volcanic soil, and were subsequently submerged below sea-level, and covered with sedimentary beds and lava-flows[107].” A striking example of the occurrence of numerous petrified plant stems has been described by Holmes from the Tertiary forests of the Yellowstone Park. From the face of a cliff on the north side of Amethyst mountain “rows of upright trunks stand out on the ledges like the columns of a ruined temple. On the more gentle slopes farther down, but where it is still too steep to support vegetation, save a few pines, the petrified trunks fairly cover the surface, and were at first supposed by us to be the shattered remains of a recent forest[108].” Marsh[109] and Conwentz[110] have described silicified trees more than fifty feet in length from a locality in California where several large forest trees of Tertiary age have been preserved in volcanic strata. In South Africa on the Drakenberg hills there occur numerous silicified trunks, occasionally erect and often lying on the ground, probably of Triassic age[111]. In some instances the specimens measure several feet in length and diameter. Some of the coniferous stems seen in Portland, and occasionally met with reared up against a house side, illustrate the silicification of plant structure on a large scale. These are of Upper Jurassic (Purbeck) age. From Grand’Croix in France a silicified stem of _Cordaites_ of Palaeozoic age has been recorded with a length of twenty meters. The preservation of plants by siliceous infiltrations has long been known. One of the earliest descriptions of this form of petrifaction in the British Isles is that of stems found in Lough Neagh, Ireland. In his lectures on Natural Philosophy, published at Dublin in 1751, Barton gives several figures of Irish silicified wood, and records the following occurrence in illustration of the peculiar properties erroneously attributed to the waters of Lough Neagh. Describing a certain specimen (No. XXVI), he writes:— “This is a whetstone, which as Mr Anthony Shane, apothecary, who was born very near the lake, and is now alive, relates, he made by putting a piece of holly in the water of the lake near his father’s house, and fixing it so as to withstand the motion of the water, and marking the place so as to distinguish it, he went to Scotland to pursue his studies, and seven years after took up a stone instead of holly, the metamorphosis having been made in that time. This account he gave under his handwriting. The shore thereabouts is altogether loose sand, and two rivers discharge themselves into the lake very near that place[112].” The well-known petrified trees from the neighbourhood of Lough Neagh are probably of Pliocene age, but their exact source has been a matter of dispute[113]. [Sidenote: PETRIFIED WOOD.]