 feet in diameter may be compared with its living relative of the Pacific coast in order to calculate its probable height. The living redwood is usually 10 to 15 feet in diameter and ranges in height from 200 to 310 feet, and as the two are so very closely related there is no reason to suppose that the fossil trunk was of less height, but by a moderate estimate it may be accredited with a minimum height of 200 feet.

TOWER FALLS.

The most accessible fossil forest, marked "Petrified Trees" on the map, is west of the Tower Falls Ranger Station and Camp Roosevelt on the road from the Grand Canyon to Mammoth Hot Springs, by way of Mount Washburn. It is on the middle slope of a hill that rises about 1,000 feet above the little valley and may be reached by a branch road from the main loop road. As the traveler approaches the forest he will observe a number of trunks standing upright among the stumps and trunks of living trees, and so much resembling them that a near view is necessary to convince him that they are really fossil trunks. Only two rise to a considerable height above the surface. The larger one is about 15 feet high and 13 feet in circumference (fig. 11): the other is a little smaller. As the roots are not exposed, it is impossible to determine the position of the part in view or the original diameter of the trees, as the bark is nowhere preserved.

Above these standing trunks lie many others, which the disintegrating forces of nature break up into small fragments and keep at about the same level as that of their surrounding matrix. Some of these trunks rise only a few inches from the surface: others are nearly covered by shifting débris. Their diameter ranges from 1 to 14 feet, and they are so perfectly preserved that the rings of growth can easily be counted. The internal structure is also in most trunks nearly as perfect as when the trees were living.

CACHE CREEK.

The forest that is next in size to the one a mile southeast of Junction Butte is on Cache Creek, about 7 miles above its mouth. It is on the south bank of the creek and covers several acres. The trunks are scattered from bottom to top of the slopes through a height of probably 800 feet. Most of the trunks are upright, but only a few project more than 2 or 3 feet above the surface. The largest one observed was 6 feet in height and 4 feet in diameter. Most of these trunks appear to the naked eye to be conifers, but a number are obviously dicotyledons—that is, they were deciduous-leaved trees. The conifers, however, were the predominant element in this as in the other fossil forests.

The slopes of the Thunderer, the mountain so prominently in view from Soda Butte on the south, also bear numerous fossil trunks. Most of them are upright, but only a very few project more than 2 feet above the surface. No remarkably large trunks were observed at this locality, the average diameter being perhaps less than 2 feet.

OTHER LOCALITIES.

Mount Norris, which is hardly to be separated from the Thunderer, also bears a small fossil forest. The trees are of about the same size and character as those in the larger mountain. Fossil forests of greater or less extent, composed mainly of upright trunks, are exposed also on Baronett Peak, Bison Peak, Abiathar Peak, Crescent Hill, and Miller Creek. In fact, there is hardly a square mile of the area of the northeastern portion of the park that is without its fossil forest, scattered trunks, or erratic fragments.

The vast area east of the Yellowstone Lake and the region still farther east, beyond the limits of the park, have not been thoroughly explored, but enough is known to make it certain that these areas contain more or less fossil wood. The stream beds in these areas in many places contain fragments of fossil wood, which indicates that trunks of trees must be near at hand.

THE PROCESS OF FOSSILIZATION.

The manner in which these forests were fossilized may next be considered. Though the whole history of the process is not fully understood, it was undoubtedly dependent on or at least greatly facilitated by the presence of volcanic and hydrothermal activity, which was doubtless then, as it is to some extent now, a marked feature of the park region. At least a hint of the probable process is afforded by the action now going on in the hot spring areas. Many of those areas are closely surrounded by forests, and unless the action of the springs is very violent the trees may be growing only a short distance away. Occasionally a hot spring may break out near the edge of a forest, the first effect being, of course, to kill the trees. In a few years, by the action of the ordinary processes of decay, a tree so killed may have lost its bark and most of its smaller branches. The hot water which constantly or intermittently surrounds the tree contains a considerable amount of silica in solution, and as this hot silica-charged water is drawn up into the wood by capillarity the silica may be deposited in the cells of the wood after the water cools or evaporates. The first result will be a more or less complete cast of the interior of the cells and vessels of the wood. This much of the process has actually been observed, but as decay is more rapid than silicification, the wood crumbles to dust before petrifaction is complete. If the trunk could be surrounded by ashes or mud and thus protected from atmospheric action, it might in time be completely turned to stone.

The fossil forests are surrounded by a matrix that is known as an acidic lava—that is, a siliceous lava—which contains abundant silica in solution. The first part of the process of silicification may well have been that above described as taking place in the hot spring areas at the present day—that is, the silica would be deposited in all the cells and vessels of the wood, making an accurate cast of all open spaces. Then, while the slow process of decay went on, as each particle of organic matter was removed its place was taken by the silica, until, finally, all the wood substance had disappeared and its place atom by atom had been taken by silica.

By this or a similar process the wood has been preserved or fossilized with remarkable fidelity: in fact, thin sections or slices of the fossil wood may be studied under the higher powers of the microscope with almost or quite as much completeness and satisfaction as if they were sections cut from a piece of living wood. Each cell and vessel, with its characteristic pits and markings, is preserved exactly as it grew. Some of the wood, however, was evidently more or less decayed before it was fossilized, or else decay worked faster than replacement, so that in some fragments the structure is not so clearly preserved. Many of the trunks were subjected to pressure before replacement was complete, and this has crushed or distorted the cells. On the whole, however, the wood is exceptionally well preserved, as may be seen in figures 12, 13, 14, and 15. These are all magnified 100 diameters and were photographed directly from the thin sections—that is, they are photomicrographs—and have not been retouched in any manner. Figure 12 shows a transverse section of the wood of the large redwood trunk that has been so often mentioned (see title page). The section is cut through one of the growth rings, which consists of 12 or 15 rows of very thick-walled cells. The large, regular thin-walled cells, which begin abruptly above the growth ring, belong to the spring wood—that is, the wood first formed after growth starts in spring, when the supply of nourishment is abundant. If there is sufficient moisture and all conditions are favorable this vigorous growth of wood cells may continue without interruption until the approach of cold or dry weather, but not infrequently there may be a brief shortage of moisture, and this is reflected in the formation of a few rows of thicker-walled cells. Such a condition may be observed in the present specimen, in which a slight, partial ring may be seen at some distance above the main ring.

The very perfect preservation of the wood of one of the pines (Pityoxylon amethystinum) is shown in figure 13, a view of a section cut through a part of a growth ring and into the spring and summer wood, the rings in this species being so broad that it is impossible to show a complete one. The opening near the bottom of the figure shows one of the large resin ducts, which, in the living wood, is filled with the “pitch” that so readily exudes when a branch is cut or broken. A longitudinal section of the other species of pine (Pityoxylon aldersoni) is shown in figure 14. The many little rows of superimposed cells in the midst of the long wood cells are the cut-off ends of what are known as medullary rays—that is, the little plates of cells that connect pith and bark. One of the resin cells cut in the long direction is shown near the center of the figure; the contents are much darker than that of the wood cells.

The very great difference between the sections of coniferous wood just described and the wood of a deciduous tree is brought out in figure 15, which is a longitudinal section of a laurel (Laurinoxylon pulchrum). In this the wood cells are relatively much smaller and shorter, and the medullary rays are in several irregular rows. The large dotted duct near the middle of the figure is a feature not present in coniferous trees.

SPECIES REPRESENTED.

An enumeration of the kinds of trees that are represented by the woods in the fossil forests of the Yellowstone National Park will naturally be demanded. A superficial or macroscopic examination of these trunks would not permit a close decision as to the kind of wood: in fact, it would hardly be possible to do more than separate them by this means into coniferous and dicotyledonous trees. But by studying thin sections under the microscope it is possible to distinguish the different kinds with reasonable accuracy. As the result of such study the following species have been detected:

Magnificent redwood (Sequoia magnifica). Alderson’s pine (Pityoxylon aldersoni). Amethyst pine (Pityoxylon amethystinum). Laurel (Laurinoxylon pulchrum). Aromatic bay (Perseoxylon aromaticum), Hayden’s sycamore (Plantaninium haydeni). Knowlton’s sycamore (Plantaninium knowltoni). Felix’s buckthorn (Rhamnacinium radiatum). Lamar oak (Quercinium lamarense). Knowlton’s oak (Quercinium knowltoni).