The common occurrence of petrified wood in rocks of different ages is well known. Fossil stems are occasionally found in their natural position of growth, the structural details being rendered permanent by the deposition of siliceous or calcareous material from water drawn by capillarity into the dead but still sound tissues. Petrified wood from Upper Jurassic beds is abundant in the Island of Purbeck; an unusually long piece of stem may be seen in the small town of Portland fixed to the wall of a house. Some of these stems have been referred by an American author to the Araucarian family of Conifers, but the structure is as a rule hardly well enough preserved to afford satisfactory evidence for identification. In his _Testimony of the Rocks_, Hugh Miller speaks of fossil wood from the upper beds of the Jurassic system in sufficient abundance on the beach at Helmsdale in Sutherlandshire to be collected in cart-loads; it is still easy to pick up good specimens on the shingle beach a short distance north of Helmsdale, and a recent microscopical examination showed that some specimens are pieces of an Araucarian tree. Impressive examples of petrified trees on a large scale are to be seen in the United States, in Arizona and the Yellowstone Park. (Frontispiece.) In the northern part of Arizona the country for over an area of 10 square miles is covered with tree trunks, some reaching 200 feet in length and a diameter of 10 feet. The nature of the mineralising substance has given rise to the name Chalcedony Park for this Triassic forest(34). A striking example of one of the Arizona trees is exhibited in the British Museum and in a neighbouring case is a splendid petrified stem, 9 ft. in height, of a conifer discovered in Tertiary lavas in Tasmania(35). [Illustration: Fig. 6. Section of the north face of Amethyst Mountain, Yellowstone Park, including upwards of 2000 ft. of strata. The steepness of the slope is exaggerated. (After W. H. Holmes.)] Figure 6 illustrates the preservation of a series of forests of Tertiary age in the mass of volcanic sediments, 2000 feet in thickness, known as Amethyst mountain, in the Yellowstone Park district. By the weathering away of the surrounding volcanic material the tall stems of the trees are exposed in places on the mountain sides like the 'columns of a ruined temple.' The height of the river at the foot of the cliff is 6700 ft. above sea-level and the mountain rises to a height of 9400 ft. above the sea. In the lower part of the section the volcanic strata are seen to rest on a foundation of older rocks _A_, and these in turn were laid down on the eroded surface of a still more ancient foundation, _B_(36).